Ce–O Covalence in Silicate Oxyapatites and Its Influence on Luminescence Dynamics

Cerium substituting gadolinium in Ca₂Gd₈(SiO₄)₆O₂ occupies two intrinsic sites of distinct coordination. The coexistence of an ionic bonding at a 4F site and an ionic–covalent mixed bonding at a 6H site in the same crystalline compound provides an ideal system for comparative studies of ion–ligand interactions. Experimentally, the spectroscopic properties and photoluminescence dynamics of this white-phosphor are investigated. An anomalous thermal quenching of the photoluminescence of Ce³⁺ at the 6H site is analyzed. Theoretically, ab initio calculations are conducted to reveal the distinctive properties of the Ce–O coordination at the two Ce³⁺ sites. The calculated eigenstates of Ce³⁺ at the 6H site suggest a weak Ce–O covalent bond formed between Ce³⁺ and one of the coordinated oxygen ions not bonded with Si⁴⁺. The electronic energy levels and frequencies of local vibrational modes are correlated with specific Ce–O pairs to provide a comparative understanding of the site-resolved experimental results. On the basis of the calculated results, we propose a model of charge transfer and vibronic coupling for interpretation of the anomalous thermal quenching of the Ce³⁺ luminescence. The combination of experimental and theoretical studies in the present work provides a comprehensive understanding of the spectroscopy and luminescence dynamics of Ce³⁺ in crystals of ionic–covalent coordination

Three-in-One: Sensing, Self-Assembly, and Cascade Catalysis of Cyclodextrin Modified Gold Nanoparticles

We herein present a three-in-one nanoplatform for sensing, self-assembly, and cascade catalysis, enabled by cyclodextrin modified gold nanoparticles (CD@AuNPs). Monodisperse AuNPs 15–20 nm in diameter are fabricated in an eco-friendly way by the proposed one-step colloidal synthesis method using CD as both reducing agents and stabilizers. First, the as-prepared AuNPs are employed as not only scaffolds but energy acceptors for turn-on fluorescent sensing based on guest replacement reaction. Then, the macrocyclic supramolecule functionalized AuNPs can be controllably assembled and form well-defined one- and two-dimensional architectures using tetrakis­(4-carboxyphenyl)­porphyrin as mediator. Finally, in addition to conventional host–guest interaction based properties, the CD@AuNPs possess unpredictable catalytic activity and exhibit mimicking properties of both glucose oxidase and horseradish peroxidase simultaneously. Especially, the cascade reaction (glucose is first catalytically oxidized and generates gluconic acid and H₂O₂; then the enzymatic H₂O₂ and preadded TMB (3,3′,5,5′-tetramethylbenzidine) are further catalyzed into H₂O and oxTMB, respectively) is well-achieved using the AuNPs as the sole catalyst. By employing a joint experimental–theoretical study, we reveal that the unique catalytic properties of the CD@AuNPs probably derive from the special topological structures of CD molecules and the resulting electron transfer effect from the AuNP surface to the appended CD molecules

Effects of Si Codoping on Optical Properties of Ce-Doped Ca₆BaP₄O₁₇: Insights from First-Principles Calculations

It was recently reported that Ce-doped Ca₆BaP₄O₁₇ displayed blue-green emission under excitation in the near-ultraviolet (UV) region and that luminescence intensities can be greatly improved by codoping with Si. Here, a combination of hybrid density functional theory (DFT) and wave function-based CASSCF/CASPT2 calculations at the spin–orbit level has been performed on geometric and electronic structures of the material to gain insights into effects of Si codoping on its optical properties. It is found that the observed luminescence arises from 4f–5d transitions of Ce³⁺ occupying the two crystallograhically distinct Ca1 and Ca2 sites of the host compound with comparable probabilities, with the energy of the lowest 4f → 5d transition of Ce_Ca1 being slightly higher than that of Ce_ca2. The codopant Si prefers to substitute for the nearest-neighbor (NN) P1 atom over the NN P2 atom around Ce³⁺, and this preference induces a blueshift of the lowest-energy 4f → 5d transition, consistent with experimental observations. The blueshift originates from a reduction in 5d crystal field splitting of Ce³⁺ associated mainly with electronic effects of the NN Si_P1 substitution, while the contribution from the change in 5d centroid energy is negligible. On the basis of calculated results, the energy-level diagram for the 4f ground states and the lowest 5d states of all trivalent and divalent lanthanide ions on the Ca²⁺ sites of Ca₆BaP₄O₁₇ is constructed and discussed in connection with experimental findings

