Ultrathin Two-dimensional Layered Composite Carbosilicates from in situ Unzipped Carbon Nanotubes and Exfoliated Bulk Silica.

A key task in today's inorganic synthetic chemistry is to develop effective reactions, routes, and associated techniques aiming to create new functional materials with specifically desired multilevel structures and properties. Herein, we report an ultrathin two-dimensional layered composite of graphene ribbon and silicate via a simple and scalable one-pot reaction, which leads to the creation of a novel carbon-metal-silicate hybrid family: carbosilicate. The graphene ribbon is in situ formed by unzipping carbon nanotubes, while the ultrathin silicate is in situ obtained from bulk silica or commercial glass; transition metals (Fe or Ni) oxidized by water act as bridging agent, covalently bonding the two structures. The unprecedented structure combines the superior properties of the silicate and the nanocarbon, which triggers some specific novel properties. All processes during synthesis are complementary to each other. The associated synergistic chemistry could stimulate the discovery of a large class of more interesting, functionalized structures and materials.

Carbon-Free, Binder-Free MnO2@Mn Catalyst for Oxygen Reduction Reaction.

Reasonable design and feasible preparation of low-cost and stable oxygen reduction reaction (ORR) catalysts with excellent performance play a key role in the development of fuel cells and metal-air batteries. A 3D porous superimposed nanosheet catalyst composed of metal manganese covered with MnO2 nanofilms (P-NS-MnO2@Mn) was designed and synthesized by rotating disk electrodes (RDEs) through one-step electrodeposition. The catalyst contains no carbon material. Therefore, the oxidation and corrosion of the carbon material during use can be avoided, resulting in excellent stability. The structural and composition characterizations indicate that the nanosheets with sharp edges exist on the surface of the wall surrounding the macropore (diameter ∼ 5.07 µm) and they connect tightly. Both the nanosheets and the wall of the macropore are composed of metal manganese covered completely with MnO2 film with a thickness of less than 5 nm. The half-wave potential of the synthesized P-NS-MnO2@Mn catalyst is 0.86 V. Besides, the catalyst exhibits good stability with almost no decay after a 30 h chronoamperometric test. Finite element analysis (FEA) simulation reveals the high local electric field intensity surrounding the sharp edges of the nanosheets. Density functional theory (DFT) calculations reveal that the novel nanosheet structure composed of MnO2 nanofilms covered on the surface of the Mn matrix accelerates the electronic transfer of the MnO2 nanofilms during the ORR process. The high local electric field intensity near the sharp edge of the nanosheets effectively promotes the orbital hybridization and strengthens the adsorbing Mn-O bond between the active site Mn in the nanosheets and the intermediate OOH* during the ORR process. This study provides a new strategy for preparing transition metal oxide catalysts and a novel idea about the key factors affecting the catalytic activity of transition metal oxides for the ORR.

Active Interphase Enables Stable Performance for an All-Phosphate-Based Composite Cathode in an All-Solid-State Battery.

High interfacial resistance and unstable interphase between cathode active materials (CAMs) and solid-state electrolytes (SSEs) in the composite cathode are two of the main challenges in current all-solid-state batteries (ASSBs). In this work, the all-phosphate-based LiFePO4 (LFP) and Li1.3 Al0.3 Ti1.7 (PO4 )3 (LATP) composite cathode is obtained by a co-firing technique. Benefiting from the densified structure and the formed redox-active Li3- x Fe2- x - y Tix Aly (PO4 )3 (LFTAP) interphase, the mixed ion- and electron-conductive LFP/LATP composite cathode facilitates the stable operation of bulk-type ASSBs in different voltage ranges with almost no capacity degradation upon cycling. Particularly, both the LFTAP interphase and LATP electrolyte can be activated. The cell cycled between 4.1 and 2.2 V achieves a high reversible capacity of 2.8 mAh cm-2 (36 µA cm-2 , 60 °C). Furthermore, it is demonstrated that the asymmetric charge/discharge behaviors of the cells are attributed to the existence of the electrochemically active LFTAP interphase, which results in more sluggish Li+ kinetics and more expansive LFTAP plateaus during discharge compared with that of charge. This work demonstrates a simple but effective strategy to stabilize the CAM/SSE interface in high mass loading ASSBs.

