Ultrafast Kinetics Research Articles

A comprehensive sintering mechanism for TBCs‐Part II: Multiscale multipoint interconnection‐enhanced initial kinetics

AbstractDuring thermal exposure, the sintering process of the plasma‐sprayed thermal barrier coatings (PS‐TBCs) presents significant two‐stage kinetics, as reported in the Part I. As a companion study, this part aims to reveal the mechanism responsible for the ultrafast sintering kinetics when initially exposed, since the initial short stage (stage‐I) often finishes most increment of the mechanical and thermal properties. A detailed examination of the pore healing behavior was carried out, based on the experimental result in Part I that the dominant structural change at the stage‐I is the significant healing at intersplat pore tips. Results show that the smooth splat surface, which made up the wedge‐shaped intersplat pores, changed to a rough surface with multiscale convexs after high‐temperature thermal exposure. Moreover, the multiscale roughening is highly sensitive to various orientations of the columnar grains inside the splats. Subsequently, multipoint connection between the counter‐surfaces of the intersplat pores was formed by the bridging of multiscale convexs. This multiscale multipoint connection between pore surfaces significantly accelerated the healing of intersplat pores, which can be dominantly responsible for the ultrafast sintering kinetics at the stage‐I. This comprehensive sintering mechanism could offer some positive suggestions toward prolonging the thermal‐insulating effect of TBCs.

Nano-engineered Mg–MgH2 system for solar thermal energy storage

The solar thermal energy could be stored and reused at a desired locations and conditions. The prerequisite is to develop a suitable media which could able to store the solar thermal energy reversibly. The metal-metal hydride system could be one of the option to store the thermal energy in the form of metal and hydrogen which on recombination will form metal- hydride and release the stored thermal energy with high efficiency. Besides the high hydrogen storage capacity, the ultrafast hydrogenation-dehydration kinetics is desirable for the viable commercial applications. In connection to this, magnesium – magnesium hydride system has been considered as a potentials candidate. However, the sluggish hydrogenation-dehydrogenation kinetics is an issue. In the present study nano-engineered Mg-V composite has been developed using MgH2 and V2O5 as a precursor for magnesium and vanadium, respectively. The composite has shown an ultrafast hydrogenation-dehydrogenation kinetics at remarkable low temperature. The hydrogenation of composite has efficiently released the thermal energy. The hydrogenated composite could be dehydrogenated using compact solar power (CSP) even below 200°C.

Femtosecond single-shot imaging and control of a laser-induced first-order phase transition in HoFeO3

Excitation of antiferromagnetic HoFeO3 with a single 80 fs laser pulse triggers a first-order spin-reorientation phase transition. In the ultrafast kinetics of the transition one can distinguish the processes of impulsive excitation of spin precession, nucleation of the new domain and growth of the nuclei. The orientation of the spins in the nuclei is defined by the phase of the laser-induced coherent spin precession. The growth of the nuclei is further promoted by heating induced by the laser excitation. Hereby we demonstrate that in HoFeO3 coherent control of the spin precession allows an effective control of the route of the heat-induced first-order magnetic phase transition. The theoretical description of the excitation of the spin precession by linearly-polarized ultrashort laser pulses is developed with the sigma model. The analysis showed high sensitivity of the excited dynamics to the initial spin orientations with respect to the crystallographic axes of the material.

Verification of Nonequilibrium Mechanism of Ultrafast Charge Recombination in Excited Donor–Acceptor Complexes

Control of charge transfer requires knowledge of its detailed mechanism. Due to the large number of known mechanisms, the identification of the mechanism in specific systems is a challenge so far. In this article we propose the idea of how to distinguish between thermal and nonequilibrium modes of charge recombination in excited donor-acceptor complexes. Simulations of the effect of solvent relaxation time scale on ultrafast charge recombination kinetics in photoexcited donor-acceptor complexes within the framework of the multichannel stochastic model have shown that a series of regularities inherent to the thermal and nonequilibrium charge transfer can strongly differ. Among them there are opposite regularities, for example, the dependence of the dynamic solvent effect on the free energy gap. In particular, theory predicts that in ultrafast charge recombination of excited donor-acceptor complexes the dynamic solvent effect is weak in the area of weak exergonicity and becomes stronger in the area of stronger exergonicity whereas for the thermal reactions an opposite trend is expected. Comparison of such trends with experimental data implemented in this article allowed establishing the regime in which the reaction proceeds. It is shown that observation of dynamic solvent effect in the region of strong exergonicity for ultrafast charge recombination is decisive evidence in favor of nonequilibrium mechanism.

