Yawen Zhan

Hierarchical assembly of Ti(IV)/Sn(II) co-doped SnO2 nanosheets along sacrificial titanate nanowires: synthesis, characterization and electrochemical properties

Hierarchical assembly of Ti(IV)/Sn(II)-doped SnO₂ nanosheets along titanate nanowires serving as both sacrificial templates and a Ti(IV) source is demonstrated, using SnCl2 as a tin precursor and Sn(II) dopants and NaF as the morphology controlling agent. Excess fluoride inhibits the hydrolysis of SnCl2, promoting heterogeneous nucleation of Sn(II)-doped SnO₂ on the titanate nanowires due to the insufficient oxidization of Sn(II) to Sn(IV). Simultaneously, titanate nanowires are dissolved forming Ti(4+) species under the etching effect of in situ generated HF resulting in spontaneous Ti(4+) ion doping of SnO₂ nanosheets formed under hydrothermal conditions. Compositional analysis indicates that Ti(4+) ions are incorporated by substitution of Sn sites at a high level (16-18 at.%), with uniform distribution and no phase separation. Mössbauer spectroscopy quantified the relative content of Sn(II) and Sn(IV) in both Sn(II)-doped and Ti(IV)/Sn(II) co-doped SnO₂ samples. Electrochemical properties were investigated as an anode material in lithium ion batteries, demonstrating that Ti-doped SnO₂ nanosheets show improved cycle performance, which is attributed to the alleviation of inherent volume expansion of the SnO₂-based anode materials by substituting part of Sn sites with Ti dopants.

Facile fabrication of N/S-doped carbon nanotubes with Fe3O4 nanocrystals enchased for lasting synergy as efficient oxygen reduction catalysts

Transition metal-doped carbon materials are regarded as a promising replacement of commercial Pt/C catalysts for the oxygen reduction reaction (ORR) in polymer-electrolyte-membrane fuel cells and metal–air batteries. The current fabrication methods are generally very complex and involve the introduction of foreign species onto the surface or into the voids of carbon nanostructures; this leads to loose attachment and severe aggregation over long term usage, weakening the synergetic effects between the host and guest species. Herein, we report a facile and scalable method to fabricate Fe, N, and S co-doped carbon nanotubes (Fe-NSCNT). Specifically, iron species were precipitated in situ and further converted to Fe3O4 nanoparticles enchased in the wall structures of N/S-doped CNTs (NSCNTs), resulting in a greatly reinforced synergistic effect. The Fe-NSCNT catalysts thus obtained showed excellent ORR performance, with a four-electron selectivity, high methanol tolerance, enhanced stability (no significant loss after 6 h, cf. 19% loss for 20% Pt/C), and high diffusion-limited current density (6.01 mA cm−2, higher than 5.79 mA cm−2 of the commercial Pt/C), comparable to that of the state-of-the-art Pt/C catalyst in alkaline media. Furthermore, when used as Zn–air battery cathode materials, the Fe-NSCNT catalyst enabled the same voltage (1.17 V at 20 mA cm−2) and specific capacity comparable (∼720 mA h gZn−1 at 10 mA cm−2) to that of the commercial Pt/C (∼735 mA h gZn−1 at 10 mA cm−2), indicating its great potential in replacing Pt/C for the practical applications in noble metal-free Zn–air batteries.

Wide angle and narrow-band asymmetric absorption in visible and near-infrared regime through lossy Bragg stacks.

Absorber is an important component in various optical devices. Here we report a novel type of asymmetric absorber in the visible and near-infrared spectrum which is based on lossy Bragg stacks. The lossy Bragg stacks can achieve near-perfect absorption at one side and high reflection at the other within the narrow bands (several nm) of resonance wavelengths, whereas display almost identical absorption/reflection responses for the rest of the spectrum. Meanwhile, this interesting wavelength-selective asymmetric absorption behavior persists for wide angles, does not depend on polarization, and can be ascribed to the lossy characteristics of the Bragg stacks. Moreover, interesting Fano resonance with easily tailorable peak profiles can be realized using the lossy Bragg stacks.

Bestow metal foams with nanostructured surfaces via a convenient electrochemical method for improved device performance

Metal foams have been intensively studied as three-dimensional (3-D) bulk mass-support for various applications because of their high conductivities and attractive mechanical properties. However, the relatively low surface area of conventional metal foams largely limits their performance in applications such as charge storage. Here, we present a convenient electrochemical method for addressing this problem using Cu foams as an example. High surface area Cu foams are fabricated in a one-pot one-step manner by repetitive electrodeposition and dealloying treatments. The obtained Cu foams exhibit greatly improved performance for different applications like surface enhanced Raman spectroscopy (SERS) substrates and 3-D bulk supercapacitor electrodes.