A doping strategy to regulate the adsorption energy of Li2S4 and Li2S to promote sulfur reduction on Chevrel phase Mo6Se8 in lithium-sulfur batteries.

Atomic doping in catalysts is an effective strategy for adjusting their catalytic activity, which has recently been applied to promote sulfur reduction reactions (SRRs) on the cathode of lithium-sulfur (Li-S) batteries. Herein, the electrocatalytic SRR mechanism of eight metal atom (Ca, Ti, V, Cr, Mn, Fe, Co or Ni) doped Chevrel phase Mo6Se8 were investigated using density functional theory (DFT) calculations. The results reveal that the interaction between polysulfides and the catalyst mainly originates from the coupling of dz2 and dxz orbitals of doped metals and the 3p orbitals of S. The Ti-doped Mo6Se8 system significantly reduces the overpotential of the SRR to only 0.21 V. After analyzing SRR processes over doped and undoped Mo6Se8, no scalar relationship was found between the adsorption energies (Ead) of various polysulfides. Instead, a linear relationship is established between 4Ead-Li2S* - Ead-Li2S4* and overpotential. Finally, a linear relationship between overpotential and descriptors was established based on a machine learning (ML) method, which can accurately predict the overpotential of the SRR over the Mo6Se8 catalyst. This work provides new theoretical insights into the SRR mechanism over metal-selenides and the rational design of a catalyst for Li-S batteries.

Theoretical Insights into Single-Atom Catalysts Supported on N-Doped Defective Graphene for Fast Reaction Redox Kinetics in Lithium-Sulfur Batteries.

Single-atom catalysts are proven to be an effective strategy for suppressing shuttle effect at the source by accelerating the redox kinetics of intermediate polysulfides in lithium-sulfur (Li-S) batteries. However, only a few 3d transition metal single-atom catalysts (Ti, Fe, Co, Ni) are currently applied for sulfur reduction/oxidation reactions (SRR/SOR), which remains challenging for screening new efficient catalysts and understanding the relationship between structure-activity of catalysts. Herein, N-doped defective graphene (NG) supported 3d, 4d, and 5d transition metals are used as single-atom catalyst models to explore electrocatalytic SRR/SOR in Li-S batteries by using density functional theory calculations. The results show that M1 /NG (M1 = Ru, Rh, Ir, Os) exhibits lower free energy change of rate-determining step ( Δ G Li 2 S ∗ ) $( {\Delta {G}_{{\mathrm{Li}}_{\mathrm{2}}{{\mathrm{S}}}^{\mathrm{*}}\ }} )$ and Li2 S decomposition energy barrier, which significantly enhance the SRR and SOR activity compared to other single-atom catalysts. Furthermore, the study accurately predicts the Δ G Li 2 S ∗ $\Delta {G}_{{\mathrm{Li}}_{\mathrm{2}}{{\mathrm{S}}}^{\mathrm{*}}\ }$ by machine learning based on various descriptors and reveals the origin of the catalyst activity by analyzing the importance of the descriptors. This work provides great significance for understanding the relationships between the structure-activity of catalysts, and manifests that the employed machine learning approach is instructive for theoretical studies of single-atom catalytic reactions.

Li ion diffusion behavior of Li3OCl solid-state electrolytes with different defect structures: insights from the deep potential model.

Li3OX (X = Cl, Br), a lithium-rich anti-perovskite material developed in recent years, has received tremendous attention due to its high ionic conductivity of >10-3 S cm-1 at room temperature. However, the origin of the high ionic conductivity of the material at the atomic level is still not clear. In this work, we investigated the dynamic behavior of the Li3OCl system with three different defect structures (Li-Frenkel, LiCl-Schottky, and Cl-O anti-site disorder) at seven temperature intervals and calculated its ionic conductivity using the deep potential (DP) model. The results show that the presence of LiCl-Schottky defects is the main reason for the high performance of the Li3OCl system, and the Li vacancy is the main carrier. The ionic conductivity obtained from the DP model is 0.49 × 10-3 S cm-1 at room temperature and it can reach 10-2 S cm-1 above the melting point, which is in the same order of magnitude as the experimentally reported results. We also explored the effect of different defect concentrations on the ionic conductivity and migration activation energy. This work also demonstrates the feasibility of the DP method for solving the accuracy-efficiency dilemma of ab initio molecular dynamics (AIMD) and classical molecular dynamics simulations.

