Stretched Central Double Bonds in Dialumene and Disilene by Amino Substituents: A Case of Lone Pair Repulsion.

There have been remarkable advances in the syntheses and applications of groups 13 and 14 homonuclear ethene analogues. However, successes are largely limited to aryl- and/or silyl-substituted species. Analogues bearing two or more heteroatoms are still scarce. In this work, the block-localized wavefunction (BLW) method at the density functional theory (DFT) level was employed to study dialumene and disilene bearing two amino substituents whose optimal geometries exhibit significantly stretched central M=M (M=Al or Si) double bonds compared with aryl- and/or silyl-substituted species. Computational analyses showed that the repulsion between the lone electron pairs of amino substituents and M=M π bond plays a critical role in the elongation of the M=M bonds. Evidently, replacing the substituent groups -NH2 with -BH2 can enhance the planarity and shorten the central double bonds due to the absence of lone pair electrons in BH2 .

Captodative Effect Facilitates the Excitation in Diboron Molecule (CAAC)2 B2 (SH)2.

Given the extraordinary versatility in chemical reactions and applications, boron compounds have gained increasing attentions in the past two decades. One of the remarkable advances is the unprecedented preparation of unsaturated boron species. Notably, Braunschweig et al. found that the cyclic (alkyl)(amino) carbenes (CAACs) stabilized diboron molecules (CAAC)2 B2 (SR)2 host unpaired electrons and exist in the 90°-twisted diradical form, while other analogues, such as N-heterocyclic carbenes (NHCs), stabilized diboron molecules prefer a conventional B=B double bond. Since previous studies recognized the differences in the steric effect between CAAC and NHC carbenes, here we focused on the role of thiol substituents in (CAAC)2 B2 (SR)2 by gradually localizing involved electrons. The co-planarity of the thiol groups and the consequent captodative effect were found to be the culprit for the 90°-twisted diradical form of (CAAC)2 B2 (SR)2 . Computational analyses identified two forces contributing to the π electron movements. One is the "push" effect of lone pairs on the sulfur atoms which boosts the π electron delocalization between the BB center and CAACs. The other is the π electron delocalization within each (CAAC)B(SR) fragment where the pull effect originates from the π electron withdrawal by CAACs. There are two such independent and orthogonal push-pull channels which function mainly in individual (CAAC)B(SR) fragments. This enhanced π push-pull effect in the triplet state facilitates the electronic excitation in (CAAC)2 B2 (SR)2 by reducing the singlet-triplet gap.

Cyclized Polyacrylonitrile as a Promising Support for Single Atom Metal Catalyst with Synergistic Active Site.

Metal single atom catalysts (SAC) have been successfully used in heterogeneous catalysis but developing a scalable and economic support for SAC is still a great challenge. Here, cyclized polyacrylonitrile (CPAN) is proposed as a promising support for single atom metal catalysts. CPAN can be easily prepared from cheap industrial product polyacrylonitrile (PAN), which has excellent processability. A series of SAC on CPAN (M/CPAN, M = Ag, Cu, Ru) are designed and the catalytic activities of the as synthesized M/CPAN are investigated by the model reduction reaction of p-nitrophenol (4-NP). M/CPAN presents excellent catalytic performance with high stability and theoretical calculations elucidate that Ag/CPAN synergistically catalyze 4-NP reduction following the Langmuir-Hinshelwood (L-H) mechanism with 4-NP preferentially adsorbing at the Ag sites and H adsorbing at the bridge C sites. These results, for the first time, reveal that the single atom on CPAN can catalyze 4-NP reduction efficiently. This methodology provides a convenient route for the preparation of a variety of SAC, and this strategy is readily scalable and holds great potential in catalytic applications.

Ultrafast solid-liquid intercalation enabled by targeted microwave energy delivery.

