Jing Ju

Microporous Aluminoborates with Large Channels: Structural and Catalytic Properties

Channel zapping: PKU-1 and newly synthesized PKU-2 (Al(2)B(5)O(9)(OH)(3)⋅n H(2)O; see picture) possess microporous structures with 18-ring and 24-ring channels, respectively. They show high reactivity and size selectivity in the cyanosilylation of aldehydes as heterogeneous Lewis acid catalysts. The different channel sizes determine the substrate selectivity. These examples demonstrate the potential of octahedron-based aluminoborate channels in catalysis.

In situ grown graphene-encapsulated germanium nanowires for superior lithium-ion storage properties

Alloying anode materials (Si, Ge, Sn etc.) in lithium-ion batteries usually suffer from a remarkable loss of capacity during the charge–discharge cycling. Herein, homogeneous in situ-grown graphene-encapsulated Ge nanowires are successfully achieved by a simple, completely catalyst-free route via arc-discharge. The Ge@G composite is composed of a graphene sheath and a metallic Ge nanowire core. This unique composite of graphene-encapsulated Ge nanowires is an ideal anode material for lithium ion storage. It can exhibit excellent electrochemical performance with a reversible specific capacity of 1400 mA h g−1 after 50 cycles at a current density of 1600 mA g−1 (the theoretical specific capacity of Ge is 1624 mA h g−1). These encouraging results demonstrate that arc-discharge synthesis provides efficient graphene encapsulation of Ge nanowires, and graphene encapsulation is a feasible solution to protect electrode materials for lithium-ion storage.

Multicolour and up-conversion fluorescence of lanthanide doped Vernier phase yttrium oxyfluoride nanocrystals

Multicolour and up-conversion fluorescent nanocrystals of lanthanide doped orthorhombic yttrium oxyfluorides were prepared via a facile precursor route for the first time. The stepwise fabrication procedure involves room-temperature coprecipitation of Y3+ ions with CO32− and F− in water and subsequent thermal decomposition. Powder X-ray diffraction studies confirmed that all of the undoped and lanthanide ion-doped samples annealed at 400 °C were pure Vernier phase adopting an orthorhombic structure. The crystal structure, synthetic details, morphologies, and luminescence properties of the nanocrystals were investigated by means of TEM, thermal and spectroscopic measurements. In the structure, Y atoms occupy four crystalline sites with different coordination by O2−/F−, giving rise to complex environments for the doping of rare earth ions. Green emission bands were observed from the Tb3+ doped samples under UV excitation (λex = 284, 377 nm), while the Eu3+ doped nanocrystals exhibited bright red emission upon both UV and blue irradiation (λex = 381, 465 nm). Those nanocrystals codoped with Yb3+/Er3+, Yb3+/Ho3+ and Yb3+/Tm3+ show intense red, green and blue up-conversion fluorescence respectively upon irradiation at 980 nm with a diode laser. The Vernier phase yttrium oxyfluoride phosphors have the potential for use in applications such as bioprobes, optical nanodevices, and chromatic displays.

Rare earth induced formation of δ-BiB3O6 at ambient pressure with strong second harmonic generation

Pure δ-BiB3O6, which is the polymorph of the most promising nonlinear optical (NLO) crystal α-BiB3O6 (BIBO), was first synthesized at high pressure/high temperature. δ-BiB3O6 possesses a non-centrosymmetric structure and moreover, its three-dimensional (3D) structure rather than 2D in BIBO suggests a high possibility to grow optical single-crystals. As an important candidate for NLO materials, we performed theoretical calculations of δ-BiB3O6 based on first principles. Surprisingly, it gives a significant higher second-harmonic generation (SHG) coefficient than previous experimental results (comparable to KDP), which greatly stimulates us to find a convenient synthesis method of the δ-phase at ambient pressure. In the present work, we report that rare-earth doping would stabilize the δ-phase at ambient pressure via a sol–gel method and the doping would reach quite a high level, δ-Bi1−xRExB3O6 (RE = La, Ce, Pr, Nd, 0 < x ≤ 0.15). The large miscibility is explained based on the structural similarity between δ-BiB3O6 and γ-REB3O6. The observed powder SHG signals are 4–6 times of KDP, consistent with our theoretical calculations. Furthermore the high level of Nd3+-doping might extend its applications to SFD or SSFM laser materials.

PKU-3: An HCl-Inclusive Aluminoborate for Strecker Reaction Solved by Combining RED and PXRD

A novel microporous aluminoborate, denoted as PKU-3, was prepared by the boric acid flux method. The structure of PKU-3 was determined by combining the rotation electron diffraction and synchrotron powder X-ray diffraction data with well resolved ordered Cl(-) ions in the channel. Composition and crystal structure analysis showed that there are both proton and chlorine ions in the channels. Part of these protons and chlorine ions can be washed away by basic solutions to activate the open pores. The washed PKU-3 can be used as an efficient catalyst in the Strecker reaction with yields higher than 90%.

Robust and stable intercalated graphene encapsulation of tin nanorods for enhanced cycle and capacity performance for lithium storage

Anode materials (Si, Ge, Sn, etc.) in lithium-ion batteries suffer from a remarkable loss of capacity during the charge–discharge cycle, and various carbon-based additions have been widely added to address this issue. In this work, we report on a simple, one-step approach for an intercalated graphene encapsulation of tin nanorods fabricated via the arc-discharge method. Our method is fast, robust, straight-forward and completely catalyst-free. The result is a Sn@G nanocomposite comprising of a single-crystalline Sn nanorod core tightly bonded to a highly-crystalline graphene shell. The as-synthesized Sn@G exhibits superior cycling and rate performance compared to Sn@C nanocomposite: a reversible specific capacity of 846 mA h g−1 after 100 cycles at the current density of 200 mA g−1 and a reversible rate capacity of 488 mA h g−1 at 2 C for Sn@G, in contrast to the 130 mA h g−1 and 48 mA h g−1, respectively for Sn@C. Further, the Sn@G possesses excellent thermal and chemical stabilities against a 950 °C N2 annealing and a 12 h etching in hydrochloric acid, respectively, and has a high mechanical strength as evident by the intercalated core–shell structure maintaining its shape intact in resisting the volume expansion during the 950 °C annealing. These encouraging results indicate that an in situ graphene encapsulated tin rods nanocomposite constitutes a highly feasible candidate for use as an anode material.