A New Ratiometric Design Strategy Based on Modulation of π-Conjugation Unit for Developing Fluorescent Probe and Imaging of Cellular Peroxynitrite.

Ratiometric fluorescent probes could effectively offset the changes of the autofluorescence and compartmental localization. FRET, ICT, etc. are the common strategies to design probes for biosensing, but these strategies have some deficiencies. Here, we proposed a new design strategy based on π-conjugation modulation, giving two different emission bands in the absence and presence of the target. The new fluorescence probe named Rhod-DCM-B was rationally designed and synthesized, which displayed a fluorescence emission peak at 670 nm because the electron cloud focuses on the conjugated DCM unit. With the addition of ONOO-, the fluorescence emission at 570 nm increased, accompanied by the decrease of fluorescence emission at 670 nm, showing a ratiometric signal change attributed to the opened spirane structure making the electron cloud concentrated on the xanthene core. The mechanism is well confirmed by MS and DFT calculations. Rhod-DCM-B exhibited outstanding sensitivity and excellent selectivity toward ONOO-. Moreover, Rhod-DCM-B was effectively employed to determine endogenous and exogenous ONOO- in living cells. As a marker for inflammation and drug-induced liver injury (DILI) process, ONOO- in vivo was successfully monitored by Rhod-DCM-B and presented a dramatic ratiometric response.

Low Bandgap Donor-Acceptor π-Conjugated Polymers From Diarylcyclopentadienone-Fused Naphthalimides.

Two novel aromatic imides, diarylcyclopentadienone-fused naphthalimides (BCPONI-2Br and TCPONI-2Br), are designed and synthesized by condensation coupling cyclopentadienone derivatives at the lateral position of naphthalimide skeleton. It has been found that BCPONI-2Br and TCPONI-2Br are highly electron-withdrawing acceptor moieties, which possess broad absorption bands and very low-lying LUMO energy levels, as low as -4.02 eV. On the basis of both building blocks, six low bandgap D-A copolymers (P1-P6) are prepared via Suzuki or Stille coupling reactions. The optical and electrochemical properties of the polymers are fine-tuned by the variations of donors (carbazole, benzodithiophene, and dithienopyrrole) and π-conjugation linkers (thiophene and benzene). All polymers exhibit several attractive photophysical and electrochemical properties, i.e., broad near-infrared (NIR) absorption, deep-lying LUMO levels (between -3.88 and -3.76 eV), and a very small optical bandgap ( E g opt ) as low as 0.81 eV, which represents the first aromatic diimide-based polymer with an E g opt of <1.0 eV. An investigation of charge carrier transport properties shows that P5 exhibits a moderately high hole mobility of 0.02 cm2 V-1 s-1 in bottom-gate field-effect transistors (FETs) and a typical ambipolar transport behavior in top-gate FETs. The findings suggest that BCPONI-2Br, TCPONI-2Br, and the other similar acceptor units are promising building blocks for novel organic semiconductors with outstanding NIR activity, high electron affinity, and low bandgap, which can be extended to various next-generation optoelectronic devices.

Growth-Rule-Guided Structural Exploration of Thiolate-Protected Gold Nanoclusters.

