Y.C. Wu

Enhanced electrochemical performance and thermal stability of a CePO4-coated Li1.2Ni0.13Co0.13Mn0.54O2 cathode material for lithium-ion batteries

A layered Li1.2Ni0.13Co0.13Mn0.54O2 cathode is coated with a CePO4 layer via a simple precipitation method. The pristine and CePO4-coated Li1.2Ni0.13Co0.13Mn0.54O2 are characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), high resolution transmission electron microscope (HR-TEM) and X-ray photoelectron spectroscopy (XPS), and the results indicate that CePO4 has been uniformly coated on the Li1.2Ni0.13Co0.13Mn0.54O2. Charge–discharge tests show that the CePO4-coated Li1.2Ni0.13Co0.13Mn0.54O2 has an obviously enhanced electrochemical performance compared with the pristine sample: the initial coulombic efficiency from 88.26% to 92.19%, rate capability from 6 to 110 mA h g−1 at 10 C, high-temperature performance from 59.5 to 219.6 mA h g−1 at 55 °C after 20 cycles, and low-temperature performance from 128.3 to 246.7 mA h g−1 at −20 °C. According to the analysis from dc impedance and electrochemical impedance spectra, the improvements on the electrochemical performance are mainly because the coated CePO4 layer can reduce side reactions of Li1.2Ni0.13Co0.13Mn0.54O2 with the electrolyte, and thus form the cathode–electrolyte interface (CEI) layer with enhanced Li+ diffusion. In addition, the CePO4 layer significantly improves the thermal stability of the coexisting systems of the charged cathode with the electrolyte. Therefore, CePO4 coating will be a promising approach to improve the electrochemical performance and thermal stability of Li-rich layered oxide cathode materials.

Supercapacitive performance of hydrogenated TiO2 nanotube arrays decorated with nickel oxide nanoparticles

Highly ordered self-organized TiO2 nanotube arrays (TNTAs) could not only be used as current collectors, but also adopted as highly ion-accessible and charge transfer-channels for the construction of supercapacitors. In this paper, hydrogenated TNTAs were obtained and then nickel oxide (NiOx) nanoparticles were successfully deposited onto the inner surface and interface of HTNTAs through a cyclic voltammetry electrochemical deposition process (NiOx/HTNTAs). The FESEM images of the samples showed that the diameter of the NiOx nanoparticles ranged from 7 to 60 nm. The as-fabricated NiOx/HTNTAs exhibited an obviously pseudocapacitive performance with a specific capacitance of 689.28 F g−1 at a current density of 1.5 A g−1 and 91.9% of the initial capacitance remaining after 5000 charge/discharge cycles at a current density of 3 A g−1 in 1 M KOH. This work reveals a feasible and green method for the fabrication of TNTAs modified with electroactive metal oxide nanoparticles as functional electrode materials for supercapacitors.

Microstructure and magnetism of Co-doped PbPdO2 films with different grain sizes

Single-phase Co-doped PbPdO2 films with grain sizes ranging from 30 nm to 120 nm were synthesized using a sol–gel spin-coating method. The doped Co ions exhibit a mixed state of 2+ and 3+, which is caused by the existence of Pb vacancies and the low electronegativity of Co. Ferromagnetism, paramagnetism, antiferromagnetism and a charge ordering state were observed in all of the Co-doped PbPdO2 films. Additionally, the charge ordering signal determined by the surface state of the nanograins is gradually weakened with decreasing grain size. The carrier-mediated mechanism bridged to the bound magnetic polaron model was used to explain the coexistence of the ferromagnetism, paramagnetism and antiferromagnetism. For the ferromagnetism, the tendency of the saturation magnetization to increase with temperature was found in all the Co-doped PbPdO2 films with a nanograin structure. This abnormal magnetic behavior is believed to be one of the peculiar features of spin gapless semiconductors.

