Fabrication and luminescence of BiPO₄:Tb³⁺/Ce³⁺ nanofibers by electrospinning

Highlights

• Successful synthesis of BiPO₄:Tb³⁺/Ce³⁺ nano fibers by electrospinning.

• Characteristic emissions originating from Tb³⁺ ions in BiPO₄:Tb³⁺/Ce³⁺.

• Energy transfer efficiency with a value of 45.9% between Tb³⁺ and Ce³⁺ ions.

Abstract

BiPO₄:Tb³⁺/Ce³⁺ nanofibers are synthesized by a simple electrospinning process and followed by a calcination treatment. The nanofibers are characterized by TG-DSC, XRD, FT-IR, SEM, TEM, and spectrophotometer. The calcined BiPO₄:Tb³⁺/Ce³⁺ nanofibers have a monoclinic phase and the diameters are in the range of 80–200 nm. The SEM and TEM images show that the nanofibers are mesoporous and composed of lots of linked nanoparticles. Under an excitation at 369 nm, BiPO₄:Tb³⁺ and BiPO₄:Tb³⁺/Ce³⁺ nanofibers show characteristic emission bands originating from the ⁵D₄ → ⁷F_j (j = 6, 5, 4, and 3) transitions of Tb³⁺ ions. The luminescent properties of BiPO₄:Tb³⁺ indicate that the quenching concentration for Tb³⁺ ions in BiPO₄ host is about 15 mol%. The co-doped Ce³⁺ ions can improve the emission intensity through the energy transfer from Ce³⁺ to Tb³⁺ ions.

Introduction

Bismuth phosphate (BiPO₄) is one of important inorganic phosphates and has three different modifications including hexagonal phase, low-temperature monoclinic phase, and high-temperature monoclinic phase [1], which has applications in fields of catalysts [2], ion sensing [3], and separating radioactive elements [4]. Also, it is reported as a better luminescent host for rare earth ions [5]. Such rare earth ions doped BiPO₄ powders are potential candidates for photonic and luminescent applications. As a result, rare earth ions doped BiPO₄ nano/micro crystals with different phases and morphologies are synthesized by different methods [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. Generally speaking, rare earth ions occupy the sites of Bi³⁺ ions because of the comparable ionic radius of Bi³⁺ ions with those of rare earth ions. And the main difference between hexagonal and monoclinic phase of BiPO₄ is the number of near neighbor oxygen atoms around Bi³⁺ ions. In the case of hexagonal and monoclinic BiPO₄, Bi³⁺ ions are surrounded by eight and nine near neighbor oxygen atoms, respectively [1]. Since luminescent properties of nano/micro crystals have close relation with their phase, morphology, and particle size [16], rare earth ions doped BiPO₄ nano/micro crystals often show different luminescent properties.

In recent years, much attention has been paid to the synthesis of one-dimensional (1D) nano/micro crystals, which exhibit novel physical and chemical properties due to their unique and fascinating characteristics for huge ratio of diameter to length and superior mechanical toughness. These fibers have potential applications in textiles, optical detectors, filtration, and vivo imaging. For example, some scientists as well as artists have developed much simple color-changing system that can apply to textiles using various nanofibers [17]. Also, fibrous materials used for filter provide advantages of high filtration efficiency and low air resistance, because of the very high surface area to volume ratio and resulting high surface cohesion, as well as the possibility of recycle usage of electrospun membrane [18]. It has been reported that Eu³⁺ ions show higher quantum efficiency values in 1D Y₂O₃ fibers than those of bulk powders [19]. The mesoporous structure of fibers will influence the electro–hole pairs' recombination rates, which further influences the luminescent properties of samples [20], [21]. Therefore, inorganic rare earth ions doped luminescent fibers are drawn great attention because of their geometry advantage and excellent luminescent efficiency. For the synthesis of nano/micro fibers, electrospinning is an effective route. Some inorganic fibers have been synthesized by this method [22], [23], [24], [25], [26], [27], [28]. Herein, we report on the synthesis of BiPO₄:Tb³⁺/Ce³⁺ fibers by electrospinning. As we all know, the particle size plays a key role in the dynamics of electron–hole recombination process [29]. The synthesis and luminescence of hexagonal BiPO₄:Tb³⁺/Ce³⁺ nanoparticles have been reported [30], [31]. In this work, we synthesize monoclinic BiPO₄:Tb³⁺/Ce³⁺ nanofibers comprising of lots of small nanoparticles by electrospinning process. The phase, morphology, and luminescence of synthesized fibers are studied carefully. To the best of our knowledge, it is the first time to report the fabrication of BiPO₄:Tb³⁺/Ce³⁺ nanofibers by electrospinning process and relating luminescent properties.