Understanding the structure and structure-property relationship of atomic and ligated clusters is one of the central research tasks in the field of cluster research. In chemistry, empirical rules such as the polyhedral skeleton electron pair theory (PSEPT) approach had been outlined to account for skeleton structures of many main-group atomic and ligand-protected transition metal clusters. Nonetheless, because of the diversity of cluster structures and compositions, no uniform structural and electronic rule is available for various cluster compounds. Exploring new cluster structures and their evolution is a hot topic in the field of cluster research for both experiment and theory. In this Account, we introduce our recent progress in the theoretical exploration of structures and evolution patterns of a class of atomically precise thiolate-protected gold nanoclusters using density functional theory computations. Unlike the conventional ligand-protected transition metal compounds, the thiolate-protected gold clusters demonstrate novel metal core/ligand shell interfacial structures in which the Au m(SR) n clusters can be divided into an ordered Au(0) core and a group of oligomeric SR[Au(SR)] x ( x = 0, 1, 2, 3, ...) protection motifs. Guided by this "inherent structure rule", we have devised theoretical methods to rapidly explore cluster structures that do not necessarily require laborious global potential energy surface searches. The structural predictions of Au38(SR)24, Au24(SR)20, and Au44(SR)28 nanoclusters were completely or partially verified by the later X-ray crystallography studies. On the basis of the analysis of cluster structures determined by X-ray crystallography and theoretical prediction, a structural evolution diagram for the face-centered-cubic (fcc)-type Au m(SR) n clusters with m up to 92 has been preliminarily established. The structural evolution diagram indicates some basic structural and electronic evolution patterns of thiolate-protected gold nanoclusters. The fcc Au m(SR) n clusters show a genetic structural evolution pattern in which each step of cluster size increase results in the formation of another Au4 tetrahedron or Au3 triangle unit in the Au core, and every increase of a structural unit in the Au core leads to an increase of two electrons in the whole cluster. The unique one- or two-dimensional cluster size evolution, the isomerism of the Au-S framework, and the formation of a double-helical and cyclic tetrahedron network in the fcc Au m(SR) n clusters all can be addressed from this evolution pattern. The summarized cluster structural evolution diagrams enable us to further explore more stable cluster structures and understand their structure-electronic structure-property relationships.

A revisit to the structure of Au20(SCH2CH2Ph)16: a cubic nanocrystal-like gold kernel.

Coinage metal clusters stabilized by organic ligands such as phosphine or organothiolate are well known to possess multi-twinned gold cores, and the face-centered-cubic (fcc) metal atom packing is unstable until the cluster size reaches a certain threshold. In this study, we searched for the smallest size gold nanocrystal protected by thiolate ligands by means of the crystal facet cleavage (CFC) method. Starting from the nanocrystal-like Au28(SR)20 cluster, after cleaving two different crystal facets and patching the ligand shells, we obtained five nanocrystal-like Au20(SR)16 isomers. These fcc-structured Au20 clusters were quite different from non-fcc Au20(SPh-tBu)16; the latter's total structure was determined by single X-ray diffraction. By employing dispersion correction density functional theory (DFT-D) calculations and considering ligand effects, we found that fcc-structured Au20(SR)16 isomers had comparable or even lower energies when compared with the non-fcc structure found in Au20(SPh-tBu)16. Furthermore, the calculation of optical absorption spectra based on predicted fcc isomers indicated that the cubic nanocrystal-like isomer structure is a good candidate to understand the structure of the Au20(SCH2CH2Ph)16 cluster.

Exploring the structure evolution and core/ligand structure patterns of a series of large sized thiolate-protected gold clusters Au145-3N(SR)60-2N (N = 1-8): a first principles study.