Dependence of magnetism on the doping level of Zn1−xMnxTe nanoparticles synthesized by a hydrothermal method

Mn-doped ZnTe nanoparticles with various degrees of Mn-doping from 3 at.% to 18 at.% were synthesized by a hydrothermal method. A single-phase zinc-blende-structured sample was obtained only at the Mn-doping level of 3 at.%, while the impurity phase of MnO was present in the samples with higher degrees of Mn-doping. In all of the samples, the Mn valence states were lower than 2+. All samples exhibited both ferromagnetism and paramagnetism. The ferromagnetism is mainly attributable to the overlapping of the bound magnetic polarons and the paramagnetism is mainly caused by the isolated polarons and Mn ions. Samples that were doped with 3 at.% and 6 at.% Mn also exhibited superparamagnetism and antiferromagnetism. The superparamagnetism may have arisen from the presence of some tiny ferromagnetic nanoparticles. The antiferromagnetism is believed to have arisen mainly from the carrier-mediated interaction among Mn ions.

Anti-site defect effect on the electronic structure of a Bi2Te3 topological insulator

Tuning the Fermi level (EF) in Bi2Te3 topological-insulator (TI) films is demonstrated on controlling the temperature of growth with molecular-beam epitaxy (MBE). Angle-resolved photoemission spectra (ARPES) reveal that EF of Bi2Te3 thin films shifts systematically with the growth temperature (Tg). The key role that a Bi-on-Te(1) (BiTe1) antisite defect plays in the electronic structure is identified through extended X-ray-absorption fine-structure (EXAFS) spectra at the Bi L3-edge. Calculations of electronic structure support the results of fitting the EXAFS, indicating that the variation of EF is due to the formation and suppression of BiTe1 that is tunable with the growth temperature. Our findings provide not only insight into the correlation between the defect structure and electronic properties but also a simple route to control the intrinsic topological surface states, which could be useful for applications in TI-based advanced electronic and spintronic devices.

Ion valence state and magnetic origin of PbPd 1−x Ni x O 2 nanograin films with a high-temperature ferromagnetism

The PbPd1u2009−u2009xNixO2 nanograin films with different Ni-doping levels (xu2009=u20090.037–0.150) were synthesized by the sol–gel spin-coating method and an oxidation treatment. The films with a thickness of about 210xa0nm were found to be single phase with a body-centered orthorhombic structure and to possess a large number of Pb vacancies. High-temperature ferromagnetism discovered in these films can be maintained above 380xa0K. Paramagnetism was also found in the films with high Ni-doping levels. The analysis on the XANES spectra and their first derivatives offered the evidences for the facts that the film’s ferromagnetism is intrinsic and the increase in the Pb valence from 2+ towards 4+, caused by the appearance of large amount of Pb vacancies and low electronegativity of Pb2+ ion, provides magnetic moments to the film’s magnetism. A carrier-mediated mechanism bridged to the bound magnetic polaron model based on the Pb vacancies, the doped Ni ions and the Pb ions with a valence higher than 2+ was adopted to interpret the origin of the magnetism within these PbPd1u2009−u2009xNixO2 nanograin films. The variation of the specific area of the grain boundary was believed to have a great influence on the proportions of the film’s ferromagnetism and paramagnetism.

High-temperature ferromagnetism of Cu-doped PbPdO2 nanograin films

The single-phase, body-centered orthorhombic-structured, Cu-doped PbPdO2 films with a nanograin structure were synthesized by using the sol–gel spin-coating method and an oxidation treatment. It was found that large amounts of Pb vacancies exist in the nanograin film and Pb ions possess a mixed valence state of 2+ and 4+. Besides the superparamagnetism and the charge ordering state, the magnetic studies displayed that ferromagnetism, which can persist up to 380u2009K, exists in the Cu-doped PbPdO2 nanograin film. As a peculiar characteristic of the spin gapless semiconductor, the ferromagnetic saturation magnetization increasing unusually with temperatures above 300u2009K was also discovered. The bound magnetic polarons based on the Pb vacancies, the doped Cu2+ ions, and the Pb ions with a valence higher than 2+ were believed to be the origin of such high-temperature ferromagnetism.Graphical abstract