Single-crystal Nanoflakes Research Articles

Highly Sensitive Long‐Wave Infrared Photodetector Based on Two‐Dimensional Hematite α‐Fe2O3

AbstractAmbient temperature long‐wavelength infrared (LWIR) photodetection has many critical applications ranging from thermal imaging and optical communications to night vision cameras. Currently, high performance, low cost, air stability, and ultra‐broadband response still constitute challenges. Here, the study demonstrates a high‐performance uncooled LWIR photodetector using 2D hexagonal α‐Fe2O3 single crystal nanoflakes, which are synthesized via chemical vapor deposition. The 2D α‐Fe2O3 photodetector realizes high specific detectivity (D*) of 3.28×109 cmHz1/2 W−1 and very low noise equivalent power of 8.94×10−14 W Hz−1/2 with 10.6 µm laser. The photo bolometer effect is responsible for the photoresponse in the infrared range with a large temperature coefficient resistance of −241.2% K−1. This study shows a new type of potential broadband response material and provides a promising LWIR optoelectrical device with good air stability.

F-doped orthorhombic Nb2O5 exposed with 97% (100) facet for fast reversible Li+-Intercalation

Orthorhombic Nb2O5 (T-Nb2O5) is attractive for fast-charging Li-ion batteries, but it is still hard to realize rapid charge transfer kinetics for Li-ion storage. Herein, F-doped T-Nb2O5 microflowers (F-Nb2O5) are rationally synthesized through topotactic conversion. Specifically, F-Nb2O5 are assembled by single-crystal nanoflakes with nearly 97% exposed (100) facet, which maximizes the exposure of the feasible Li+ transport pathways along loosely packed 4g atomic layers to the electrolytes, thus effectively enhancing the Li+-intercalation performance. Besides, the band gap of F-Nb2O5 is reduced to 2.87 eV due to the doping of F atoms, leading to enhanced electrical conductivity. The synergetic effects between tailored exposed crystal facets, F-doping, and ultrathin building blocks, speed up the Li+/electron transfer kinetics and improve the pseudocapacitive properties of F-Nb2O5. Therefore, F-Nb2O5 exhibit superior rate capability (210.8 and 164.9 mAh g−1 at 1 and 10 C, respectively) and good long-term 10 C cycling performance (132.7 mAh g−1 after 1500 cycles).

Low-crystalline Ni/Co-oxyhydroxides nanoarrays on carbon cloth with high mass loading and hierarchical structure as cathode for supercapacitors

Compared with the crystalline transition metal oxyhydroxide, low-crystalline structure is conducive to high electrochemical performance due to its more grain boundaries and ion diffusion channels. However, the manufacture of low-crystalline oxyhydroxides remains challenging, especially those with multiple transition metals. Herein, high mass loading (~9 mg cm−2) and low-crystalline Ni/Co-oxyhydroxides nanoarrays on carbon cloth (CC) were prepared by electrochemical reconstruction. The low-crystalline Ni/Co-oxyhydroxides nanoarrays possessed a hierarchical structure, which were consist of flake-like (Co0.25, Ni0.75)O(OH) single-crystals and needle-like Ni-doped CoO(OH) polycrystalline. Both single crystal nanoflakes and polycrystalline nanoneedles contained a lot of defects. Owing to the unique structure, the mutual doping of Ni and Co atoms, and the abundant defects, the nanoarrays of Ni/Co-3500 showed excellent electrochemical performance with a high specific capacitance of 8.87 F cm−2 at 5 mA cm−2 and a high rate retention of 69.33% at 50 mA cm−2, as well as a high cycle stability remaining 91.6% of initial capacitance after 2000 galvanostatic charge–discharge cycles at a current density of 20 mA cm−2. Furthermore, the assembled asymmetric supercapacitor of Ni/Co-3500//oxidized CC delivered a high areal capacitance of 1.48 F cm−2 and a volumetric capacitance of 14.8 F cm−3 at 8 mA cm−2, as well as 1.11 F cm−2 (11.1 F cm−3) even at 50 mA cm−2. The energy density of Ni/Co-3500//oxidized CC device was up to 4.35 mWh cm−3 at the powder density of 60.1 mW cm−3. It indicates that the as-prepared low-crystalline Ni/Co-oxyhydroxides arrays on CC is a promising electrode for high energy density supercapacitors.

Low-temperature conversion synthesis of M(OH) 2 (M=Ni, Co, Fe) nanoflakes and nanorods

Single crystal nanoflakes and nanorods of Ni(OH) 2, Co(OH) 2 and Fe 3O 4 were prepared by a controlled hydrothermal conversion route from MC 2O 4·2H 2O in the NaOH solution. The structure and morphology of the final products and their oxalate precursors were characterized by X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). The formation mechanism of conversion was suggested in this paper, accompanied the morphology control hypothesis.