Shiman He

Chestnut-Like TiO2@α-Fe2O3 Core–Shell Nanostructures with Abundant Interfaces for Efficient and Ultralong Life Lithium-Ion Storage

Transition metal oxides caused much attention owing to the scientific interests and potential applications in energy storage systems. In this study, a free-standing three-dimensional (3D) chestnut-like TiO2@α-Fe2O3 core-shell nanostructure (TFN) is rationally synthesized and utilized as a carbon-free electrode for lithium-ion batteries (LIBs). Two new interfaces between anatase TiO2 and α-Fe2O3 are observed and supposed to provide synergistic effect. The TiO2 microsphere framework significantly improves the mechanical stability, while the α-Fe2O3 provides large capacity. The abundant boundary structures offer the possibility for interfacial lithium storage and electron transport. The as-prepared TFN delivers a high capacity of 820 mAh g-1 even after 1000 continuous cycles with a Coulombic efficiency of ca. 99% at a current of 500 mA g-1, which is better than the works reported previously. A thin gel-like SEI (solid electrolyte interphase) film and Fe0 phase yielded during charge/discharge cycling have been confirmed which makes it possible to alleviate the volumetric change and enhance the electronic conductivity. This confirmation is helpful for understanding the mechanism of lithium-ion storage in α-Fe2O3-based materials. The as-prepared free-standing TFN with excellent stability and high capacity can be an appropriate candidate for carbon-free anode material in LIBs.

Anatase TiO2 single crystal hollow nanoparticles: their facile synthesis and high-performance in dye-sensitized solar cells

In this paper, we successfully synthesized anatase TiO2 hierarchical microspheres (S0), anatase TiO2 sub-micro hollow mesospheres (S50), anatase TiO2 single crystal hollow nanoparticles (S100), nanoparticles (S250) and (S500) by using different amounts of hydrofluoric acid (HF) versus titanium n-tetrabutoxide (TBT) and acetic acid (AcOH). The structure and morphology of the as-prepared materials were confirmed by X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The DSSCs (dye-sensitized solar cells) based on anatase single crystal hollow TiO2 nanoparticles (S100) as a photoanode showed an efficient power conversion efficiency of 8.94% along with a current density of 17.39 mA cm−2 and an open circuit voltage of 778 mV, which is higher than the DSSCs based on S0 (8.10%), S50 (8.57%), S250 (7.25%) and S500 (6.12%). The high performance of S100 as a DSSC is attributed to their hollow structure which might help to harvest more light, higher light scattering and trapping abilities and comparatively higher surface area. Therefore, we can expect that our materials are promising for assembling superior photoelectrodes for future preparation of highly-efficient DSSCs and may lead to applications for energy storage, water splitting, catalysis, and gas sensing.

Hollow nanocubes constructed from oriented anatase TiO2 nanoarrays: topotactic conversion and fast lithium-ion storage

Mechanically stable titanium dioxide (TiO2) with the abilities of rapidly storing and releasing Li+ can be potentially applied in electric and hybrid electric vehicles, due to its ability to enhance the stability and safety, as well as the high current performance, of lithium ion batteries (LIBs). Herein, we rationally and facilely synthesized oriented anatase TiO2 nanoarrays (OATNs) from the NH4TiOF3 mesocrystal precursor through topotactic conversion and in situ epitaxial growth under moderate conditions. This study proves that the crystallization, porous structure, and orientation of OATNs are controllable, which affect the electronic and electrochemical properties and the Li+ diffusion coefficient. The optimal OATNs formed by hydrothermally treating NH4TiOF3 mesocrystals with an H3BO3 aqueous solution for 10 h (OATNs-10) delivered a high capacity of ca. 115 mA h g−1 at a current density of 50 C (170 mA g−1 of 1 C) even after continuous 2000 cycles with a Coulombic efficiency of ca. 100%. This indicates a high current rate performance and excellent stability. The unique properties of OATNs-10 make them a promising candidate for practical applications in LIBs.

Ultrathin nanobelts-assembled Chinese knot-like 3D TiO 2 for fast and stable lithium storage

Nanostructured TiO2 has applications in solar cells, photocatalysts, and fast-charging, safe lithium ion batteries (LIBs). To meet the demand of high-capacity and high-rate LIBs with TiO2-based anodes, it is important to fine-tune the nanoarchitecture using a well-controlled synthesis approach. Herein, we report a new approach that involves epitaxial growth combined with topotactic conversion to synthesize a unique type of 3D TiO2 nanoarchitecture that is assembled by well-oriented ultrathin nanobelts. The whole nanoarchitecture displays a 3D Chinese knot-like morphology; the core consists of robust perpendicular interwoven nanobelts and the shell is made of extended nanobelts. The nanobelts oriented in three perpendicular [001]A directions facilitate Li+ penetration and diffusion. Abundant anatase/TiO2-B interfaces provide a large amount of interfacial pseudocapacitance. A high and stable capacity of 130 mA·h·g−1 was obtained after 3,000 cycles at 10 A·g−1 (50 C), and the high-rate property of our material was greater than that of many recently reported high-rate TiO2 anodes. Our result provides, not only a novel synthesis strategy, but also a new type of 3D anatase TiO2 anode that may be useful in developing long-lasting and fast-charging batteries.

Hexagonal β-Na(Y,Yb)F4 based core/shell nanorods: epitaxial growth, enhanced and tailored up-conversion emission

To meet the increasing requirement, much effort has been devoted to enhance the emission intensity and tailor the emission color of rare earth phosphors. However, limited contributions have been made to the up-conversion (UC) of nanorods by complete epitaxial growth on each facet to achieve this requirement. In this study, we propose a facile epitaxial growth route to grow anisotropic hexagonal β-NaYF4:Yb3+/Ho3+@β-NaYbF4:Er3+, β-NaYF4:Yb3+/Ho3+@β-NaYF4, and β-NaYbF4:Er3+@β-NaYF4 core/shell nanorods, which were realized by adding hexagonal β-NaYF4:Yb3+/Ho3+ or β-NaYbF4:Er3+ nanorods as a core-nanostructure into a solution containing cubic α-NaYbF4:Er3+ or α-NaYF4 nanoparticles as the shell-precursor. During epitaxial growth-induced phase transformation, the precursor nanoparticles disappeared gradually in the solution and consequently corresponding β-phased shell yielded on each outer facet of each β-phased nanorod core. Eventually, the nanorod core was covered completely with a uniformly grown β-NaYbF4:Er3+ or β-NaYF4 shell. The UC emission of either β-NaYF4:Yb3+/Ho3+ or β-NaYbF4:Er3+ core can be enhanced by the outer shell due to the decrease in the number of surface defects. In addition, tailored UC emissions could be obtained by controlling the shell components and thickness, typically in the core/shell nanorods of β-NaYF4:Yb3+/Ho3+@β-NaYbF4:Er3+. The tunable colors with improved emission in these core/shell nanorods may find wider applications in multicolor labeling and anti-counterfeiting.