Single-Ion-Conducting "Polymer-in-Ceramic" Hybrid Electrolyte with an Intertwined NASICON-Type Nanofiber Skeleton.

The fast Li+ transportation of "polymer-in-ceramic" electrolytes is highly dependent on the long-range Li+ migration pathways, which are determined by the structure and chemistry of the electrolytes. Besides, Li dendrite growth may be promoted in the soft polymer region due to the inhomogeneous electric field caused by the commonly low Li+ transference number of the polymer. Herein, a single-ion-conducting polymer electrolyte is infiltrated into intertwined Li1.3Al0.3Ti1.7(PO4)3 (LATP) nanofibers to construct free-standing electrolyte membranes. The composite electrolyte possesses a large electrochemical window exceeding 5 V, a high ionic conductivity of 0.31 mS cm-1 at ambient temperature, and an extraordinary Li+ transference number of 0.94. The hybrid electrolyte in the lithium symmetric cell shows stable Li plating/stripping up to 2000 h under 0.1 mA cm-2 without dendrite formation. The Li|hybrid electrolyte|LiFePO4 battery exhibits enhanced rate capability up to 1 C and a stable cycling performance with an initial discharge capacity of 131.8 mA h g-1 and a retention capacity of 122.7 mA h g-1 after 500 cycles at 0.5 C at ambient temperature. The improved electrochemical performance is attributed to the synergistic effects of the LATP nanofibers and the single-ion-conducting polymer. The fibrous fast ion conductors provide continuous ion transport channels, and the polymer improves the interfacial contact with the electrodes and helps to suppress the Li dendrites.

A new type of noncovalent surface-π stacking interaction occurring on peroxide-modified titania nanosheets driven by vertical π-state polarization.

Noncovalent π stacking of aromatic molecules is a universal form of noncovalent interactions normally occurring on planar structures (such as aromatic molecules and graphene) based on sp2-hybridized atoms. Here we reveal a new type of noncovalent surface-π stacking unusually occurring between aromatic groups and peroxide-modified titania (PMT) nanosheets, which can drive versatile aromatic adsorptions. We experimentally explore the underlying electronic-level origin by probing the perturbed changes of unoccupied Ti 3d states with near-edge X-ray absorption fine structures (NEXAFS), and find that aromatic groups can vertically attract π electrons in the surface peroxo-Ti states and increase their delocalization regions. Our discovery updates the concept of noncovalent π-stacking interactions by extending the substrates from carbon-based structures to a transition metal oxide, and presents an approach to exploit the surface chemistry of nanomaterials based on noncovalent interactions.

Ultrathin 2D Fe-Nanosheets Stabilized by 2D Mesoporous Silica: Synthesis and Application in Ammonia Synthesis.

Developing high-performance Fe-based ammonia catalysts through simple and cost-efficient methods has received an increased level of attention. Herein, we report for the first time, the synthesis of two-dimensional (2D) FeOOH nanoflakes encapsulated by mesoporous SiO2 (mSiO2) via a simple solution-based method for ammonia synthesis. Due to the sticking of the mSiO2 coating layers and the limited spaces in between, the Fe after reduction retains the 2D morphology, showing high resistance against the sintering in the harsh Haber-Bosch process. Compared to supported Fe particles dispersed on mSiO2 spheres, the coated catalyst shows a significantly improved catalytic activity by 50% at 425 °C. Thermal desorption spectroscopy (TDS) reveals the existence of a higher density of reactive sites for N2 activation in the 2D Fe catalyst, which is possibly coupled to a larger density of surface defect sites (kinks, steps, point defects) that are generally considered as active centers in ammonia synthesis. Besides the structural impact of the coating on the 2D Fe, the electronic one is elucidated by partially substituting Si with Al in the coating, confirmed by 29Si and 27Al magic-angle spinning nuclear magnetic resonance (MAS NMR). An increased apparent activation energy (Ea) of the Al-containing catalyst evidences an influence on the nature of the active site. The herein-developed stable 2D Fe nanostructures can serve as an example of a 2D material applied in catalysis, offering the chance of a rational catalyst design based on a stepwise introduction of various promoters, in the coating and on the metal, maintaining the spatial control of the active centers.