In chemical reactions, the breaking and formation of chemical bonds usually need external energy to overcome the activation barriers. Conventional energy delivery transfers energy from heating sources via various media, hence losing efficiency and inducing side reactions. In contrast, microwave (MW) heating is known to be highly energy efficient through dipole interaction with polar media, but how exactly it transmits energy to initiate chemical reactions has been unknown. Here, we report a rigorous determination of energy delivery mechanisms underlying MW-enabled rapid hydrothermal synthesis, by monitoring the structure and temperature of all the involved components as solid-liquid intercalation reaction occurs using in situ synchrotron techniques. We reveal a hitherto unknown direct energy transmission between MW irradiation source and the targeted reactants, leading to greatly reduced energy waste, and so the ultrafast kinetics at low temperature. These findings open up new horizons for designing material synthesis reactions of high efficiency and precision.

Multi-electron transfer enabled by topotactic reaction in magnetite.

A bottleneck for the large-scale application of today's batteries is low lithium storage capacity, largely due to the use of intercalation-type electrodes that allow one or less electron transfer per redox center. An appealing alternative is multi-electron transfer electrodes, offering excess capacity, which, however, involves conversion reaction; according to conventional wisdom, the host would collapse during the process, causing cycling instability. Here, we report real-time observation of topotactic reaction throughout the multi-electron transfer process in magnetite, unveiled by in situ single-crystal crystallography with corroboration of first principles calculations. Contradicting the traditional belief of causing structural breakdown, conversion in magnetite resembles an intercalation process-proceeding via topotactic reaction with the cubic close packed oxygen-anion framework retained. The findings from this study, with unique insights into enabling  multi-electron transfer via topotactic reaction, and its implications to the cyclability and rate capability, shed light on designing viable multi-electron transfer electrodes for high energy batteries.

Cationic Ordering Coupled to Reconstruction of Basic Building Units during Synthesis of High-Ni Layered Oxides.

Metal (M) oxides are one of the most interesting and widely used solids, and many of their properties can be directly correlated to the local structural ordering within basic building units (BBUs). One particular example is the high-Ni transition metal layered oxides, potential cathode materials for Li-ion batteries whose electrochemical activity is largely determined by the cationic ordering in octahedra (e.g., the BBUs in such systems). Yet to be firmly established is how the BBUs are inherited from precursors and subsequently evolve into the desired ordering during synthesis. Herein, a multimodal in situ X-ray characterization approach is employed to investigate the synthesis process in preparing LiNi0.77Mn0.13Co0.10O2 from its hydroxide counterpart, at scales varying from the long-range to local individual octahedral units. Real-time observation corroborated by first-principles calculations reveals a topotactic transformation throughout the entire process, during which the layered framework is retained; however, due to preferential oxidation of Co and Mn over Ni, significant changes happen locally within NiO6 octahedra. Specifically, oxygen loss and the associated symmetry breaking occur in NiO6; as a consequence, Ni2+ ions become highly mobile and tend to mix with Li, causing high cationic disordering upon formation of the layered oxides. Only through high-temperature heat treatment, Ni is further oxidized, thereby inducing symmetry reconstruction and, concomitantly, cationic ordering within NiO6 octahedra. Findings from this study shed light on designing high-Ni layered oxide cathodes and, more broadly, various functional materials through synthetic control of the constituent BBUs.

Identification of dopant site and its effect on electrochemical activity in Mn-doped lithium titanate.

Doped metal oxide materials are commonly used for applications in energy storage and conversion, such as batteries and solid oxide fuel cells. The knowledge of the electronic properties of dopants and their local environment is essential for understanding the effects of doping on the electrochemical properties. Using a combination of X-ray absorption near-edge structure spectroscopy (XANES) experiment and theoretical modeling we demonstrate that in the dilute (1 at. %) Mn-doped lithium titanate (Li4/3Ti5/3O4, or LTO) the dopant Mn2+ ions reside on tetrahedral (8a) sites. First-principles Mn K-edge XANES calculations revealed the spectral signature of the tetrahedrally coordinated Mn as a sharp peak in the middle of the absorption edge rise, caused by the 1s → 4p transition, and it is important to include the effective electron-core hole Coulomb interaction in order to calculate the intenisty of this peak accurately. This dopant location explains the impedance of Li migration through the LTO lattice during the charge-discharge process, and, as a result - the observed remarkable 20% decrease in electrochemical rate performance of the 1% Mn-doped LTO compared to the pristine LTO.