Matrix-Free Polymer Nanocomposite Thermoplastic Elastomers

Thermoplastic elastomer (TPE) grafted nanoparticles were prepared by grafting block copolymer poly­(styrene-<i>block</i>-(<i>n</i>-butyl acrylate)) onto silica nanoparticles (NPs) via surface-initiated reversible addition–fragmentation chain transfer (RAFT) polymerization. The effects of polymer chain length and graft density on the mechanical properties were investigated using films made solely from the grafted NPs. The ultimate tensile stress and elastic modulus increased with increasing PS chain length. The dispersion of the silica NPs and the microphase separation of the block copolymer in the matrix-free polymer nanocomposite were investigated using small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA). The higher polymer graft density TPEs exhibited better microphase separation of the block copolymers and more uniform silica NP dispersion than lower polymer graft density TPEs with similar polymer chain length and composition

Matrix-Free Polymer Nanocomposite Thermoplastic Elastomers

Thermoplastic elastomer (TPE) grafted nanoparticles were prepared by grafting block copolymer poly­(styrene-<i>block</i>-(<i>n</i>-butyl acrylate)) onto silica nanoparticles (NPs) via surface-initiated reversible addition–fragmentation chain transfer (RAFT) polymerization. The effects of polymer chain length and graft density on the mechanical properties were investigated using films made solely from the grafted NPs. The ultimate tensile stress and elastic modulus increased with increasing PS chain length. The dispersion of the silica NPs and the microphase separation of the block copolymer in the matrix-free polymer nanocomposite were investigated using small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA). The higher polymer graft density TPEs exhibited better microphase separation of the block copolymers and more uniform silica NP dispersion than lower polymer graft density TPEs with similar polymer chain length and composition

Hydrogen Activation on the Promoted and Unpromoted ReS₂ (001) Surfaces under the Sulfidation Conditions: A First-Principles Study

Hydrogen activation on the promoted and promoter-free ReS₂(001) surfaces under the sulfidation conditions is studied by means of periodic density function theory (DFT) calculations within the generalized gradient approximation. First, surface-phase diagrams are investigated by plotting the surface free energy as a function of the chemical potential of S (μ_S) on the unpromoted and promoted ReS₂ (001) surfaces with different loadings of nickel, cobalt, tungsten, and tantalum. The results show that on the unpromoted surface sulfur coverage of 25% and on the promoted surfaces sulfur coverage of 25% as well as 25% promoter modification are the most stable conditions, respectively, under hydrodesulfurization (HDS) reaction conditions. Second, hydrogen adsorption and dissociation are explored on these preferred surfaces. It is found that hydrogen adsorbs weakly on all the surfaces studied. The physical adsorption character makes its diffusion favorable, resulting in various adsorption sites and dissociation pathways, i.e., dissociation at surface Re or promote atom, at the interlayer, as well as at the adsorbed S atom. Calculated results show that hydrogen dissociation at the surface Re site is always kinetically favorable. All of the studied dopants can largely activate the adsorbed S but display distinct roles toward the activity of the nearest Re atom; i.e., Co/Ni dopant passivates the nearest surface Re while W/Ta activates it. The activity difference is found to be closely associated with the difference in the bond strength of metal–S and the resultant difference in the induced surface geometry. Moreover, promoter effect is localized because it seems nominal when the reaction occurs at a Re atom with one dopant atom separation. The present results provide a rational understanding of the activity difference between the promoter-free and the promoted surfaces, which would be helpful to further understand the mechanism of HDS and to enhance the development of highly active and selective hydrotreating catalysts

Generating Electric Current Based on the Solvent-Dependent Charging Effects of Defective Boron Nitride Nanosheets