Solid-State NMR and MRI Spectroscopy for Li/Na Batteries: Materials, Interface, and In Situ Characterization.

Enhancing the electrochemical performance of batteries, including the lifespan, energy, and power densities, is an everlasting quest for the rechargeable battery community. However, the dynamic and coupled (electro)chemical processes that occur in the electrode materials as well as at the electrode/electrolyte interfaces complicate the investigation of their working and decay mechanisms. Herein, the recent developments and applications of solid-state nuclear magnetic resonance (ssNMR) and magnetic resonance imaging (MRI) techniques in Li/Na batteries are reviewed. Several typical cases including the applications of NMR spectroscopy for the investigation of the pristine structure and the dynamic structural evolution of materials are first emphasized. The NMR applications in analyzing the solid electrolyte interfaces (SEI) on the electrode are further concluded, involving the identification of SEI components and investigation of ionic motion through the interfaces. Beyond, the new development of in situ NMR and MRI techniques are highlighted, including their advantages, challenges, applications and the design principle of in situ cell. In the end, a prospect about how to use ssNMR in battery research from the perspectives of materials, interface, and in situ NMR, aiming at obtaining deeper insight of batteries with the assistance of ssNMR is represented.

Reconstructing 1D Fe Single-atom Catalytic Structure on 2D Graphene Film for High-Efficiency Oxygen Reduction Reaction.

The ordinary intrinsic activity and disordered distribution of metal sites in zero/one-dimensional (0D/1D) single-atom catalysts (SACs) lead to inferior catalytic efficiency and short-term endurance in the oxygen reduction reaction (ORR), which restricts the large-scale application of hydrogen-oxygen fuel cells and metal-air batteries. To improve the activity of SACs, a mild synthesis method was chosen to conjugate 1D Fe SACs with 2D graphene film (Fe SAC@G) that realized a composite structure with well-ordered atomic-Fe coordination configuration. The product exhibits outstanding ORR electrocatalytic efficiency and stability in 0.1 M KOH aqueous solution. DFT-D computational results manifest the intrinsic ORR activity of Fe SAC@G originated from the newly-formed FeN4 -O-FeN4 bridge structure with moderate adsorption ability towards ORR intermediates. These findings provide new ways for designing SACs with high activity and long-term stability.

Overall Oxygen Electrocatalysis on Nitrogen-Modified Carbon Catalysts: Identification of Active Sites and In†Situ Observation of Reactive Intermediates.

The recent mechanistic understanding of active sites, adsorbed intermediate products, and rate-determining steps (RDS) of nitrogen (N)-modified carbon catalysts in electrocatalytic oxygen reduction (ORR) and oxygen evolution reaction (OER) are still rife with controversy because of the inevitable coexistence of diverse N configurations and the technical limitations for the observation of formed intermediates. Herein, seven kinds of aromatic molecules with designated single N species are used as model structures to investigate the explicit role of each common N group in both ORR and OER. Specifically, dynamic evolution of active sites and key adsorbed intermediate products including O2 (ads), superoxide anion O2 - *, and OOH* are monitored with in situ spectroscopy. We propose that the formation of *OOH species from O2 - * (O2 - *+H2 O→OOH*+OH- ) is a possible RDS during the ORR process, whereas the generation of O2 from OOH* species is the most likely RDS during the OER process.

Ultrasensitive ultrasound detection using an intracavity phase-shifted fiber Bragg grating in a self-injection-locked diode laser.

We report a high-sensitivity fiber-optic ultrasonic sensor system using a self-injection-locked distributed-feedback (DFB) diode laser where a π-phase-shifted fiber Bragg grating (πFBG) serves as both the locking resonator and the sensing element in a fiber ring feedback loop. By controlling the delay time of the feedback light through a fiber stretcher, the laser wavelength is locked to an external cavity mode on the spectral slope of the πFBG, and the ultrasound-induced wavelength shifts of the πFBG are converted to laser intensity variation. The ultrasonic sensing scheme simplifies the feedback control because the self-injection locking automatically pulls the laser wavelength to the πFBG resonant wavelength. In addition, it improves the detection sensitivity because the frequency noise of the DFB laser is drastically reduced. We show that the sensor system achieves a strain sensitivity of 78 fε/Hz1/2 at around 200 kHz.

