Journal of Luminescence 167 (2015) 126–131 Contents lists available at ScienceDirect Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin The influence of boric acid on improved persistent luminescence 2 þ and thermal oxidation resistance of SrAl2O4:Eu Songhak Yoon a,n, Jakob Bierwagen b, Matthias Trottmann a, Bernhard Walfort c, Nando Gartmann c, Anke Weidenkaff d, Hans Hagemann b, Simone Pokrant a a Laboratory Materials for Energy Conversion, Empa-Swiss Federal Laboratories for Materials Science and Technology, Ueberlandstrasse 129, CH-8600 Dübendorf, Switzerland b Department of Physical Chemistry, University of Geneva, Quai Ernest Ansermet 30, CH-1211 Geneva 4, Switzerland c LumiNova AG, Switzerland, Speicherstrasse 60a, CH-9053 Teufen, Switzerland d Institute for Materials Science, University of Stuttgart, Heisenbergstrasse 3, DE-70569 Stuttgart, Germany article info abstract 2þ Article history: Persistent luminescence of SrAl2O4:Eu has attracted considerable attention due to their high initial Received 27 March 2015 brightness, long-lasting time and excellent thermal stability. Here the influence of boric acid on the Received in revised form 2þ persistent luminescence and thermal oxidation resistance of SrAl2O4:Eu was investigated in detail. 26 May 2015 Crystal structural analysis and scanning electron microscopy revealed that with the addition of boron, Accepted 15 June 2015 the unit cell volume decreased and the morphology of the particles became more irregular with sharp Available online 23 June 2015 edges. Thermogravimetric analysis showed better thermal oxidation resistance accompanied by a change Keywords: in oxygen vacancy concentration when boron acid is used. Photoluminescence spectra and afterglow Persistent luminescence 2þ decay curves confirm an improved afterglow performance for boron-added SrAl2O4:Eu . Thermo- SrAl O 2 4 luminesence allowed monitoring the changes in the trap states due to the presence of B. Our results Boric acid imply that the substantial improvement of afterglow performance and the thermal stability in Thermal oxidation resistance 2þ Oxygen vacancies SrAl2O4:Eu can be attributed to the incorporation of boron into the aluminate network. & 2015 Elsevier B.V. All rights reserved. 1. Introduction Reportedly, when boron is added during the synthesis of strontium aluminate phosphors, improved emission intensity and 2 þ SrAl2O4:Eu has been well known as an afterglow phosphor, enhanced afterglow efficiency with better stability could be which is widely used in safety and emergency signs as well as in obtained [7–11]. The role of boric acid was primarily discussed as a luminous watches and clocks for lighting in the dark [1,2]. The simple flux during the sintering process. Boron flux enhances the afterglow (or persistent luminescence) is the emission of light formation and crystal growth of strontium aluminate [9,10]. Luitel usually in the visible range remaining for several hours after the claimed that boric acid decreases the decomposition temperature excitation source is turned off. Matsuzawa et al. first reported on a of the precursor SrCO3 and greatly reduces the phase formation 2 þ 3 þ temperature of strontium aluminates [12]. On the other hand, green phosphor of SrAl2O4:Eu ,Dy which showed superior brightness and long duration [3]. After being irradiated with Chen and Chen reported that the added boron in the synthetic þ 2 reaction of SrAl2O4:Eu,Dy does not only act as a flux, but also forms ultraviolet (UV) light, SrAl2O4:Eu exhibits an intense and stable 5− fi a BO4 tetrahedra network, which was con rmed by infrared (IR) green emission band with a maximum at ca. 520 nm due to the 27 þ absorption and solid state Al magic angle spinning nuclear 4f65d1-4f7 transition of Eu2 [4–6]. The long decay time of per- þ magnetic resonance (MAS NMR) spectra [7]. Following this study, sistent luminescence in SrAl O :Eu2 is found to be related to the 2 4 Nag and Kutty also identified the incorporation of borate slow thermal de-trapping process of the electrons stored in traps BO5−species in SrAl O by IR and NMR [8]. As an evidence for after photo-excitation by UV-irradiation. Since then, both sig- 4 2 4 boron incorporation in strontium aluminate, Chang et al. showed nificant improvement in the afterglow properties and a better unit cell contraction resulting in the shift of emission peak to understanding of the persistent luminescence mechanism have shorter wavelength, when Sr4Al14O25 is sintered with the aid of continuously gained special attention [1,2]. B2O3 [13]. Tang et al. also reported that the introduction of B2O3 dopant in the SrAl2O4 resulted in the shrinkage of SrAl2O4 crystal n Corresponding author. Tel.: þ41 58 765 46 02; fax þ41 58 765 69 19. lattice, inducing a higher number of crystal defects with a stronger E-mail address: [email protected] (S. Yoon). PL intensity [14]. Furthermore, the generation of deep trap levels http://dx.doi.org/10.1016/j.jlumin.2015.06.021 0022-2313/& 2015 Elsevier B.V. All rights reserved. S. Yoon et al. / Journal of Luminescence 167 (2015) 126–131 127 which are beneficial for the enhanced persistent luminescence of Differential scanning calorimetry (DSC) and thermogravimetric 2 þ SrAl2O4:Eu has been reported either due to the incorporation of analysis (TGA) have been carried out separately using a NETZSCH 3− boron into the crystal lattice forming planar BO3 or tetra- DSC-404C and a NETZSCH STA 409CD thermobalance, respectively. 