Insights into the Ultra-Fast Li Storage Properties of Organic Polymers

Organic materials have long been recognized as resource-abundant, sustainable, and potentially low-cost electrode materials that will handily satisfy the ever-growing demand for batteries. Although once had the reputation as being less stable and kinetically slow compared with their traditional inorganic counterparts, organic materials have been steadily improving on performance to be competent electrode candidates in recent years. We have recently reported a π-conjugated redox polymer with high cycling stability, high capacity utilization, and ultra-fast Li storage kinetics. The polymer delivers 80% of its theoretical capacity at a rate of 50C even with a high active mass ratio of 80 wt.% at practical mass loading, which is unusual for organic electrode materials. There is no established theory as for how such performance can be achieved and optimized via molecular design and microstructure engineering. We set out to scrutinize the charge storage mechanism of polymer electrodes and investigate the influence of molecular electronic structure and electrode microstructure on electrochemical performance. We have designed and synthesized a series of polymers with similar molecular structure but varying degree of conjugation in the backbone. The polymers are evaluated as Li storage electrode materials. The morphology and its evolution, charge transfer process, and state-of-charge-dependent electron and ion transport of the polymer electrodes are monitored and correlated to the cell performance. We found that regardless of the degree of conjugation, the redox reaction and the bulk ionic diffusion in the polymers proceed at comparable rates. We developed a procedure to directly measure the electrical conductivity of electrochemically lithiated polymers. This conductivity increases by 100-fold as the π-conjugation degree of the polymers increases. The structural order and morphology of the polymers differ widely with the conjugation length, leading to significantly different microstructure. The electrochemical surface area of the polymer electrode is continuously altered by the π-conjugation degree, which in turn impacts the ion transport model in the electrode. Overall, our study establishes general analysis tools that elucidate the underlying charge storage mechanism of polymer electrode materials. The results shown here provide guidance for further advancing high-energy/power energy battery based on organic materials.

Ultrafast Double Optical Pump Technique for Probing Charge Transfer Processes at the Electrode-Electrolyte Interface

The electrode-electrolyte interface is one of the most technologically relevant there is, as it is where charge transfer occurs in driven electrochemical reactions. The short times after heterogeneous charge transfer witness the occurrence of primordial processes, such as intramolecular rearrangement and solvation shell reorganization. Yet, probing the evolution of this interface with ultrafine time resolution (10-15—10-12 s) is highly challenging, if not impossible, with electronics-based techniques. [1] Photoinjection techniques have been used to trigger charge capture by acceptors at the electrode-electrolyte interface, which was observed with > 10-9 s time resolution from photocurrent. [2] We aim to use the aqueous electron as a test bed for probing the heterogeneous charge transfer with ultrafine time resolution. The ultrafast kinetics of this fundamental species have been studied in the bulk [3], but it was never observed at the interface. We have used ultrafast laser pulses in the ultraviolet (UV) range with the intent of triggering photoinjection from a 200 nm thick polycrystalline gold electrode to the 0.5 M Na2SO4 electrolyte. The energy of the ultraviolet photons (4.64 eV) is high enough for excited electrons to reach the LUMO of water, and exposure of the electrode to the UV laser beam indeed creates a large photovoltage. A second ultrafast pulse of lower energy (ranging from 0.31 to 1.85 eV) is used to resonantly pump the excitation after a controlled delay varying from -4 ps to 100 ps with the smaller steps being of 50 fs. This second pump pulse also causes a photovoltage change, but smaller by an order of magnitude than the photovoltage caused by the first. The photovoltage due to the second pump pulse decays at a rate that is dependent on the photon energy, which yields a resonant profile that matches the absorption profile of hot electrons in water during the solvation process. [4] For instance, the decay rate at 0.31 eV is very fast, being comparable or smaller than our 50 fs minimum step size, and is independent of pump power. At 1.85 eV, however, the photovoltage lasts for longer than 100 ps, showing that a long-lasting species endures at the gold-electrolyte interface. In between, we capture the smooth evolution of the decay kinetics. We expect this technique to enable the study of ultrafast kinetics of relevant processes directly at the electrode-electrolyte interface, giving valuable mechanistic insight on important electrochemical reactions such as the oxygen reduction reactions (ORR).