The atomic structures of many atomically precise nanosized ligand protected gold clusters have been resolved recently. However, the determination of the atomic structures of large sized ligand protected gold clusters containing metal atoms over ∼100 is still a grand challenge. The lack of structural information of these larger sized clusters has greatly hindered the understanding of the structure evolution and structure-property relations of ligand protected gold nanoclusters. In this work, we theoretically studied the structure evolution of a series of large sized Au145-3N(SR)60-2N (N = 1-8) clusters based on an "[Au2@Au(SR)2] fragmentation" pathway starting from a model Au145(SR)60 cluster. Through comprehensively searching the atomic structure of various clusters and evaluating their stabilities by means of first principles calculations, the stabilization mechanism of experimentally reported Au130(SR)50 and Au133(SR)52 clusters is first rationalized. Our studies indicated that Au130(SR)50 and Au133(SR)52 are two critical sized clusters on which the gold cores underwent configuration transitions between decahedral and icosahedral cores. The energy comparisons of various cluster isomer structures indicated that the Au130(SR)50, Au127(SR)48, Au124(SR)46 and Au121(SR)44 clusters favored a decahedral core, while the Au133(SR)52, Au136(SR)54, Au139(SR)56, and Au142(SR)58 clusters preferred icosahedral gold cores. Furthermore, we also find that the cuboctahedral gold core is less stable in the cluster size region between ∼120 and ∼140 gold atoms. The optical absorption properties and relative thermodynamic stabilities of the Au145-3N(SR)60-2N (N = 1-8) clusters are also surveyed by density functional theory (DFT) and time-dependent DFT calculations.

Theoretical Predictions of a New ∼14 kDa Core-Mass Thiolate-Protected Gold Nanoparticle: Au68(SR)36.

As an important intermediate link between the smaller and larger size thiolate-protected gold nanoparticles (RS-AuNPs), the molecular formula and atomic structure of the ∼14 kDa core-mass RS-AuNP species (containing around 70 core gold atoms) have not been determined unambiguously. In this work, we theoretically predict an unprecedented ∼14 kDa core-mass AuNP species, denoted as Au68(SR)36, which is composed a symmetric, face-centered-cubic (fcc) 68-gold atom framework. The fcc gold kernel in the Au68(SR)36 is made of eight 13-atom Au-cubotahedrons sharing 12 square faces, showing a standard 2 × 2 × 2 magic cube formula. The Au68(SR)36 is thought to be a key intermediate NP bridging the evolution of Au44(SR)28 and Au92(SR)44. The DFT calculations indicate the Au68(SR)36 has a sizable HOMO-LUMO gap of 0.98 eV and relative high thermodynamic stability. The fcc 68-atom gold framework in the Au68(SR)36 also presents a new candidate to address the atomic structure of recently reported water-soluble mercaptobenzoic acid protected Au68NPs.

A ten-electron (10e) thiolate-protected Au29(SR)19 cluster: structure prediction and a 'gold-atom insertion, thiolate-group elimination' mechanism.

The atomic structure of a ten-electron (10e) thiolate-protected gold cluster, denoted as Au29(SR)19, was theoretically predicted. Based on the prediction of the atomic structure of the 10e Au29(SR)19 cluster, we proposed a novel 'gold-atom insertion, thiolate-group elimination' mechanism to understand the structural evolution of the face-centered-cubic (fcc) thiolate-protected gold clusters. The key step of structural evolution from Au28(SR)20 to Au29(SR)19 and then to the Au30(SR)18 cluster, i.e. the growth of triangle-Au3 units in the gold cores, is understood from the first insertion of an exterior Au(0) atom into the ligand shell and then cleavage of µ3-SR groups.

Novel D-A-π-A-Type Organic Dyes Containing a Ladderlike Dithienocyclopentacarbazole Donor for Effective Dye-Sensitized Solar Cells.

