Sol gel synthesis and pH effect on the luminescent and structural properties of YPO₄: Pr³⁺ nanophosphors

Highlights

• Pr³⁺ -doped YPO₄ nanophosphors prepared by simple sol gel route with different pH values were synthesized.

• The effect pH values on structural, steady, time resolved photoluminescence spectroscopy have been investigated.

• The grain size of samples increases with increases in pH value.

• The highest luminescent intensity was obtained for samples prepared at pH = 4.

Abstract

Pr³⁺ -doped YPO₄ nanophosphors prepared by simple sol gel method with different pH values (2, 4, 7 and 11) were obtained. The nanopowders samples were characterized by X-ray diffraction (XRD), room temperature steady and time resolved photoluminescence spectroscopy. The thorough study of pH influence on particle's structure and luminescence of YPO4: 1 at. Pr³⁺ is presented. It was found that the grain size of samples increases with increases in pH value and obtained particles crystallize in a tetragonal phase with xenotime structure. Under 4f5d excitation (230 nm), all emission spectra show the inter-configurational 4f²→4f5d and under ³P₂ excitation (449 nm), only the intra-configurational ¹D₂→³H₄ red emission transition between 580 nm and 620 nm are observed. The highest luminescent intensity was obtained for samples prepared at pH = 4. Furthermore, it was found that the pH of solution has no effect of ¹D₂ lifetime.

Introduction

Yttrium orthophosphate YPO₄ doped with Ce³⁺ and Pr³⁺ions are attractive material for many important applications such as scintillation detectors, medical imaging, display device [1], [2] which present a high chemical and thermal stability [3], [4]. Lanthanide phosphates, LnPO₄ (Ln = Y, Gd, Lu, and La), crystallize in two polymorphs, that is, tetragonal phase with xenotime structure and monoclinic phase with monazite structure. YPO₄ crystallizes with the zircon structure (xenotime-type) with a tetragonal symmetry (a = b = 0.6894 nm and c = 0.6027 nm) and space group I4₁/amd. [5], where the site symmetry for Y³⁺ ions is D_2d point-group [6]. The structure can be described as chains parallel to the c axis of corner-sharing structural units built of (YO₈) dodecahedron and a (PO₄) tetrahedron linked together by an edge [7]. YPO₄ matrix has excellent optical and physical properties: large indirect band gap (∼8.6 eV), high dielectric constant (∼7–10), refractive index (∼1.72), high melting point (∼1600 °C) and phonon energy (∼1080 cm⁻¹) [8]. Both bulk and nanoscale forms of YPO₄ can be easily doped and co-doped with trivalent rare earth ions [9], [10] due to similar ionic radii and valence between them and Y³⁺. Among rare earth ions, extensive studies are focused on Pr³⁺ ion for potential use in a variety of applications such as scintillators, lasers, display devices and biological labeling. In fact, Pr³⁺ offers rich emissions from the visible to near-infrared region due to the wealth of its energy levels [11]. In addition, depending on the host material, Pr³⁺ ion can provide several types of emissions related to spectroscopic processes namely: (1) the allowed electric-dipole inter-configurational transitions to 4f5d → 4f², (2) intra-configurational transitions from ³P₀ or ¹D₂, giving rise to greenish-blue and red emissions, (3) the photon cascade emission (PCE) and (4) the up-conversion process. Srivastava et al. [12] have investigated the conditions requirements that 4f¹5d¹→4f² interconfigurational emission transitions of Pr³⁺ ion in a solid become dominant. Several experimental and theoretical works dealing with rare earth doped YPO₄ single crystal can be found in the literature [13], [14], [15], [16]. Also, Srivastava [13] studied in details Pr³⁺ 4f5d energy position, relative to the intraconfigurational excited energy levels ¹S₀, ³P_J and the conduction band of host material, affect optical properties of Pr³⁺ ion.

It is well-known that optical and physical proprieties of materials depend not only on their composition, but also on their structure, size, and shape [17]. It is also known that the use of appropriate synthesis method may provide materials with targeted optical and physical properties, since most of the contemporary synthesis procedures allow control of the material's structure, morphology, and size by alternation of synthesis conditions [18]. Several methods have been developed so far for the preparation of nanosized phosphor materials. These include hydrothermal method, sol gel, and co-precipitation process [19], [20]. The sol gel method is frequently a method of choice, since it provides nanomaterials with high homogeneity and purity, and does not require high-temperature treatments [21].

Nanoscale Pr³⁺ -doped YPO₄ material has been studied by several authors [22], [23]. It has been found and reported that the pH value of the reaction solution is a crucial factor which determines the phase structure and morphology of YPO₄:Eu³⁺ nanocystals [10]. This fact has been confirmed for materials doped with other rare earth ions, namely for YPO₄: Yb³⁺/Er³⁺ particles [24]. In this paper, we report effects of the reaction solution pH on the structure and luminescence of Pr³⁺ doped YPO₄ nanopowders synthesized by sol gel method.