Controlled synthesis of microsheets of ZnAl layered double hydroxides hexagonal nanoplates for efficient removal of Cr(VI) ions and anionic dye from water

The removal of effluents from water is necessary for environmental concerns. The common approach to the development of methodologies for water purification is to search for excellent materials with optimised chemical compositions and unique structural dimension. Here a facile chemical approach has been developed for the synthesis of microsheets of chlorine interlayer ZnAl layered double hydroxides (Cl-ZnAl LDHs) hexagonal nanoplates by simply varying the molar ratio of salts precursors in the presence of hexamethylenetetramine (HMTA). The possible formation mechanism of microsheets of Cl-ZnAl LDHs hexagonal nanoplates has been described on the basis of different characterization. The prepared unique structural microsheets exhibited ultrafast adsorption rates and high adsorption capacities in the removal of anionic methyl orange (MO) dye and Cr(VI) ions from aqueous solution. The adsorption isotherms, kinetics models and mechanisms for Cr(VI) ions and MO onto Cl-ZnAl LDHs were also investigated in detail. The high uptake capability, reusability and ultrafast kinetics make Cl-ZnAl LDHs unique structural microsheets a potentially attractive low-cost adsorbent for the efficient removal of metal ions and toxic dye from contaminated water. Furthermore, the simple synthesis strategy might be envisaged for another type of LDHs, and consequently, a variety of applications can target due to their specific structural dimensions.

Phase formation and microstructure of PbTiO3 and CaTiO3 ceramics prepared by a direct current arc discharge technique

ABSTRACTPbTiO3 and CaTiO3 ceramics are prepared at temperatures above 2000°C by a direct current arc discharge technique under an argon atmosphere, and the microstructure, components and morphology of each material are investigated. Both samples show the formation of a perovskite structure. Moreover, a tetragonal phase is found in the PbTiO3 ceramic material, and a cubical phase is formed in the CaTiO3 ceramic material. In this work, PbTiO3 and CaTiO3 ceramics were rapidly prepared using a simple DC arc discharge technique; we found this technique of ceramic material synthesis was much faster and better than conventional methods. The whole reaction process only takes 30 seconds because of ultrafast kinetics of crystallization.

Fabrication of a phosphorylated graphene oxide–chitosan composite for highly effective and selective capture of U(vi)

The synthesized GO–CS–P exhibits ultrafast removal kinetics, high sorption capacity and great selectivity towards U(vi).

Synergistic hydrogen desorption properties of the 4LiAlH4 + Mg2NiH4 composite

Mg2NiH4 was prepared by hydriding combustion synthesis (HCS) method firstly. Then, the as prepared Mg2NiH4 was mechanically milled with LiAlH4 to form a novel composite of 4LiAlH4 + Mg2NiH4. Hydrogen storage properties and reaction mechanism of the 4LiAlH4 + Mg2NiH4 composite during hydrogenation/dehydrogenation have been investigated systematically by pressure-composition-temperature (PCT), differential scanning calorimetry (DSC) and X-ray diffraction (XRD) measurements. The microstructure of the composite has been investigated by scanning electron microscopy (SEM). The experimental results show that there is a mutual destabilization effect between Mg2NiH4 and LiAlH4 during hydrogen desorption. Mg2NiH4 can promote the decomposition of LiAlH4 in both of the ball milling process and the thermal desorption process. Conversely, the in situ formed Al from the decomposition of LiAlH4 can destabilize Mg2NiH4 via a new reaction pathway with lower activation energy. Moreover, the 4LiAlH4 + Mg2NiH4 composite shows ultrafast re-hydrogenation kinetics, and the re-hydrogenation mechanism has been revealed.

Co3O4 Nanospheres Embedded in a Nitrogen-Doped Carbon Framework: An Electrode with Fast Surface-Controlled Redox Kinetics for Lithium Storage

Herein, we develop a Co3O4-based anode material with a hierarchical structure similar to that of a lotus pod, where single yolk–shell-structured Co3O4@Co3O4 nanospheres are well embedded in a nitrogen-doped carbon (N–C) conductive framework (Co3O4@Co3O4/N–C). This distinctive architecture contains multiple advantages of both the yolk–shell structure and conductive N–C framework to improve the Li ion storage performance. Especially, the doping of the N atom in N–C increases the interaction between the carbon and adsorbents, which is confirmed by the theoretical calculations in this work, making the carbon framework much more electrochemically active. As a result, the Co3O4@Co3O4/N–C exhibits fast surface-controlled kinetics, which corroborate the high counterion mobility and the ultrafast electron-transfer kinetics of the electrode. Due to these synergetic effects, desired capacity stability (1169.6 mAh g–1 at 200 mA g–1 after 100 cycles) and superior rate performance (633.4 mAh g–1 at 10 A g–1) have been realized in this Co3O4@Co3O4/N–C electrode.