A novel ladderlike fused-ring donor, dithienocyclopentacarbazole (DTCC) derivative, is used to design and synthesize three novel donor-acceptor-π-acceptor-type organic dyes (C1-C3) via facile direct arylation reactions, in which the DTCC derivative substituted by four p-octyloxyphenyl groups is served as the electron donor and the carboxylic acid group is used as the electron acceptor or anchoring group. To fine-tune the optical, electrochemical, and photovoltaic properties of the three dyes, various auxiliary acceptors, including benzo[2,1,3]thiadiazole (BT), 5,6-difluorobenzo[2,1,3]thiadiazole (DFBT), and pyridal[2,1,3]thiadiazole (PT), are incorporated into the dye backbones. The results indicate that all of the three dyes exhibit strong light-capturing ability in the visible region and obtain relatively high molar extinction coefficients (>31 000 M-1 cm-1) due to their strong charge transfer (CT) from donor to acceptor. Moreover, theoretical model calculations demonstrate fully separated highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels for the three dyes, which is helpful for efficient charge separation and electron injection. Using the three dyes as sensitizers, conventional dye-sensitized solar cells (DSSCs) based on liquid iodide/triiodide redox electrolytes are fabricated. Our results indicate that the BT-containing dye C1 affords the highest power conversion efficiency of up to 6.75%, much higher than that of the DFBT-containing dye C2 (5.40%) and the PT-containing dye C3 (1.85%). To our knowledge, this is the first example reported in the literature where the DTCC unit has been used to develop novel organic dyes for DSSC applications. Our work unambiguously demonstrates that the ladderlike DTCC derivatives are the superb electron-donating blocks for the development of high-performance organic dyes.

Geometric structure, electronic structure and optical absorption properties of one-dimensional thiolate-protected gold clusters containing a quasi-face-centered-cubic (quasi-fcc) Au-core: a density-functional theoretical study.

Based on the recently reported atomic structures of thiolate-protected Au28(SR)20, Au36(SR)24, Au44(SR)28, and Au52(SR)32 clusters, a family of homogeneous, linear, thiolate-protected gold superstructures containing novel quasi-face-centered-cubic (quasi-fcc) Au-cores is theoretically envisioned, denoted as the Au20+8N(SR)16+4N cluster. By means of density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations, a unified view of the geometric structure, electronic structure, magic stable size and size-dependent NIR absorption properties of Au20+8N(SR)16+4N clusters is provided. We find that the Au20+8N(SR)16+4N clusters demonstrate oscillating transformation energies dependent on N. The odd-N clusters show more favorable (negative) reaction energies than the even-N clusters. The magic stability of recently reported Au28(SR)20, Au36(SR)24, Au44(SR)28, Au52(SR)32 and Au76(SR)44 clusters can be addressed from the relative reaction energies and geometric distortion of Au-cores. A novel 4N + 4 magic electron-number is suggested for the Au20+8N(SR)16+4N cluster. Using the polyhedral skeletal electron pair theory (PSEPT) and the extended Hückel molecular orbital (EHMO) calculations, we suggest that the magic 4N + 4 electron number is correlated with the quasi-fcc Au-cores, which can be viewed as double helical tetrahedron-Au4 chains. The size-dependent optical absorption properties of Au20+8N(SR)16+4N clusters are revealed based on TD-DFT calculations. We propose that these clusters are potential candidates for the experimental synthesis of atomically precise one-dimensional ligand protected gold superstructures with tunable NIR absorption properties.

Total Structure Determination of Au21(S-Adm)15 and Geometrical/Electronic Structure Evolution of Thiolated Gold Nanoclusters.

The larger size gold nanoparticles typically adopt a face-centered cubic (fcc) atomic packing, while in the ultrasmall nanoclusters the packing styles of Au atoms are diverse, including fcc, hexagonal close packing (hcp), and body-centered cubic (bcc), depending on the ligand protection. The possible conversion between these packing structures is largely unknown. Herein, we report the growth of a new Au21(S-Adm)15 nanocluster (S-Adm = adamantanethiolate) from Au18(SR)14 (SR = cyclohexylthiol), with the total structure determined by X-ray crystallography. It is discovered that the hcp Au9-core in Au18(SR)14 is transformed to a fcc Au10-core in Au21(S-Adm)15. Combining with density functional theory (DFT) calculations, we provide critical information about the growth mechanism (geometrical and electronic structure) and the origin of fcc-structure formation for the thiolate-protected gold nanoclusters.

