Synthesis and optical properties of chromium-doped spinel hollow nanofibers by single-nozzle electrospinning

Abstract

Novel spinel hollow nanofibers were synthesized by a convenient single-nozzle electrospinning technique. Based on the composition and morphology results by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), a 'gas-push' model was proposed to elucidate the formation mechanism of hollow nanofibers during calcination. Photoluminescence (PL) properties of calcined ZnAl 2 O 4 : xCr 3+ and MgAl 2 O 4 : xCr 3+ nanofibers were investigated systematically. Under an excitation of 400 nm or 530-550 nm (4 A 2g A 4 T 1g or 4 A 2g A 4 T 2g transitions of Cr 3+ ions), an emission band at 675-725 nm including a sharp emission line (R-line) at y687 nm with several vibrational sidebands was obtained. Compared with the spectroscopic properties of ZnAl 2 O 4 : xCr 3+ nanofibers, the emission band was broadened and the decay lifetime was shortened for MgAl 2 O 4 : xCr 3+ nanofibers. By calculation of crystal field strength, the value of D q /B is estimated to be 3.25 and 2.72 for ZnAl 2 O 4 : xCr 3+ and MgAl 2 O 4 : xCr 3+ nanofibers, respectively. The difference of crystal field strength gives a good explanation of the spectroscopic and decay evolution between ZnAl 2 O 4 : xCr 3+ and MgAl 2 O 4 : xCr 3+ nanofibers.

Cr‐doped ZnAl2O4: Microwave solution route for ceramic nanoparticles from metalorganic complexes in minutes

“…Diffused reflectance spectra (Figure ) show three noticeable bands at 270 nm, 370 nm, and 540 nm except for the as‐prepared sample, 2A, which can be assigned to the 4 A 2g (F) → 4 T 1g (P), 4 A 2g (F) → 4 T 1g (F), and 4 A 2g (F) → 4 T 2g (F) spin‐allowed Cr 3+ transitions . The band at 370 nm intensifies with annealing up to 800°C, beyond which it gradually disappears.…”

“…The emission spectra (Figure ) consist of several narrow bands due to the presence of Cr 3+ in a strong field with octahedral coordination. These bands correspond to the general 2 E → 4 A 2 transitions of the Cr 3+ present in an unperturbed or perturbed environment with perturbations due to crystal defects . Cr 3+ emissions contain an unresolved zero phonon line at 688 nm (ZPL) (R line) and multi‐phonon side bands on either side of the ZPL .…”

“…In high crystal fields, the five‐fold‐degenerate d 3 state splits into two energy levels t 2g and e 2g . In the excitation spectra, ν 1 gives the strength of the crystal field‐splitting parameter, D q , as 10.15 . From the excitation spectra, it is possible to calculate the electronic repulsion factor B (Racah parameter)—according to Equation below: …”

“…Among metal oxides, ZnAl 2 O 4 with a band gap of 3.8 eV and transparent to visible light, is a popular phosphor host and used as a UV‐optoelectronic material, optomechanical stress sensor, and in stress imaging . The ZnAl 2 O 4 phosphor has emissions in the blue and, when strategically doped with transition or rare‐earth elements, can have emissions ranging from green to red . When doped with Cr 3+ , ZnAl 2 O 4 gives emission in the red and the near infrared region due to transitions of Cr 3+ in the octahedral sites .…”

“…The ZnAl 2 O 4 phosphor has emissions in the blue and, when strategically doped with transition or rare‐earth elements, can have emissions ranging from green to red . When doped with Cr 3+ , ZnAl 2 O 4 gives emission in the red and the near infrared region due to transitions of Cr 3+ in the octahedral sites . Cr 3+ ‐doped spinel phosphors have also been employed as biomarkers to detect cancer cells and to visualize tissues of interest …”