Beibei He

Room temperature multiferroic behavior of Cr-doped ZnO films

Single-phase 9u2002at.u2009% Cr-doped ZnO film has been prepared on Pt(111)/Ti/SiO2/Si(100) substrates by reactive sputtering method. The film is found to present ferroelectric and ferromagnetic behaviors simultaneously at room temperature, and it undergoes transitions to paraelectric and paramagnetic phases at ∼368–373 and ∼495u2002K, respectively. It is considered that the local electric dipoles induced by the distortions of CrO4 tetrahedra should be responsible for the ferroelectricity. On the other hand, the ferromagnetic ordering could be explained by the interaction of the localized spins with statically occupied polaron states. The multiferroic behavior adds a dimension to the multifunction of ZnO.

Strain-induced ferromagnetism enhancement in Co:ZnO films

The structural and ferromagnetic properties of Zn0.95Co0.05O films grown on Si and LiNbO3 (LNO) substrates have been studied as a function of thickness (15–900 nm). The structural characterizations indicate that the c-axis lattice constant and Co–O bond length slightly decrease with the increase in film thickness, implying the progressive relaxation of the tensile strain. The magnetic measurements show that a larger strain can result in an enhancement of room temperature ferromagnetism. The thinnest films (15 nm) with the largest lattice strains possess the highest saturated magnetic moments, i.e., 5.52 and 2.96μB/Co in Co:ZnO/LNO and Co:ZnO/Si films, respectively. As the film becomes thicker, the saturated ferromagnetism rapidly decreases, which is about two orders of magnitude smaller than that of the 15-nm-thick film when its thickness is 900 nm. The enhancement of ferromagnetism in Co:ZnO films originates from the combination of enlarged Co–O bond length and increased defect amount induced by strain.

Tailoring of surface modified ultrathin membranes with CO2 tolerance and high oxygen permeability

We report our findings on Ce0.9Gd0.1O2−δ (GDC) modified (Pr0.9La0.1)2(Ni0.74Cu0.21Nb0.05)O4+δ (PLNCN) ultrathin membranes as highly efficient and CO2-stable oxygen transporting membranes (OTMs). Until now, such membranes have offered the highest oxygen permeability among those CO2-stable membranes reported in the literature.

Mo2C-induced solid-phase synthesis of ultrathin MoS2 nanosheet arrays on bagasse-derived porous carbon frameworks for high-energy hybrid sodium-ion capacitors

Conventional supercapacitors often suffer from low energy density. Hybrid Na-ion capacitors (NICs) are emerging as an important energy-storage device with high-energy and high-power output. They possess complementary merits of a high-capacity battery-type anode and a high-rate capacitive cathode. However, the existing anodes (e.g., carbon, TiO2, and Na2Ti2O5) are often limited by sluggish kinetics and low capacities of Na-ion storage. Here we report the fabrication of ultrathin MoS2 nanosheet arrays vertically anchored on bagasse-derived three-dimensional (3D) porous carbon frameworks (MoS2@BPC) as NIC anodes through a facile two-step solid-phase reaction strategy (BPC → Mo2C@BPC → MoS2@BPC). During the solid-phase synthesis process, the formation of Mo2C intermediates is the key to a successful growth of powder-type MoS2 nanosheet arrays on BPC. Meanwhile, the bagasse-derived porous cross-linked carbon structure can act as a 3D scaffold to effectively increase the conductivity, sodium-ion diffusion and structural stability of MoS2@BPC during charge–discharge processes. As a consequence, the MoS2@BPC electrode delivers a high specific capacity for Na-ion storage (490 mA h g−1 at 0.1 A g−1) with superior high-rate capability and cyclability (cycled over 5000 cycles at 2 A g−1). Coupled with BPC as the cathode, the hybrid electrode made of MoS2@BPC enables the NIC to deliver both high energy density (112.2 W h kg−1 at 55 W kg−1) and power density (8333 W kg−1 at 53.2 W h kg−1) as well as long cycling stability, which may bridge the supercapacitor–battery divide.

Correlation between donor defects and ferromagnetism in insulating Sn1−xCoxO2 films

Sn1−xCoxO2 films have been fabricated to study the local structure of Co dopant and the mediation effects of donor defects (oxygen vacancies and Sn interstitials) on magnetic properties. Compared to as-grown film, the ferromagnetism is evidently enhanced after annealing in vacuum at 400u2009°C due to the increase in oxygen vacancies. While annealing at higher temperature, the ferromagnetism declines because of the domination of decrease in Sn interstitials over increase in oxygen vacancies in the films. The incorporation of Co dopant as well as the presence of oxygen vacancies and Sn interstitials is verified using x-ray absorption fine structure spectroscopy. The variations in the concentration of defects as a function of annealing temperature are obtained by positron annihilation spectroscopy technique. Additionally, the changes in structure and ferromagnetism after annealing in different atmospheres further demonstrate the crucial roles of oxygen vacancies and Sn interstitials in tuning ferromagnetism.

Nickel-Based Bicarbonates as Bifunctional Catalysts for Oxygen Evolution and Reduction Reaction in Alkaline Media

Oxygen electrocatalysis, including the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), is one of the most important electrochemical processes for sustainable energy conversion and storage technologies. Herein, nickel-based bicarbonates are, for the first time, developed as catalysts for oxygen electrocatalysis, and demonstrate superior electrocatalytic performance in alkaline media. Iron doping can significantly tune the real valence of nickel ions, and consequently tailor the electrocatalytic ability of bicarbonates. Among the nickel-based bicarbonates, Ni0.9 Fe0.1 (HCO3 )2 exhibits the highest bifunctional catalytic activity, with a potential difference of 0.86u2005V between the OER potential at a current density of 10u2005mAu2009cm-2 and the ORR potential at a current density of -1u2005mAu2009cm-2 , which outperforms most of the reported precious-metal-free catalysts. The present work provides new insights into exploring efficient catalysts for oxygen electrocatalysis, and it suggests that, in addition to the extensively studied transition metal hydroxides and oxides, bicarbonates and carbonates also show great potential as precious metal-free catalysts.

Effects of Buffer Layer on Structure and Ferromagnetism of Co-Doped SnO

The thickness of SnO₂ buffer layers has been found to considerably affect the structure and magnetic properties of Sn_1-xCo_xO₂ films. As the buffer-layer thickness increases, the lattice constant <i>a</i> and Co-O bond length of the Sn_1-xCo_xO₂ films contract, which suggest the gradual relaxation of the accumulated tensile strain. Simultaneously, the amount of structural defects in Sn_1-xCo_xO₂ films decreases significantly and the grain size increases with increasing buffer-layer thickness. All the Sn_1-xCo_xO₂ films exhibit intrinsic ferromagnetism at room temperature and the saturated magnetic moment varies with the buffer-layer thickness. The dependence of ferromagnetism on the buffer-layer thickness can be explained on the basis of the variations in the density of structural defects and Co-O bond length in Sn_1-xCo_xO₂ films.