A structural study of the hole-doped superconductors Pr1−xSrxFeAsO

The structural details of the Pr1−xSrxFeAsO (1111) superconducting system are analyzed using data obtained from synchrotron x-ray diffraction and the structural parameters are carefully studied as the system moves from non-superconducting to hole-doped superconducting with an increase of the Sr concentration. Superconductivity emerges when the Sr doping amount reaches 0.221. The linear increase of the lattice constants proves that Sr is successfully introduced into the system, and the Sr concentration can be accurately determined by electron density analyses. The evolution of structural parameters with Sr concentration in Pr1−xSrxFeAsO and comparison of them to other similar structural parameters of the related Fe-based superconductors suggest that the interlayer space between the conducting As–Fe–As layer and the insulating Pr–O–Pr layer is important for improving Tc in hole-doped (1111) superconductors, which seems to be a different trend from that encountered in the electron-doped systems.

Systematic Study of Cr3+ Substitution into Octahedra-Based Microporous Aluminoborates

Single crystals of pure aluminoborate PKU-1 (Al3B6O11(OH)5·nH2O) were obtained, and the structure was redetermined by X-ray diffraction. There are three independent Al atoms in the R3 structure model, and Al3 locates in a quite distorted octahedral environment, which was evidenced by (27)Al NMR results. This distortion of Al3O6 octahedra release the strong static stress of the main framework and leads to a symmetry lowering from the previously reported R3 to the presently reported R3. We applied a pretreatment to prepare Al(3+)/Cr(3+) aqueous solutions; as a consecquence, a very high Cr(3+)-to-Al(3+) substitution content (∼50 atom %) in PKU-1 can be achieved, which is far more than enough for catalytic purposes. Additionally, the preference for Cr(3+) substitution at the Al1 and Al2 sites was observed in the Rietveld refinements of the powder X-ray data of PKU-1:0.32Cr(3+). We also systematically investigated the thermal behaviors of PKU-1:xCr(3+) (0 ≤ x ≤ 0.50) by thermogravimetric-differential scanning calorimetry, in situ high-temperature XRD in vacuum, and postannealing experiments in furnace. The main framework of Cr(3+)-substituted PKU-1 could be partially retained at 1100 °C in vacuum. When 0.04 ≤ x ≤ 0.20, PKU-1:xCr(3+) transferred to the PKU-5:xCr(3+) (Al4B6O15:xCr(3+)) structure at ∼750 °C by a 5 h annealing in air. Further elevating the temperature led to a decomposition into the mullite phase, Al4B2O9:xCr(3+). For x > 0.20 in PKU-1:xCr(3+), the heat treatment led to a composite of Cr(3+)-substituted PKU-5 and Cr2O3, so the doping upper limit of Cr(3+) in PKU-5 structure is around 20 atom %.

Superconductivity induced by doping platinum in BaFe2As2

By substituting Fe with the 5d-transition metal Pt in BaFe2As2, we have successfully synthesized the superconductors BaFe2-xPtxAs2. The systematic evolution of the lattice constants indicates that the Fe ions were successfully replaced by Pt ions. By increasing the doping content of Pt, the antiferromagnetic order and structural transition of the parent phase is suppressed and superconductivity emerges at a doping level of about x=0.02. At a doping level of x=0.1, we get a maximum transition temperature T-c of about 25 K. While even for this optimally doped sample, the residual resistivity ratio is only about 1.35, indicating a strong impurity scattering effect. We thus argue that the doping to the Fe sites naturally leads to a high level impurity scattering, although the superconductivity can still survive at about 25 K. The synchrotron powder x-ray diffraction shows that the resistivity anomaly is in good agreement with the structural transition. The superconducting transitions at different magnetic fields were also measured at the doping level of about x=0.1, yielding a slope of -dH(c2)/dT=5.4 T/K near T-c. Finally a phase diagram was established for the Pt-doped 122 system. Our results suggest that superconductivity can also be easily induced in the FeAs family by substituting the Fe with Pt with almost the similar maximum transition temperatures as doping Ni, Co, Rh, and Ir.

Soft x-ray photoelectron spectroscopy study of type-I clathrates

Abstract Extensive soft x-ray photoelectron spectroscopy studies are performed on Ba8 Ga16 Ge30 (BGG) and Sr8Ga16Ge30 (SGG) single crystals ranging from Fermi to core levels, at a high-energy facility. Valence band x-ray photoelectron spectroscopy (XPS) experiments with theoretical calculations revealed that the valence band is mainly constructed by the Ge/Ga 4s and 4p wave functions with little contribution of the Ba/Sr atomic orbitals. Surprisingly, unexpected features evidencing the different shift for the 2a- and 6d- sites between Ba 4d and Sr 3d are observed. The detailed analyses including theoretical support by first-principles band-structure calculations lead to the conclusion that the component distributions of the larger tetrakaidecahedral cage are different depending on the endohedral atoms, which contrasts with the past consensus that BGG and SGG have the same framework structure. This may give thorough reconsiderations on earlier interpretations of experimental data.