Self-assembled N-graphene nanohollows enabling ultrahigh energy density cathode for Li-S batteries.

Functional porous carbon materials are widely used to solve the low conductivity and shuttle effect of Li-S batteries; however, the common carbon/sulfur composite electrodes based on traditional technology (with conducting agents and binders) make it difficult for a battery to work stably at an ultra-high sulfur loading of 10 mg cm-2. Herein, an appropriate content of sulfur was injected into a pomegranate-like structure self-assembled with nanohollows (PSSN) of N-graphene. The Li-PSSN/S battery based on traditional technology displays a large-capacity, high-rate and long-life at an ultra-high areal-sulfur loading of 10.1 mg cm-2. The excellent performance with ultra-high areal-sulfur loading can be attributed to the hierarchal nanohollows with graphene-shells being in close contact to build a 3D-electronic conduction network and promoting electrolyte adsorption into the entire electrode to maintain rapid Li-ion transport, while stopping the shuttle-effect via the strong interaction of polysulfide with the doped N elements on graphene-shells. In addition, the exact sulfur content can provide just enough space to maintain the huge volume change and constant thickness of the S-electrodes during the charge-discharge process to enhance the cycling stability.

Excess Li-Ion Storage on Reconstructed Surfaces of Nanocrystals To Boost Battery Performance.

Because of their enhanced kinetic properties, nanocrystallites have received much attention as potential electrode materials for energy storage. However, because of the large specific surface areas of nanocrystallites, they usually suffer from decreased energy density, cycling stability, and effective electrode capacity. In this work, we report a size-dependent excess capacity beyond theoretical value (170 mA h g-1) by introducing extra lithium storage at the reconstructed surface in nanosized LiFePO4 (LFP) cathode materials (186 and 207 mA h g-1 in samples with mean particle sizes of 83 and 42 nm, respectively). Moreover, this LFP composite also shows excellent cycling stability and high rate performance. Our multimodal experimental characterizations and ab initio calculations reveal that the surface extra lithium storage is mainly attributed to the charge passivation of Fe by the surface C-O-Fe bonds, which can enhance binding energy for surface lithium by compensating surface Fe truncated symmetry to create two types of extra positions for Li-ion storage at the reconstructed surfaces. Such surface reconstruction nanotechnology for excess Li-ion storage makes full use of the large specific surface area of the nanocrystallites, which can maintain the fast Li-ion transport and greatly enhance the capacity. This discovery and nanotechnology can be used for the design of high-capacity and efficient lithium ion batteries.

Controllable Formation of (004)-Orientated Nb:TiO2 for High-Performance Transparent Conductive Oxide Thin Films with Tunable Near-Infrared Transmittance.

A niobium-doped titanium dioxide (Nb:TiO2, NTO) film is a promising candidate material for indium-free transparent conductive oxide (TCO) films. It is challenging and interesting to control (004)-oriented growth to decrease resistivity. In this work, NTO films with different fractions of preferential (004) orientation (η(004)) were controllably prepared by direct current sputtering. Notably, the direction of local-ordering of ions-packing could be adjusted by slightly changing the angle between the sputtering source and the glass substrate, which is identified as a key factor to determine the growth direction of a columnar crystal as well as the η(004) of films. Hall effect measurements indicate that NTO films with the highest η(004) present the lowest resistivity (6.4 × 10-4 Ω cm), which originates from super-high carrier concentration (2.9 × 1021 cm-3) and mobility (3.4 cm2 V-1 s-1). The corresponding low sheet resistance (10.3 Ω sq-1) makes it a potential material for commercial TCO films. We also observe that films with higher η(004) show lower transmittance in the near-infrared region.