This work presents a method of generating electric current based on the defects of few-layer boron nitride nanosheets (BNNSs). The density functional theory calculations showed that the atomic charge of the B atom in acetone was more positive than in water. The electrostatic force microscopy measurements illustrated that the local electrical potential was 0.35 mV in acetone, while the potential signal was very difficult to capture when using water as the dispersant. This effect was further demonstrated by the performance of the acoustic energy-harvesting nanogenerator: the BNNSs were assembled into a film after being dispersed in acetone and then integrated into the generator device, generating average output current of ∼0.98 nA, which was much better than 0.2 nA, the average output current of another device with water as the dispersant. These results demonstrated that solvent effects made the as-prepared BNNSs carry net charges, which could be utilized to harvest acoustic energy and generate current

Mechanical Properties, Electronic Structures, and Potential Applications in Lithium Ion Batteries: A First-Principles Study toward SnSe₂ Nanotubes

First-principles calculations were carried out to investigate the mechanical and electronic properties as well as the potential application of SnSe₂ nanotubes. It was found that the mechanical properties are closely dependent on diameter and chirality: the Young’s modulus (<i>Y</i>) increases with the enlargement of diameter and converges to the monolayer limit when the diameter reaches a certain degree; with a comparable diameter, the armchair nanotube has a larger Young’s modulus than the zigzag one. The significantly higher Young’s modulus of SnSe₂ nanotubes with the larger diameter demonstrates that the deformation does not easily occur, which is beneficial to the application as anode materials in lithium ion batteries because a large volume expansion during charge–discharge cycling will result in serious pulverization of the electrodes and thus rapid capacity degradation. On the other hand, band structure calculations unveiled that SnSe₂ nanotubes display a diversity of electronic properties, which are also diameter- and chirality-dependent: armchair nanotubes (ANTs) are indirect bandgap semiconductors, and the energy gaps increase monotonously with the increase of tube diameter, while zigzag nanotubes (ZNTs) are metals. The metallic SnSe₂ ZNTs exhibit terrific performance for the adsorption and diffusion of Li atom, thus they are very promising as anode materials in the Li-ion batteries

Precise Engineering of the Electrocatalytic Activity of FeN₄‑Embedded Graphene on Oxygen Electrode Reactions by Attaching Electrides

Using first-principles calculations combined with a constant-potential implicit solvent model, we comprehensively studied the activity of oxygen electrode reactions catalyzed by electride-supported FeN4-embedded graphene (FeN4Cx). The physical quantities in FeN4Cx/electrides, i.e., work function of electrides, interlayer spacing, stability of heterostructures, charge transferred to Fe, d-band center of Fe, and adsorption free energy of O, are highly intercorrelated, resulting in activity being fully expressed by the nature of the electrides themselves, thereby achieving a precise modulation in activity by selecting different electrides. Strikingly, the FeN4PDCx/Ca2N and FeN4PDCx/Y2C systems maintain a high oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) activity with the overpotential less than 0.46 and 0.62 V in a wide pH range. This work provides an effective strategy for the rational design of efficient bifunctional catalysts as well as a model system with a simple activity-descriptor, helping to realize significant advances in energy devices

Two-Dimensional Be₂C with Octacoordinate Carbons and Negative Poisson’s Ratio

In this work, we predicted three new two-dimensional (2D) Be₂C structures, namely, α-Be₂C, β-Be₂C, and γ-Be₂C, on the basis of density functional theory (DFT) computations and the particle-swarm optimization (PSO) method. In α-Be₂C, a carbon atom binds to eight Be atoms, forming an octacoordinate carbon moiety. This is the first example of an octacoordinate carbon containing material. The other two structures, β-Be₂C and γ-Be₂C, are quasi planar hexacoordinate carbon (phC) containing 2D materials. Good stability with these three phases is revealed by their lower cohesive energy and positive phonon modes. More interestingly, these predicted new phases of Be₂C are all semiconductors and have unusual negative Poisson’s ratios (NPRs). If synthesized, 2D Be₂C materials will have a broad range of applications in electronics and mechanics