Decoupling Interfacial Charge Transfer from Bulk Diffusion Unravels Its Intrinsic Role for Efficient Charge Extraction in Perovskite Solar Cells.

In a perovskite solar cell, the overall photoinduced charge-transfer (CT) process comprises both charge diffusion through the bulk to perovskite/electrode interfaces and interfacial electron and hole transfer to electrodes. In this study, we decoupled these two entangled processes by investigating the film thickness-dependent CT dynamics from CH3NH3PbI3 perovskites to [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) (electron acceptor) and spiro-OMeTAD (hole acceptor). By fitting ultrafast transient absorption kinetics to an explicit "diffusion-coupled charge-transfer" model, we found that the charge diffusion from the film interior to perovskite/electrode interfaces took ∼200 ps to a few nanoseconds, depending on the thickness of perovskite film; the subsequent interfacial charge transfer was ultrafast, ∼6 ps for electron transfer to PCBM and ∼8 ps for hole transfer to spiro-OMeTAD, and led to efficient charge extraction (>90%) to electrodes in a 400 nm thick film. Our results indicate that the picosecond interfacial charge transfer is a key to high-performance perovskite solar cells.

Slow Singlet Fission Observed in a Polycrystalline Perylenediimide Thin Film.

Singlet exciton fission (SF) is a process wherein an exciton in an organic semiconductor divides its energy to form two excitations. This process can offset thermalization losses in light harvesting technologies, but requires photostable materials with high SF efficiency. We report ultrafast kinetics of polycrystalline films of N-N'-dioctyl-3,4,9,10-perylenedicarboximide (C8-PDI), a chromophore predicted to undergo SF on picosecond time scales. While transient absorption measurements display picosecond dynamics, such kinetics are absent from low-fluence time-resolved emission experiments, indicating they result from singlet-singlet exciton annihilation. A model that accounts for annihilation can reproduce both measurements and highlights that care must be taken when extracting SF rates from time-resolved data. Our model also reveals SF proceeds in C8-PDI over 3.8 ns. Despite this slow rate, SF occurs in high yield (51%) due to a lack of competing singlet deactivation pathways. Our results show perylenediimides are a promising class of SF materials that merit further study.

Modulation of Crystal Surface and Lattice by Doping: Achieving Ultrafast Metal-Ion Insertion in Anatase TiO2.

We report that an ultrafast kinetics of reversible metal-ion insertion can be realized in anatase titanium dioxide (TiO2). Niobium ions (Nb5+) were carefully chosen to dope and drive anatase TiO2 into very thin nanosheets standing perpendicularly onto transparent conductive electrode (TCE) and simultaneously construct TiO2 with an ion-conducting surface together with expanded ion diffusion channels, which enabled ultrafast metal ions to diffuse across the electrolyte/solid interface and into the bulk of TiO2. To demonstrate the superior metal-ion insertion rate, the electrochromic features induced by ion intercalation were examined, which exhibited the best color switching speed of 4.82 s for coloration and 0.91 s for bleaching among all reported nanosized TiO2 devices. When performed as the anode for the secondary battery, the modified TiO2 was capable to deliver a highly reversible capacity of 61.2 mAh/g at an ultrahigh specific current rate of 60 C (10.2 A/g). This fast metal-ion insertion behavior was systematically investigated by the well-controlled electrochemical approaches, which quantitatively revealed both the enhanced surface kinetics and bulk ion diffusion rate. Our study could provide a facile methodology to modulate the ion diffusion kinetics for metal oxides.

Evaluation of Surface State Mediated Charge Recombination in Anatase and Rutile TiO2.

