Photoelectric properties of Sr₂MgSi₂O₇: Eu²⁺ phosphors produced by co-precipitation method

Highlights

• The blue phosphor Sr₂MgSi₂O₇: Eu²⁺was synthesized by the co-precipitation method.

• This phosphor used to find the latent fingerprints printed on different surfaces.

• Three models mimicked the crystal host and doped phosphors were studied by CASTEP.

• Doped Eu²⁺ ion forms the coordinate covalent bond with its neighboring oxygen atoms.

• Oxygen vacancy causes obvious changes in the electron orbitals of Si and O atoms.

Abstract

Eu²⁺-doped phosphors Sr₂MgSi₂O₇: xEu²⁺ (x = 1–5 mol%) with blue color are synthesized by the co-precipitation method. The X-ray diffraction patterns of these phosphors indicate a tetragonal crystal structure. The emission peak centered at 486 nm is attributed to the 4 F⁷→ 4F⁶5D¹ transitions of Eu²⁺ ions with CIE coordinates of (x = 0.176, y = 0.637). We use Sr₂MgSi₂O₇: 4 mol% Eu²⁺ phosphors to find latent fingerprints (LFPs) with visible patterns on different surfaces under 337 nm UV light. The results indicate that this phosphor may possess the potential to be used for LFP recognition. Three calculated models mimicking the crystal host, Eu²⁺ ion-doped phosphor, and Eu²⁺ ion-doped phosphor with oxygen vacancy are built using CASTEP software to perform density functional theory (DFT) calculations. The geometry optimization results of the Eu²⁺ doping ion reveal that a coordinate covalent bond is formed by the Eu²⁺ doping ion with its neighboring oxygen atoms. The oxygen vacancy V_O causes obvious changes in the electron orbitals of Si and O atoms as well as the 5d empty orbital of Eu²⁺ ions.

Introduction

Long afterglow phosphors have great potential for application in many fields such as lighting [[1], [2], [3]], traffic signs [4,5], watches and clocks [6,7], luminous paints [8], sensors and detectors [9,10], biomedical imaging [11,12], photocatalysis [13,14], and solar cells [15,16]. Among various kinds of phosphors, silicates can be used in a wide range of applications due to their easy preparation [17], low cost [18], stable crystal structure [19], long persistence time [[20], [21], [22]], physical and chemical stabilities [23,24], excellent water resistance [25], environment friendliness [26], varied luminescent colors [27], and strong absorption in the near-UV region.

The family of materials M₂MgSi₂O₇ (M = Ca, Sr, Ba), which are called alkaline earth akermanites, are the most widely studied persistent luminescent silicates [[28], [29], [30], [31], [32], [33], [34]]. Ions of rare earth elements are the most common doped ion to be used as an activator in phosphors. Luminescent Eu²⁺ ions can emit light from the UV to the infrared region on different host matrices because the involved 5d orbital of a Eu²⁺ ion is external and strongly influenced by the crystal field. Activator Eu²⁺ ions are used to make Sr₂MgSi₂O₇ based blue phosphors, and their broadband emission usually consists of transitions from 4f⁶5 d¹ to the 4f⁷ ground state.

The host of Sr₂MgSi₂O₇ (SMSO) has a tetragonal crystal structure. The Sr²⁺ ion in this crystal structure occupies a position that is connected with eight neighboring O²⁻ ions. Obviously, when Eu²⁺ is doped into the SMSO crystal, the electronic structure of SMSO is not changed because their ionic radii are almost the same, and no charge compensation is required for the replacement of Sr²⁺ with Eu²⁺. Therefore, we adopted the density functional theory (DFT) calculation in the Cambridge Sequential Total Energy Package (CASTEP) software and used a simple primitive crystal model to study how the Eu²⁺ doping and oxygen vacancies affect the electronic orbitals in the crystal structure.

Among the various synthesis methods, the co-precipitation method [[35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53]] is known to produce phosphor powders with uniform, narrow size distribution, and a homogeneous distribution of the activator ions. Therefore, the co-precipitation method was chosen to prepare Sr₂MgSi₂O₇: Eu²⁺ phosphors with various doping concentrations of Eu²⁺ ions. According to our experimental results, the optimum Eu²⁺ doping concentration was 4 mol%, and the optimum sintered temperature and time were 1150 °C and 11 h, respectively.

In addition, the crystal phases and structures of these synthesized phosphors were studied by X-ray wide-angle diffraction (XRD) and field emission scanning electron microscopy (FESEM). The photoluminescence excitation (PLE) and photoluminescence (PL) spectra of these phosphors were studied using a fluorescence spectrophotometer. The binding energies of each surface composition of this phosphor were determined by X-ray photoelectron spectroscopy (XPS). The loss of weight during the synthesis period was studied by thermogravimetric analysis (TGA).

Owing to the advantages of Sr₂MgSi₂O₇: Eu²⁺ blue phosphor powder such as low cost, ease of synthesis, lack of human toxicity, and environment friendliness, we try to explore the possibility and feasibility of its application to LFP discovery. For this, the pattern identification of LFPs on the surfaces of various materials is evaluated.