Highly Efficient Metal-Free Nitrogen-Doped Nanocarbons with Unexpected Active Sites for Aerobic Catalytic Reactions.

Nitrogen (N)-doped nanocarbons (NDN) as metal-free catalysts have elicited considerable attention toward selective oxidation of alcohols with easily oxidizable groups to aldehydes in the past few years. However, finding a new NDN catalytic material that can meet the requirement of the feasibility on the aerobic catalytics for other complicated alcohols is a big challenge. The real active sites and the corresponding mechanisms on NDN are still unambiguous because of inevitable coexistence of diverse edge sites and N species based on recently reported doping methods. Here, four NDN catalysts with enriched pyridinic N species and without any graphitic N species are simply fabricated via a chemical-vapor-deposition-like method. The results of X-ray photoelectron spectroscopy and X-ray absorption near-edge structure spectra suggest that the dominating N species on NDN are pyridinic N. It is demonstrated that NDN catalysts perform impressive reactivity for aerobic oxidation of complicated alcohols at an atmospheric pressure. Eleven kinds of aromatic molecules with single N species and tunable π conjugation systems are used as model catalysts to experimentally identify the actual role of each N species at a real molecular level. It is suggested that pyridinic N species play an unexpected role in catalytic reactions. Neighboring carbon atoms in pyridinic N species are responsible for facilitating the rate-determining step process clarified by kinetic isotope effects, in situ nuclear magnetic resonance, in situ attenuated total reflectance infrared, and theoretical calculation. Moreover, NDN catalysts exhibit a good catalytic feasibility on the synthesis of important natural products (e.g., intermediates of vitamin E and K3) from phenol oxidation.

Fluorine-free water-in-ionomer electrolytes for sustainable lithium-ion batteries.

The continuously increasing number and size of lithium-based batteries developed for large-scale applications raise serious environmental concerns. Herein, we address the issues related to electrolyte toxicity and safety by proposing a "water-in-ionomer" type of electrolyte which replaces organic solvents by water and expensive and toxic fluorinated lithium salts by a non-fluorinated, inexpensive and non-toxic superabsorbing ionomer, lithium polyacrylate. Interestingly, the electrochemical stability window of this electrolyte is extended greatly, even for high water contents. Particularly, the gel with 50 wt% ionomer exhibits an electrochemical stability window of 2.6 V vs. platinum and a conductivity of 6.5 mS cm-1 at 20 °C. Structural investigations suggest that the electrolytes locally self-organize and most likely switch local structures with the change of water content, leading to a 50% gel with good conductivity and elastic properties. A LiTi2(PO4)3/LiMn2O4 lithium-ion cell incorporating this electrolyte provided an average discharge voltage > 1.5 V and a specific energy of 77 Wh kg-1, while for an alternative cell chemistry, i.e., TiO2/LiMn2O4, a further enhanced average output voltage of 2.1 V and an initial specific energy of 124.2 Wh kg-1 are achieved.

Probing Ligand-Induced Cooperative Orbital Redistribution That Dominates Nanoscale Molecule-Surface Interactions with One-Unit-Thin TiO2 Nanosheets.

Understanding the general electronic principles underlying molecule-surface interactions at the nanoscale is crucial for revealing the processes based on chemisorption, like catalysis, surface ligation, surface fluorescence, etc. However, the electronic mechanisms of how surface states affect and even dominate the properties of nanomaterials have long remained unclear. Here, using one-unit-thin TiO2 nanosheet as a model surface platform, we find that surface ligands can competitively polarize and confine the valence 3d orbitals of surface Ti atoms from delocalized energy band states to localized chemisorption bonds, through probing the surface chemical interaction at the orbital level with near-edge X-ray absorption fine structure (NEXAFS). Such ligand-induced orbital redistributions, which are revealed by combining experimental discoveries, quantum calculations, and theoretical analysis, are cooperative with ligand coverages and can enhance the strength of chemisorption and ligation-induced surface effects on nanomaterials. The model and concept of nanoscale cooperative chemisorption reveal the general physical principle that drives the coverage-dependent ligand-induced surface effects on regulating the electronic structures, surface activity, optical properties, and chemisorption strength of nanomaterials.