In nanostructured thin films, photogenerated charge carriers can access the surface more easily than in dense films and thus react more readily. However, the high surface area of these films has also been associated with enhanced recombination losses via surface states. We herein use transient absorption spectroscopy to compare the ultrafast charge carrier kinetics in dense and nanostructured TiO2 films for its two most widely used polymorphs: anatase and rutile. We find that nanostructuring does not enhance recombination rates on ultrafast time scales, indicating that surface state mediated recombination is not a key loss pathway for either TiO2 polymorph. Rutile shows faster, and less intensity-dependent recombination than anatase, which we assign to its higher doping density. For both polymorphs, we conclude that bulk rather than surface recombination is the primary determinant of charge carrier lifetime.

High-Power Lithium Vanadate / Carbon Composite for Hybrid Supercapacitors

1. Introduction The next step in the improvement of the high-energy-density Li4Ti5O12(LTO) / activated carbon hybrid supercapacitor1 is to substitute the negative electrode (LTO) with other potential candidates with higher voltage and capacity. Here, we selected Li3VO4 (LVO) because of its low redox potential (1.0 V – 0.1 V vs. Li+/Li) and reversible insertion mechanism with high theoretical capacity (394 mAh g-1 in case of 2-electron reaction and 591 mAh g-1 in case of the 3-electron).2-4 To overcome the well-known drawbacks of LVO such as low electronic conductivity (<10-10 S m-1) and large voltage hysteresis (>500 mV) which prevent to achieve high rate performances5-7, we investigated the potentialities of a nanosized LVO (10-50 nm) hyper-dispersed within multi-walled carbon nanotube (MWCNT) matrix prepared by our original ultracentrifugation (UC) process. As expected, the rate performances are considerably enhanced for the uc-LVO/MWCNT, and in addition, the structural mechanism involved during the reversible insertion/extraction of Li is clarified thanks to a deep investigation using in operando XRD. The performances of the obtained composite show that it can be used as a LTO alternative for hybrid supercapacitors. 2. Experimental NH4VO3, citric acid, ethylene glycol and lithium hydroxide were dissolved into deionized water to form a clear orange solution. After the addition under stirring of MWCNT, the solution was subjected to the UC treatment at 80 ˚C for 5min. Then, the mixture was dried at 130˚C under vacuum for 12 h to accelerate the polymerization of ethylene glycol. A calcination stage at 300°C for 3h under air was applied to remove the polymer and a final sintering at 800˚C under N2 atmosphere allows obtaining the Li3VO4/MWCNT composite. Physicochemical characterizations were performed on the synthesized composites by XRD, XPS and TEM observation. Relation between electrochemical behavior and structure change were investigated by in operando XAFS and XRD measurements. 3. Results and discussion Successful synthesis of nanocrystalline LVO particles of ca. 10~50 nm on the surface of MWCNT was confirmed by the combination of HRTEM observation and XRD measurements. The LVO/MWCNT composite shows excellent electrochemical behavior such as high charge (delithiation) rate capability with 170 mAh g-1 per composite at a high rate of 20 A g-1, when charged and discharged down to 0.1 V vs. Li. In operando XRD patterns on the composites show the existence of irreversible structure changes acting as an activation process, where the reaction mechanism of LVO switches from a “two-phase” to a “solid-solution” reaction. The relationship between the hysteresis and Li insertion mechanism was further investigated by charge-discharge tests at different cut-off voltages. It was found that the “activated” LVO/MWCNT composites within the limited voltage window (0.76 -2.5 V vs. Li) can achieve higher reversibility and energy efficiency (low voltage hysteresis below 100mV) with ultrafast Li insertion kinetics and long-term cycle life for the needed hybrid supercapacitor applications. References 1) K. Naoi, W. Naoi, S. Aoyagi, J. Miyamoto and T. Kamino, Acc. Chem. Res., 46, 1075 (2013). 2) S. Ni, J. Zhang, J. Ma, X. Yang, L. Zhang and H. Zeng, Adv. Mater. Interfaces, 3, 1500340 (2016). 3) J. Zhang, S. Ni, J. Ma, X. Yang and L. Zhang, J. Power Sources, 301, 41 (2015). 4) Z. Liang, Y. Zhao, Y. Dong, Q. Kuang, X. Lin, X. Liu, D. Yan, J. Power Sources, 274, 345 (2015). 5) Y. Shi, J. Gao, H.-D. Abruna, H.-J. Li, H.-K. Liu, D. Wexler, J.-Z. Wang and Y. Wu, Chem. Eur. J., 20, 5608 (2014). 6) Y. Shi, J.-Z. Wang, S.-L. Chou, D. Wexler, H.-J. Li, K. Ozawa, H.-k. Liu and Y.-P. Wu, Nano lett., 13, 4715 (2013). 7) H. Li, X. Liu, T. Zhai, D. Li and H. Zhou, Adv. Energy Mater., 3, 428 (2012).

