Porous silicon-based anode materials have gained much interest because the porous structure can effectively accommodate volume changes and release mechanical stress, leading to improved cycling performance. Magnesiothermic reduction has emerged as an effective way to convert silica into porous silicon with a good electrochemical performance. However, corrosive HF etching is normally a mandatory step to improve the electrochemical performance of the as-synthesized silicon, which significantly increases the safety risk. This has become one of the major issues that impedes practical application of the magnesiothermic reduction synthesis of the porous silicon anode. Here, a facile HF-free method is reported to synthesize macro-/mesoporous silicon with good cyclic and rate performance by simply increasing the reduction temperature from 700 °C to 800 °C and 900 °C. The mechanism for the structure change resulting from the increased temperature is elaborated. A finite element simulation indicated that the 3D continuous structure formed by the magnesiothermic reduction at 800 °C and 900 °C could undertake the mechanical stress effectively and was responsible for an improved cyclic stability compared to the silicon synthesized at 700 °C.
In this study, a series of near-infrared (NIR) KZnF3:Cr3+ phosphors was prepared by different chromium sources using a HF-free hydrothermal method. The influence of different chromium sources on the crystal structure, morphology, and luminescence properties of Cr3+-doped KZnF3 phosphors were systematically investigated. The results showed that the chromium source changed from CrF3·xH2O to (NH4)3CrF6 and leaded to an increased crystal field, resulting in a blue shift of the emission peak position from 803 nm to 753 nm, accompanied by the full-width half maximum (FWHM) reduced from 140 nm to 122 nm. Moreover, KZnF3:0.03Cr3+ (CrF3·xH2O) and KZnF3:0.03Cr3+ ((NH4)3CrF6) phosphors maintained 58% and 87% of their initial room-temperature intensity at 423 K, respectively. These results indicated that altering the synthetic raw materials provided new insights for designing NIR phosphors with highly thermal stability.
Near-infrared (NIR) phosphor-converted light-emitting diodes (pc-LEDs) have been attracting considerable attention as a new solid-state light source for night vision technology and biological imaging applications. Nevertheless, the requirement for toxic and concentrated HF in the previous synthesis of fluoride phosphors may cause environmental pollution and safety problems. Here, we report a feasible HF-free green synthesis method using a two-step process to prepare the Cr3+ source and a series of Cr3+, Li+ doped NIR phosphors using aqueous solution and methanol solution. X-ray diffraction analysis indicates that the pure target material was successfully synthesized. The luminescence intensity of K2ZnF4:Cr3+,Li+ is 2.58 times improved compared with K2ZnF4:Cr3+. Furthermore, the prepared phosphors show broadband emission in the range of 650 to 1000 nm, with a full width at half-maximum (FWHM) of 139 nm peaking at 783 nm. The NIR output power of the pc-LED fabricated with the K2ZnF4:Cr3+,Li+ phosphor and a 450 nm blue chip reaches 1007 mW at 320 mA. This work provides a promising phosphor for application as a NIR light emitting diode and has important implications for the green synthesis of HF-free Cr3+-doped fluorides in the future.
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