Song LI,Song YE,Xiao CHEN,Tianhua LIU,Zhuang GUO,Deping WANG
Yb 3+ and Er 3+ ions co-doped NaYF 4 nanocrystals were synthesized with different amounts of NaOH via oleic acid (OA)-assisted hydrothermal process. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), and photoluminescence spectra were used to characterize the products. The result indicated that the introduction of NaOH into the initial reaction solution effectively promoted the cubic (α-) to hexagonal (β-) phase transition of NaYF 4 :Yb 3+ ,Er 3+ nanocrystals, while excessive amount of NaOH favored the formation of α-NaYF 4 :Yb 3+ ,Er 3+ nanocrystals. Besides, with the increase of NaOH amount, the morphologies of β-NaYF 4 :Yb 3+ ,Er 3+ nanocrystals varied from irregular nanobranch to uniform nanorod. Further investigation revealed that the addition of NaOH could facilitate the deprotonation of OA, leading to the formation of oleate (OA − ), and meanwhile increased the concentration of OH − ions, inducing consequently the phase transition and morphology evolution of NaYF 4 :Yb 3+ ,Er 3+ nanocrystals. Moreover, the upconversion luminescence properties of NaYF 4 :Yb 3+ ,Er 3+ nanocrystals were systematically investigated. It was found that the upconversion emissions not only depended on the phase and morphology but also were influenced by the surface groups.
Yuying Zhao,Fengming Zhou,Qiuyue Liang,Bo Zhang,Peng Wang,Qingliang Lv,Caixia Li,Zhenyu Xiao,Lei Wang
Although the intrinsic layer structure of layered double hydroxide (LDH) is beneficial for more accessible active centers and faster electrolyte ions diffusion rate than closed-packed atom arrangement for achieving higher capacitance, the narrow interlaminar spacing results in inevitable “death volume” during the rapid charging-discharging process. Herein, an electrochemical assistant MOF etching strategy is developed to construct hierarchical porous LDH with pre-embed OH − ions and filled H 2 O molecule in inter-layer space (E-MOF-0.5 V) for unsealing theoretical capacity. According to the valence equilibrium theory, the increased valence of the metal center during electrochemical polarization can accelerate the filling of OH − ions and H 2 O molecules in the LDH inter-layer space. The involved OH − and water molecules can achieve rapid mass transfer processes through ‘‘Newton’s Cradle’’ and the rich pore and ultra-thin layer structure may make sure the active centers are almost completely accessible. Therefore, the optimized E-MOF-0.5 V delivers an outstanding specific capacity of 8547.2 mF cm −2 (2670.9 F g −1 ) at 4 mA cm −2 and maintains an excellent cycling performance of 88 % after 7200 cycles at 20 mA cm −2 . Besides, an E-MOF-0.5 V//rGO-Zn hybrid supercapacitor device is assembled, which presents an energy density of 7.97 mWh cm −2 and a power density of 6.63 mW cm −2 .
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