The synergistic effect achieved by combining different nitrogen-doped carbon shells for high performance capacitance.

Ellipsoid nitrogen-doped hollow carbon shells with different nitrogen contents and electrical conductivities were prepared using a simple calcination method by regulating the calcination temperature. Although a high nitrogen content promotes pseudocapacitance, it reduces the electrical conductivity of carbon, causing loss of capacitance. The best rate performance was achieved by a mixture of two types of ellipsoid nitrogen-doped hollow carbon shells, in which one contains a higher level of nitrogen with lower conductivity and higher pseudocapacitance, while the other contains a relatively lower level of nitrogen with higher conductivity. The enhanced performance can be explained by the synergistic effect of one component providing high pseudocapacitance and the other component serving as a highly electrically conductive network, which leads to activation of "nitrogen" to enhance pseudocapacitance performance. The mixed material showed a specific capacitance of 156.9 F g-1 at a high current density of 10 A g-1, with no degradation after 10 000 cycles.

Aligned Li+ Tunnels in Core-Shell Li(NixMnyCoz)O2@LiFePO4 Enhances Its High Voltage Cycling Stability as Li-ion Battery Cathode.

Layered transition-metal oxides (Li[NixMnyCoz]O2, NMC, or NMCxyz) due to their poor stability when cycled at a high operating voltage (>4.5 V) have limited their practical applications in industry. Earlier researches have identified Mn(II)-dissolution and some parasitic reactions between NMC surface and electrolyte, especially when NMC is charged to a high potential, as primarily factors responsible for the fading. In our previous work, we have achieved a capacity of NMC active material close to theoretical value and optimized its cycling performance by a depolarized carbon nanotubes (CNTs) network and an unique "pre-lithiation process" that generates an in situ organic coating (∼40 nm) to prevent Mn(II) dissolution and minimize the parasitic reactions. Unfortunately, this organic coating is not durable enough during a long-term cycling when the cathode operates at a high potential (>4.5 V). This work attempts to improve the surface protection of the NMC532 particles by applying an active inorganic coating consisting of nanosized- and crystal-orientated LiFePO4 (LFP) (about 50 nm, exposed (010) face) to generate a core-shell nanostructure of Li(NixMnyCoz)O2@LiFePO4. Transmission electron microscopy (TEM) and etching X-ray photoelectron spectroscopy have confirmed an intimate contact coating (about 50 nm) between the original structure of NMC and LFP single-particle with atomic interdiffusion at the core-shell interface, and an array of interconnected aligned Li+ tunnels are observed at the interface by cross-sectional high-resolution TEM, which were formed by ball-milling and then strictly controlling the temperature below 100 °C. Batteries based on this modified NMC cathode material show a high reversible capacity when cycled between 3.0 and 4.6 V during a long-term cycling.

Storage and Effective Migration of Li-Ion for Defected ß-LiFePO4 Phase Nanocrystals.

Lithium iron phosphate, a widely used cathode material, crystallizes typically in olivine-type phase, α-LiFePO4 (αLFP). However, the new phase ß-LiFePO4 (ßLFP), which can be transformed from αLFP under high temperature and pressure, is originally almost electrochemically inactive with no capacity for Li-ion battery, because the Li-ions are stored in the tetrahedral [LiO4] with very high activation barrier for migration and the one-dimensional (1D) migration channels for Li-ion diffusion in αLFP disappear, while the Fe ions in the ß-phase are oriented similar to the 1D arrangement instead. In this work, using experimental studies combined with density functional theory calculations, we demonstrate that ßLFP can be activated with creation of effective paths of Li-ion migration by optimized disordering. Thus, the new phase of ßLFP cathode achieved a capacity of 128 mAh g(-1) at a rate of 0.1 C (1C = 170 mA g(-1)) with extraordinary cycling performance that 94.5% of the initial capacity retains after 1000 cycles at 1 C. The activation mechanism can be attributed to that the induced disorder (such as FeLiLiFe antisite defects, crystal distortion, and amorphous domains) creates new lithium migration passages, which free the captive stored lithium atoms and facilitate their intercalation/deintercalation from the cathode. Such materials activated by disorder are promising candidate cathodes for lithium batteries, and the related mechanism of storage and effective migration of Li-ions also provides new clues for future design of disordered-electrode materials with high capacity and high energy density.