Enhanced Li storage performance of LiNi(0.5)Mn(1.5)O(4)-coated 0.4Li(2)MnO(3)·0.6LiNi(1/3)Co(1/3)Mn(1/3)O(2) cathode materials for li-ion batteries.

In this study, Li-rich cathode, 0.4Li2MnO3·0.6LiNi1/3Co1/3Mn1/3O2 was synthesized by a resorcinol formaldehyde assisted sol-gel method for the first time. Then, the surface of the as-prepared Li-rich cathode was modified with different amounts of LiNi0.5Mn1.5O4 (5, 10, and 20 wt %) through a simple dip-dry approach. The structural and electrochemical characterizations revealed that the spinel LiNi0.5Mn1.5O4 coating not only can prevent electrolytes from eroding the Li-rich core but also can facilitate fast lithium ion transportation. As a result, the 20 wt % coated sample delivered an initial discharge capacity of 298.6 mAh g(-1) with a Coulombic efficiency of 84.8%, compared to 281.1 mAh g(-1) and 70.2%, respectively, for the bare sample. Particularly, the coated sample demonstrates a Li storage capacity of 170.7 mAh g(-1) and capacity retention of 94.4% after 100 cycles at a high rate of 5 C (1250 mA g(-1)), showing a prospect for practical lithium battery applications. More significantly, the synthetic method proposed in this work is facile and low-cost and possibly could be adopted for large-scale production of surface-modified cathode materials.

Interface electronic structures of reversible double-docking self-assembled monolayers on an Au(111) surface.

Double-docking self-assembled monolayers (DDSAMs), namely self-assembled monolayers (SAMs) formed by molecules possessing two docking groups, provide great flexibility to tune the work function of metal electrodes and the tunnelling barrier between metal electrodes and the SAMs, and thus offer promising applications in both organic and molecular electronics. Based on the dispersion-corrected density functional theory (DFT) in comparison with conventional DFT, we carry out a systematic investigation on the dual configurations of a series of DDSAMs on an Au(111) surface. Through analysing the interface electronic structures, we obtain the relationship between single molecular properties and the SAM-induced work-function modification as well as the level alignment between the metal Fermi level and molecular frontier states. The two possible conformations of one type of DDSAM on a metal surface reveal a strong difference in the work-function modification and the electron/hole tunnelling barriers. Fermi-level pinning is found to be a key factor to understand the interface electronic properties.

Electronic structure of pyridine-based SAMs on flat Au(111) surfaces: extended charge rearrangements and Fermi level pinning.

Density functional theory calculations are used to investigate the electronic structure of pyridine-based self-assembled monolayers (SAMs) on an Au(111) surface. We find that, when using pyridine docking groups, the bonding-induced charge rearrangements are frequently found to extend well onto the molecular backbone. This is in contrast to previous observations for the chemisorption of other SAMs, e.g., organic thiolates on gold, and can be explained by a pinning of the lowest unoccupied states of the SAM at the metal Fermi-level. The details of the pinning process, especially the parts of the molecules most affected by the charge rearrangements, strongly depend on the length of the molecular backbone and the tail-group substituent. We also mention methodological shortcomings of conventional density functional theory that can impact the quantitative details regarding the circumstances under which pinning occurs and highlight a number of peculiarities associated with bond dipoles that arise from Fermi-level pinning.

Is there a Au-S bond dipole in self-assembled monolayers on gold?

Self-assembled monolayers (SAMs) of functionalized thiols are widely used in organic (opto)electronic devices to tune the work function, Phi, of noble-metal electrodes and, thereby, to optimize the barriers for charge-carrier injection. The achievable Phi values not only depend on the intrinsic molecular dipole moment of the thiols but, importantly, also on the bond dipole at the Au-S interface. Here, on the basis of extensive density-functional theory calculations, we clarify the ongoing controversy regarding the existence, the magnitude, and the nature of that bond dipole.