5− A baseline was measured with an empty crucible. In each mea- hedral BO4 units [15,16] or due to the easy entry of other activator ions like Dy and/or other rare earth elements in the host crystal surement around 0.05 g and 0.5 g of powders were used for DSC lattice [11]. Consequently, different models of defect complexes and TGA, respectively. The powders were heated under synthetic have been proposed so far [5,17,18]. air (50 mL/min) up to 750 °C for 15 min with a heating and cooling The global mechanism of persistent luminescence is considered rate of 5 °C/min. For DSC, the initial heating rate of 20 °C/min was to have been understood [1,2,5,19,20] qualitatively, but the cor- applied from room temperature to 550 °C. relation between the chemical composition, crystal structural The photoluminescence excitation and emission spectra were fl change, the nature of the trap states and the afterglow behavior is obtained at room temperature using a spectro uorometer (Fluor- still under discussion. Although the knowledge about the role of olog 3-22 Jobin Yvon, Edison) with a nominal resolution of 1 nm. boron is very important and represents even a prerequisite for the The afterglow intensities depend on the individual sample pre- successful realization or optimization of SrAl O :Eu2þ phosphor, paration and are not directly comparable. 2 4 fi only a few experiments were performed to study the effect of boric Phosphorescence decay pro les were measured with a photo- acid on the crystal structure, phase composition, element dis- meter (PhotoMR, RC Tritec in collaboration with Monyco) equip- 2 þ ped with a highly sensitive photomultiplier from Hamamatsu. The tribution and luminescence properties of SrAl2O4:Eu . Previous publications focused mainly on the effect of co-doping of spectra were recorded at room temperature after excitation by a 2 þ D65 fluorescence lamp for 20 min (ISO 17514). The sample pre- SrAl2O4:Eu with boron and different rare earth ions such as Dy, paration procedure is standardized such that the afterglow which complicated the differentiation of the doping effects of each intensities are comparable. element. 2 þ For measuring the thermoluminescence a custom-made setup The present study focuses on SrAl2O4:Eu with and without was employed. The sample was cooled with a stream of cold boric acid to investigate the effect of boron on the formation of nitrogen-gas from a liquid-nitrogen Dewar and heated with a lattice defects and trap states. In addition, the influence of boric heating cartridge (Omega, CSH-101100). The heating rate was acid on thermal oxidation resistance was investigated in order to 10 °C/min. The temperature was monitored with a temperature find the reason for the enhanced persistent luminescence of sensor (Omega, Type T thermocouple) and controlled with a PID SrAl O :Eu2 þ. Crystal structure, phase composition, luminescence 2 4 controller (Omega, Model CN7523). The thermoluminescence was properties and persistent luminescence of SrAl O :Eu2þ were 2 4 collected with a custom-made 1:1 telescope and detected with a systematically characterized using XRD, SEM, DSC, TGA, UV–vis PMT (Hamamatsu, R2949), equipped with a photon counting unit spectroscopy, photoluminescent excitation and emission spectro- (Hamamatsu, C3863) and a custom-made high voltage supply. The scopy (PLE and PL) and phosphorescence spectrophotometry samples were charged with a UV-LED (400 nm, Ultrafire WF 501B). paying special attention to the oxidation behavior. 100 mg of samples was mixed with 300 mg of KBr (FT-IR grade, 499%, Sigma-Aldrich). For thermoluminescence measurement, the mixture was then pressed with a hydraulic lab press (30 s, 2. Experimental procedures 150 kN) to give a nontransparent pellet with 13 mm diameter and approximately 1 mm thickness. 2þ SrAl2O4:Eu powders were synthesized by a solid-state reac- 2 þ tion with or without boric acid (denoted as SrAl2O4:Eu ,B and 2 þ SrAl2O4:Eu , respectively). Stoichiometric amounts of SrCO3 and 3. Results Al2O3 together with 1.5 mol% of Eu2O3 were thoroughly mixed, 2 þ either with 0.9 wt% of boric acid to total mass or without boric X-ray diffraction patterns of SrAl2O4:Eu with and without acid. The powder mixtures were then ground in a ball mill and boric acid, and their oxidized powders are shown in Fig.