How to Optimize the Interface between Photosensitizers and TiO2 Nanocrystals with Molecular Engineering to Enhance Performances of Dye-Sensitized Solar Cells?

In this work, the interfacial properties of a series of metal-free organic naphthodithienothiophene (NDTT)-based photosensitizers adsorbed on TiO2 surfaces were investigated by a combination of ab initio calculations and experimental measurements. The calculations and experiments reveal that because of the efficient charge transfer from the adsorbed dyes to TiO2 nanocrystal surface there is an upward shift for the energy levels of dyes and a downward shift for the conduction band of surface TiO2 and that the band gaps for both of them are also reduced. Such electronic level alignments at the interface would lead to increased light absorption range by adsorbed dyes and increased driving force for charge injection but reduced open-circuit potential (V(oc)). More interestingly, we found that molecule engineering of the donor group and introducing additional electron-withdrawing unit have little effect on the electronic level alignments at the interface (because band gaps of the dyes adsorbed on TiO2 surfaces become approximately identical when compared with those of the dyes measured in solution) but that they can affect the steric effect and the charge separation at the interface to tune V(oc) and the short-circuit current density (J(sc)) effectively. All these findings suggest that optimizing the interfacial properties of dyes adsorbed on TiO2 surfaces by synchronously modifying steric effects of dye molecules anchored on TiO2 and charge-transfer and separation properties at the interfaces is important to construct efficient dye-sensitized solar cells.

Janus Solid-Liquid Interface Enabling Ultrahigh Charging and Discharging Rate for Advanced Lithium-Ion Batteries.

LiFePO4 has long been held as one of the most promising battery cathode for its high energy storage capacity. Meanwhile, although extensive studies have been conducted on the interfacial chemistries in Li-ion batteries,1-3 little is known on the atomic level about the solid-liquid interface of LiFePO4/electrolyte. Here, we report battery cathode consisted with nanosized LiFePO4 particles in aqueous electrolyte with an high charging and discharging rate of 600 C (3600/600 = 6 s charge time, 1 C = 170 mAh g(-1)) reaching 72 mAh g(-1) energy storage (42% of the theoretical capacity). By contrast, the accessible capacity sharply decreases to 20 mAh g(-1) at 200 C in organic electrolyte. After a comprehensive electrochemistry tests and ab initio calculations of the LiFePO4-H2O and LiFePO4-EC (ethylene carbonate) systems, we identified the transient formation of a Janus hydrated interface in the LiFePO4-H2O system, where the truncated symmetry of solid LiFePO4 surface is compensated by the chemisorbed H2O molecules, forming a half-solid (LiFePO4) and half-liquid (H2O) amphiphilic coordination environment that eases the Li desolvation process near the surface, which makes a fast Li-ion transport across the solid/liquid interfaces possible.

A core-shell nanohollow-γ-Fe2O3@graphene hybrid prepared through the Kirkendall process as a high performance anode material for lithium ion batteries.

We synthesized a core-shell structure with graphene as the shell and nano-hollow γ-Fe2O3 as the core through a Kirkendall process at room temperature. When this hybrid is used as an anode material for lithium-ion batteries, it exhibits a remarkable electrochemical performance: a high reversible capacity of 1095, 833, and 551 mA h g(-1) at the current rates of 0.1 C, 1 C, and 2 C, respectively. When evaluated at 10 C, the capacity can still reach 504 mA h g(-1).

High-performance plastic platinized counter electrode via photoplatinization technique for flexible dye-sensitized solar cells.