Correction: Synthesis of Ca(PF6)2, formed via nitrosonium oxidation of calcium.

Correction for 'Synthesis of Ca(PF6)2, formed via nitrosonium oxidation of calcium' by Evan N. Keyzer et al., Chem. Commun., 2017, 53, 4573-4576.

Identifying the Structural Evolution of the Sodium Ion Battery Na2 FePO4 F Cathode.

Na2 FePO4 F is a promising cathode material for Na-ion batteries owing to its relatively high discharge voltage and excellent cycling performance. Now, the long- and short-range structural evolution of Na2 FePO4 F during cycling is studied by in situ high-energy X-ray diffraction (XRD), ex situ solid-state nuclear magnetic resonance (NMR), and first-principles DFT calculations. DFT calculations suggest that the intermediate phase, Na1.5 FePO4 F, adopts the space group of P21 /c, which is a subgroup (P21 /b11, No. 14) of Pbcn (No. 60), the space group of the starting phase, Na2 FePO4 F, and this space group provides a good fit to the experimental XRD and NMR results. The two crystallographically unique Na sites in the structure of Na2 FePO4 F behave differently during cycling, where the Na ions on the Na2 site are electrochemically active while those on the Na1 site are inert. This study determines the structural evolution and the electrochemical reaction mechanisms of Na2 FePO4 F in a Na-ion battery.

The Effect of Water on Quinone Redox Mediators in Nonaqueous Li-O2 Batteries.

The parasitic reactions associated with reduced oxygen species and the difficulty in achieving the high theoretical capacity have been major issues plaguing development of practical nonaqueous Li-O2 batteries. We hereby address the above issues by exploring the synergistic effect of 2,5-di-tert-butyl-1,4-benzoquinone and H2O on the oxygen chemistry in a nonaqueous Li-O2 battery. Water stabilizes the quinone monoanion and dianion, shifting the reduction potentials of the quinone and monoanion to more positive values (vs Li/Li+). When water and the quinone are used together in a (largely) nonaqueous Li-O2 battery, the cell discharge operates via a two-electron oxygen reduction reaction to form Li2O2, with the battery discharge voltage, rate, and capacity all being considerably increased and fewer side reactions being detected. Li2O2 crystals can grow up to 30 µm, more than an order of magnitude larger than cases with the quinone alone or without any additives, suggesting that water is essential to promoting a solution dominated process with the quinone on discharging. The catalytic reduction of O2 by the quinone monoanion is predominantly responsible for the attractive features mentioned above. Water stabilizes the quinone monoanion via hydrogen-bond formation and by coordination of the Li+ ions, and it also helps increase the solvation, concentration, lifetime, and diffusion length of reduced oxygen species that dictate the discharge voltage, rate, and capacity of the battery. When a redox mediator is also used to aid the charging process, a high-power, high energy density, rechargeable Li-O2 battery is obtained.

Understanding LiOH Chemistry in a Ruthenium-Catalyzed Li-O2 Battery.

Non-aqueous Li-O2 batteries are promising for next-generation energy storage. New battery chemistries based on LiOH, rather than Li2 O2 , have been recently reported in systems with added water, one using a soluble additive LiI and the other using solid Ru catalysts. Here, the focus is on the mechanism of Ru-catalyzed LiOH chemistry. Using nuclear magnetic resonance, operando electrochemical pressure measurements, and mass spectrometry, it is shown that on discharging LiOH forms via a 4 e- oxygen reduction reaction, the H in LiOH coming solely from added H2 O and the O from both O2 and H2 O. On charging, quantitative LiOH oxidation occurs at 3.1 V, with O being trapped in a form of dimethyl sulfone in the electrolyte. Compared to Li2 O2 , LiOH formation over Ru incurs few side reactions, a critical advantage for developing a long-lived battery. An optimized metal-catalyst-electrolyte couple needs to be sought that aids LiOH oxidation and is stable towards attack by hydroxyl radicals.

Fiber-optic anemometer based on single-walled carbon nanotube coated tilted fiber Bragg grating.