High Performance Hollow Metal–Organic Framework Nanoshell‐Based Etalons for Volatile Organic Compounds Detection

A novel high performance optical metal–organic framework (MOF) sensor is presented by means of hollow MOF nanoshell‐based etalon (HMNSE), which allows for selective volatile organic compound (VOC) detection with distinct color output and high efficiency of optical transduction. The HMNSE is constructed by integrating hollow MOF nanoshell arrays between the reflective surfaces of silicon wafer and metal layer, and features ultrathin shell thickness (&lt;100 nm) and macropores within a submicron thin film, which provides appropriate optical thickness, tunable interference color, and fast mass transport all at one time. Due to the rational design of optical motif, the HMNSE has attained the largest optical response so far for ultrathin MOF toward VOC detection. Selective detection on vapor phase alcohols, ketones, nitriles, cyclohexane/benzene, and xylene isomers has been exhibited, which reflected well the characteristic effect of size exclusion and chemical interaction between the MOF and VOCs. The synergistic combination of enhanced reflectance peak and optical shift has led to visible color change with unprecedented high saturation upon VOC exposure. Wide dynamic range, ultrafast kinetics, and excellent reversibility and stability have also been achieved, rendering HMNSE promising for low‐cost, real‐time, and on‐site detection.

Origin of Asymmetry of Paired Nanogap Voltammograms Based on Scanning Electrochemical Microscopy: Contamination Not Adsorption.

Formation of a nanometer-wide gap between tip and substrate electrodes by scanning electrochemical microscopy (SECM) enables voltammetric measurement of ultrafast electron-transfer kinetics. Herein, we demonstrate the advantage of SECM-based nanogap voltammetry to assess the cleanness of the substrate surface in solution by confirming that airborne contamination of highly oriented pyrolytic graphite (HOPG) causes the nonideal asymmetry of paired nanogap voltammograms of (ferrocenylmethyl)trimethylammonium (Fc(+)). We hypothesize that the amperometric response of a 1 μm-diameter Pt tip is less enhanced in the feedback mode, where more hydrophilic Fc(2+) is generated from Fc(+) at the tip and reduced voltammetrically at the HOPG surface covered with airborne hydrophobic contaminants. The tip current is more enhanced in the substrate generation/tip collection mode, where less charged Fc(+) is oxidized at the contaminated HOPG surface. In fact, symmetric pairs of nanogap voltammograms are obtained with the cleaner HOPG surface that is exfoliated in humidified air and covered with a nanometer-thick water adlayer to suppress airborne contamination. This result disproves a misconception that the asymmetry of paired nanogap voltammograms is due to electron exchange mediated by Fc(2+) adsorbed on the glass sheath of the tip. Moreover, weak Fc(+) adsorption on the HOPG surface causes only the small hysteresis of each voltammogram upon forward and reverse sweeps of the HOPG potential. Significantly, no Fc(2+) adsorption on the HOPG surface ensures that the simple outer-sphere pathway mediates ultrafast electron transfer of the Fc(2+/+) couple with standard rate constants of ≥12 cm/s as estimated from symmetric pairs of reversible nanogap voltammograms.

The Fine Line Between a Two-Phase and Solid-Solution Transformation in Li4+XTi5O12

