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 ZnAl₂O₄: xCr³⁺ and MgAl₂O₄: xCr³⁺ nanofibers were investigated systematically. Under an excitation of 400 nm or 530–550 nm (⁴A_2g→⁴T_1g or ⁴A_2g→⁴T_2g transitions of Cr³⁺ ions), an emission band at 675–725 nm including a sharp emission line (R-line) at ∼687 nm with several vibrational sidebands was obtained. Compared with the spectroscopic properties of ZnAl₂O₄: xCr³⁺ nanofibers, the emission band was broadened and the decay lifetime was shortened for MgAl₂O₄: xCr³⁺ nanofibers. By calculation of crystal field strength, the value of D_q/B is estimated to be 3.25 and 2.72 for ZnAl₂O₄: xCr³⁺ and MgAl₂O₄: xCr³⁺ nanofibers, respectively. The difference of crystal field strength gives a good explanation of the spectroscopic and decay evolution between ZnAl₂O₄: xCr³⁺ and MgAl₂O₄: xCr³⁺ nanofibers.