In this work, a novel and simple optical fiber hot-wire anemometer based on single-walled carbon nanotubes (SWCNTs) coated tilted fiber Bragg grating (TFBG) is proposed and demonstrated. For the hot-wire wind speed sensor design, TFBG is an ideal in-fiber sensing structure due to its unique features. It is utilized as both light coupling and temperature sensing element without using any geometry-modified or uncommon fiber, which simplifies the sensor structure. To further enhance the thermal conversion capability, SWCNTs are coated on the surface of the TFBG instead of traditional metallic materials, which have excellent thermal characteristics. When a laser light is pumped into the sensor, the pump light propagating in the core will be easily coupled into cladding of the fiber via the TFBG and strongly absorbed by the SWCNTs thin film. This absorption acts like a hot-wire raising the local temperature of the fiber, which is accurately detected by the TFBG resonance shift. In the experiments, the sensor's performances were investigated and controlled by adjusting the inherent angle of the TFBG, the thickness of SWCNTs film, and the input power of the pump laser. It was demonstrated that the developed anemometer exhibited significant light absorption efficiency up to 93%, and the maximum temperature of the local area on the fiber was heated up to 146.1°C under the relatively low pump power of 97.76 mW. The sensitivity of -0.3667 nm/(m/s) at wind speed of 1.0 m/s was measured with the selected 12° TFBG and 1.6 µm film.

A Novel Low-Power-Consumption All-Fiber-Optic Anemometer with Simple System Design.

A compact and low-power consuming fiber-optic anemometer based on single-walled carbon nanotubes (SWCNTs) coated tilted fiber Bragg grating (TFBG) is presented. TFBG as a near infrared in-fiber sensing element is able to excite a number of cladding modes and radiation modes in the fiber and effectively couple light in the core to interact with the fiber surrounding mediums. It is an ideal in-fiber device used in a fiber hot-wire anemometer (HWA) as both coupling and sensing elements to simplify the sensing head structure. The fabricated TFBG was immobilized with an SWCNT film on the fiber surface. SWCNTs, a kind of innovative nanomaterial, were utilized as light-heat conversion medium instead of traditional metallic materials, due to its excellent infrared light absorption ability and competitive thermal conductivity. When the SWCNT film strongly absorbs the light in the fiber, the sensor head can be heated and form a "hot wire". As the sensor is put into wind field, the wind will take away the heat on the sensor resulting in a temperature variation that is then accurately measured by the TFBG. Benefited from the high coupling and absorption efficiency, the heating and sensing light source was shared with only one broadband light source (BBS) without any extra pumping laser complicating the system. This not only significantly reduces power consumption, but also simplifies the whole sensing system with lower cost. In experiments, the key parameters of the sensor, such as the film thickness and the inherent angle of the TFBG, were fully investigated. It was demonstrated that, under a very low BBS input power of 9.87 mW, a 0.100 nm wavelength response can still be detected as the wind speed changed from 0 to 2 m/s. In addition, the sensitivity was found to be -0.0346 nm/(m/s) under the wind speed of 1 m/s. The proposed simple and low-power-consumption wind speed sensing system exhibits promising potential for future long-term remote monitoring and on-chip sensing in practical applications.

Simple method for self-referenced and lable-free biosensing by using a capillary sensing element.

We demonstrated a simple method for self-reference and label free biosensing based on a capillary sensing element and common optoelectronic devices. The capillary sensing element is illuminated by a light-emitting diode (LED) light source and detected by a webcam. Part of gold film that deposited on the tubing wall is functionalized to carry on the biological information in the excited SPR modes. The end face of the capillary was monitored and separate regions of interest (ROIs) were selected as the measurement channel and the reference channel. In the ROIs, the biological information can be accurately extracted from the image by simple image processing. Moreover, temperature fluctuation, bulk RI fluctuation, light source fluctuation and other factors can be effectively compensated during detection. Our biosensing device has a sensitivity of 1145%/RIU and a resolution better than 5.287 × 10-4 RIU, considering a 0.79% noise level. We apply it for concanavalin A (Con A) biological measurement, which has an approximately linear response to the specific analyte concentration. This simple method provides a new approach for multichannel SPR sensing and reference-compensated calibration of SPR signal for label-free detection.