The working of Li-ion batteries is based on the reversible insertion and extraction of Li-ions in the crystal structure of the positive and negative electrode materials resulting in highly efficient and high density energy storage. The nature of the electrode phase transitions induced by the insertion of Li-ions is of large practical importance for Li-ion battery performance. First order phase transitions result in an attractive flat electrode potential, however the phase boundaries formed are associated with poor rate performance and typically lead to poor cycle performance by mechanical failure. However, two of the most studied important electrode materials, olivine LiFePO4 1 and spinel Li4Ti5O12.2 , 3 both displaying a first order phase transition, appear exceptions showing excellent rate performance and a long cycle life. Interestingly, even in equilibrium the fundamental nature of the structural change during the first order phase transition of spinel Li4Ti5O12, is under debate, where it has been proposed to display either a complete solid solution, a phase-separation on the nano-scale or macroscopic phase separation.4-8 Despite the large number of first principle studies9-11 performed, mainly aiming to elucidate the diffusion pathways of Li, few efforts have been made to accurately describe the bulk insertion at finite temperatures from first principles. Here we report an exhaustive Li-vacancy configurational density functional theory (DFT) investigation of Li1+xTi5O12 (0 ≥ x ≥ 9) performed with different Li on 16d arrangements. The results indicate that the configurational interactions allow phase segregation between 8a and 16c occupation on a sub nanometer length scale induced by the partial replacement of Li on the 16d sites. This intimate mixing of the 8a and 16c occupancy structurally appears as a solid solution, however it should be considered as a first order phase transition with a sub nanometer domain size, the existence of which is due to the very small energy associated with the coherent interfaces. Based on our predictions of a sub-nano phase separating system we investigate kinetic properties of these phase boundaries by molecular dynamic simulations. We initialized molecular dynamic calculations in supercells that contain two phases (Li4Ti5O12 and Li7Ti5O12) directly in contact with each other. The results clearly display ultra-fast interface kinetics that rationalizes the excellent rate performance of this material. Calculated activation barriers are in good agreement with NMR relaxation measurement. The interface width between the coexisting phases is basically the distance separating the 16c and 8a atomic positions creating mixed 16c/8a occupation condition very similar to a solid solution description. In retrospect, the current work not only explains the apparent phase separation vs.solid solution contradiction in literature, it also explains the excellent rate performance of this intriguing material. (1) Padhi, A. K.; Nanjundaswamy, K. S.; Goodenough, J. B. Journal of the Electrochemical Society 1997, 144, 1188. (2) Deschanv, A.; Raveau, B.; Sekkal, Z. Mater. Res. Bull. 1971, 6, 699. (3) Colbow, K. M.; Dahn, J. R.; Haering, R. R. J. Power Sources 1989, 26, 397. (4) Wagemaker, M.; Simon, D. R.; Kelder, E. M.; Schoonman, J.; Ringpfeil, C.; Haake, U.; Lützenkirchen-Hecht, D.; Frahm, R.; Mulder, F. M. Adv. Mater. 2006, 18, 3169.(5) Wagemaker, M.; van Eck, E. R. H.; Kentgens, A. P. M.; Mulder, F. M. J. Phys. Chem. B 2009, 113, 224.(6) Lu, X.; Zhao, L.; He, X.; Xiao, R.; Gu, L.; Hu, Y.-S.; Li, H.; Wang, Z.; Duan, X.; Chen, L.; Maier, J.; Ikuhara, Y. Adv. Mater. 2012, 24, 3233.(7) Kitta, M.; Akita, T.; Tanaka, S.; Kohyama, M. J. Power Sources 2014, 257, 120.(8) Schmidt, W.; Bottke, P.; Sternad, M.; Gollob, P.; Hennige, V.; Wilkening, M. Chem. Mater. 2015, 27, 1740.(9) Ziebarth, B.; Klinsmann, M.; Eckl, T.; Elsässer, C. Phys. Rev. B 2014, 89, 174301. (10) Chen, Y.; Ouyang, C.; Song, L.; Sun, Z. Electrochim. Acta 2011, 56, 6084. (11) Bhattacharya, J.; van der Ven, A. Phys. Rev. B 2010, 81, 104304.

Preparation of a Magnetic Molecularly Imprinted Graphene Composite Highly Adsorbent for 4-Nitrophenol in Aqueous Medium

In this article, a type of magnetic molecularly imprinted graphene composite as a highly efficient adsorbent was prepared by forming a molecularly imprinted sol–gel polymer on the surface of magnetic graphene. The magnetic Fe3O4 nanoparticles were first deposited on a graphene sheet to prepare the magnetic graphene (MGR). Using the obtained magnetic graphene as a supporting matrix, 4-nitrophenol (4-NP) as template, phenyltriethoxysilane and tetramethoxysilane as functional monomers, a magnetic molecularly imprinted graphene composite (MGR@MIPs) was subsequently formed after the sol–gel polymerization and extraction of 4-NP. The preparation conditions (concentrations of monomer and template, and reaction time) were optimized. The as-prepared MGR@MIPs was characterized by FTIR, VSM, SEM, and TEM images. Under the optimized conditions, the obtained MGR@MIPs exhibited ultrafast adsorption kinetics (2 min to achieve the equilibrium state), large binding capacity (142 mg/g), and high selectivity toward 4-NP (th...