A photoplatinization technique was proposed to deposit Pt on a thin TiO(2) layer modified indium tin oxide-coated polyethylene naphthalate (ITO/PEN) substrate at low temperature (about 50 °C after 1 h of UV irradiation) for the first time. The fabrication process includes coating and hydrolyzing the tetra-n-butyl titanate to form a TiO(2)-modified layer and the photoplatinization of the modified substrate in H(2)PtCl(6)/2-propanol precursor solution under UV irradiation. The obtained platinized electrodes were used as counter electrodes (CE) for flexible dye-sensitized solar cells (FDSCs). The well-optimized platinized electrode showed high optical transmittance, up to 76.5% between 400 and 800 nm (T(av)), and the charge transfer resistance (R(ct)) was as low as 0.66 Ω cm(2). A series of characterizations also demonstrated the outstanding chemical/electrochemical durability and mechanical stability of the platinized electrode. The FDSCs with TiO(2)/Ti photoanodes and the obtained CEs achieved a power conversion efficiency (PCE) up to 8.12% under rear-side irradiation (AM 1.5 illumination, 100 mW cm(-2)). The obtained CEs were also employed in all-plastic bifacial DSCs. When irradiated from the rear side, the bifacial FDSC yielded a PCE of 6.26%, which approached 90% that of front-side irradiation (6.97%). Our study revealed that, apart from serving as a functional layer for deposition of Pt, the thin TiO(2) layer modification on ITO/PEN substrates also played an important role in improving the transparency and the mechanical properties of the CE. The effect of the thickness of the TiO(2) layer for Pt coating on the performance of the CE was also investigated.

Highly efficient visible light TiO2 photocatalyst prepared by sol-gel method at temperatures lower than 300°C.

Highly efficient visible light TiO(2) photocatalyst was prepared by the sol-gel method at lower temperature (≤ 300°C), and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR), UV-vis diffuse reflectance spectroscopy (UV-vis DRS), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and differential scanning calorimetry-thermogravimetric analysis (DSC-TGA). The effects of the heat treatment temperature and time of the as-prepared TiO(2) on its visible light photocatalytic activity were investigated by monitoring the degradation of methyl orange solution under visible light irradiation (wavelength ≥ 400 nm). Results show that the as-prepared TiO(2) nanoparticles possess an anatase phase and mesoporous structure with carbon self-doping and visible photosensitive organic groups. The visible light photocatalytic activity of the as-prepared TiO(2) is greatly higher than those of the commercial TiO(2) (P-25) and other visible photocatalysts reported in literature (such as PPy/TiO(2), P3HT/TiO(2), PANI/TiO(2), N-TiO(2) and Fe(3+)-TiO(2)) and its photocatalytic stability is excellent. The reasons for improving the visible light photocatalytic activity of the as-prepared TiO(2) can be explained by carbon self-doping and a large amount of visible photosensitive groups existing in the as-prepared TiO(2). The apparent optical thickness (τ(app)), local volumetric rate of photo absorption (LVRPA) and kinetic constant (k(T)) of the photodegradation system were calculated.

Characterization and photocatalytic activity of poly(3-hexylthiophene)-modified TiO2 for degradation of methyl orange under visible light.

Poly(3-hexylthiophene) (P3HT) was synthesized via chemical oxidative polymerization with anhydrous FeCl(3) as oxidant, 3-hexylthiophene as monomer, chloroform as solvent. TiO(2) nanoparticles modified by a small amount of P3HT (TiO(2)/P3HT) were prepared by blending TiO(2) nanoparticles and P3HT in chloroform solution. The resulting photocatalysts were characterized by the methods of TEM, XRD, FT-IR, XPS and UV-vis diffuse reflectance spectroscope. The photocatalytic activity of TiO(2)/P3HT was investigated by degrading methyl orange under visible light. The degradation rate of methyl orange was 88.5 and 13.5% when it was degradated by TiO(2)/P3HT and neat TiO(2)(P-25) for 10h, respectively. In addition, TiO(2)/P3HT nanocomposites showed excellent photocatalytic stability after 10 cycles under visible light irradiation. A possible mechanism for the photocatalytic oxidative degradation was also discussed.