Synthesis, electronic structure and photoluminescence properties of Ba₂BiV₃O₁₁: Eu³⁺ red phosphor

Highlights

• Ba₂BiV₃O₁₁: Eu³⁺ phosphors were prepared by the sol–gel method.

• The electronic structure of Ba₂BiV₃O₁₁ was estimated by density functional theory.

• Ba₂BiV₃O₁₁: Eu³⁺ can be excited at 394 and 465 nm and emit red light.

Abstract

Eu³⁺-activated Ba₂BiV₃O₁₁ red-emitting phosphors have been successfully synthesized by the sol–gel method. The electronic structure of Ba₂BiV₃O₁₁ host was estimated by density functional theory (DFT) calculation. The structure and phase purity of the samples were characterized by powder X-ray diffraction (XRD), and their photoluminescence (PL) spectra, decay curves of ⁵D₀→⁷F₂ transitions, absorption spectra, and the Commission International de I’Eclairage (CIE) color coordinates were also investigated. The thermal stabilities of the phosphors were studied using the dependence of the luminescence intensities on temperature, and thermal activation energy was also determined. The results demonstrated that Ba₂BiV₃O₁₁: Eu³⁺ was a potential red-emitting phosphor for n-UV-based white light-emitting diodes (w-LEDs).

Introduction

White light emitting diodes (w-LEDs) have gradually replaced traditional light sources for applications in display, decoration and illumination because of their energy saving, durability, high efficiency and environmental friendly merits over conventional light sources [1], [2], [3]. Generally, most w-LED commercial products are produced by combining an InGaN-based blue LED chip with a yellow phosphor YAG: Ce³⁺, which suffers from low color rendering index (CRI, < 75) and high color temperature (>4500 K) due to the deficiency of red light component [4]. At present, there are two common ways to solve the above problems: one is combining a blue LED chip (420–470 nm) with red and green phosphors; the other is fabricated by pumping tricolor (red, green, and blue) phosphors with a near-ultraviolet (n-UV) LED chip (350–410 nm) [5], [6], [7]. Both of the aforementioned methods require high efficient red emitting phosphors.

The Eu³⁺ ion is a classical activator for efficient red phosphors and has been widely investigated due to its characteristic red emissions from ⁵D₀ → ⁷F_J (J = 0−4) transitions. Among these transitions, the dominant 610–630 nm emission from ⁵D₀ → ⁷F₂ electric dipole transition is conducive to improve the color purity of the phosphor, in which Eu³⁺ occupies the lattice sites without centro-symmetry [8], [9]. In addition, the excitation spectra of Eu³⁺ ions in oxide hosts are usually composed of two parts: charge transfer band and excitation peaks around 394 and 465 nm, which match well with the emission of n-UV or blue LED chips. Up to now, the main commercial red-light emitting phosphors are still restricted to Eu²⁺ doped nitrides and Y₂O₂S: Eu³⁺, but the synthesis conditions required for the former is extreme and the latter is chemically unstable, which leads to high cost and shortening the life time of w-LEDs [10], [11]. Hence, there is requirement to search for novel environment-friendly red phosphors with remarkable chemical stability and high luminance.

As important phosphor hosts, vanadates were given more attention for their potential applications in LEDs because they not only have physical and chemical stability but also present strong absorption in blue or UV region [12], [13]. However, most of these phosphors were prepared by the conventional solid-state reaction, the conditions of which are incongruous with the concept of saving energy by using LEDs since their preparation requires high temperature, sufficient grinding and long energy consuming heating process [14], [15], [16]. The sol–gel method is a perfect way for oxide compounds to reduce the synthesis temperature and shorten the annealing time, which has been extensively used in the preparation process of phosphors. The Bi³⁺ ion usually is an efficient dopant by strengthening and broadening UV excitation bands when the doping concentration is opportune, it is interesting to investigate whether Bi³⁺ has a large effect or not if the host contains Bi³⁺ ions. Ba₂BiV₃O₁₁ with the monoclinic crystal structure has rarely been reported as a phosphor host. Herein, Eu³⁺-doped Ba₂BiV₃O₁₁ red emitting phosphors were synthesized via a sol–gel method, the structure and photoluminescence properties of the phosphors have been investigated. The series of phosphors can be efficiently excited by 394 and 465 nm light, and exhibited pure red emission peaking at 619 nm. Furthermore, the role of the Bi³⁺ in the host was also analyzed by the electronic structure of the Ba₂BiV₃O₁₁ host.