RSC Advances This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. This Accepted Manuscript will be replaced by the edited, formatted and paginated article as soon as this is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains. www.rsc.org/advances Page 1 of 20 RSC Advances Role of oxidizing agent to complete the synthesis of strontium aluminate based phosphors by the combustion method R.E. Rojas-Hernandez*, M.A. Rodriguez, J. F. Fernandez Electroceramic Department, Instituto de Cerámica y Vidrio, CSIC, Kelsen 5, 28049, Madrid, Spain. KEYWORDS: combustion synthesis; phosphorescence; nanostructure; sub-micron size; strontium aluminates Abstract The influence of processing parameters on phase formation of strontium aluminates doped with Eu2+ and Dy3+ by the combustion method has been evaluated. The addition of a slight excess of urea as fuel has an important effect on the phase formation, but a larger amount of urea inhibited the strontium incorporation into the stuffed tridymite-like structure. The experimental results show that it is possible to incorporate larger amount of urea than the theoretical one by adding nitric acid as oxidizing agent. The presence of an oxidizing agent Manuscript promotes both an effective chelation of the cations and a higher crystalline order on strontium aluminate. Nanostructured lamellar particles have been obtained in a single step and the particles are well crystallized. The combustion method avoids the standard requirements of post-thermal treatments in reducing atmosphere to promote the appearance of Eu2+ cations. A higher amount of urea in the presence of the oxidizing agent produces strontium aluminate particles with higher phosphorescence brightness, owing to the increase of the Accepted reduction process Eu3+ to Eu2+. The luminescence properties correlate with the crystallite size of the strontium aluminate. The results demonstrate a pathway to obtain phosphorescent pigments with size down to 10 µm. Advances RSC * Corresponding author. Tlf: +34 91 735 58 40; Ext 1074. Fax: +34 91 735 58 43. E-mail address:[email protected] (R.E. Rojas-Hernandez) RSC Advances Page 2 of 20 1. Introduction Phosphorescent materials have been studied due to their potential application in different areas like traffic safety and emergency signs, clocks, fluorescent devices and inks. After the discovery in 1996 of SrAl2O4: Eu2+Dy3+ as new persistent luminescent compound by Matsuzawa et al. [1], many researchers have developed some methods for preparation of strontium aluminate powders including sol-gel method, hydrothermal synthesis, chemical precipitation, solid-state reaction and so on. In recent years, rare earth-doped strontium aluminates have been intensively researched for their quantum efficiency, good stability, long-lasting time, which implies a wide application in many areas [1–3]. Commonly strontium aluminates are produced by solid state reaction, due to the refractoriness of alumina compounds; this method requires high temperatures, around 1300 – 1500ºC, and a long processing time. Some researchers included the addition of mineralizing agent as boron to decrease the synthesis temperature Manuscript [4,5], nevertheless, the powders produced by this synthesis route have a particle size within the range of 20 to 100 µm [6]; being a disadvantage for some applications. It is known that luminescent properties depend considerably on the grain size. New attractive applications will be possible if the grain size decreases down to 10 µm, particularly those related to printing technologies. Accepted 2+ 3+ Among the different synthesis methods, SrAl2O4: Eu Dy phosphors have been prepared by combustion method [7–9]. This method is suitable to obtain these functional materials due to some advantages such as low processing temperature and fast synthesis. In addition, this method produces a submicron grain size. For these reasons this synthesis method has been considered in order to obtain these phosphors. One of the advantages of the combustion method is the low temperature of synthesis. It has been reported that at Advances initiating temperature of 500ºC the SrAl2O4 a monoclinic phase begins to appear. Nonetheless, other references denote the optimal temperature should be 600ºC to achieve the best optical properties [10,11]. It is necessary to take into account that SrAl2O4 has two polymorphs: monoclinic symmetry that it is stable at RSC temperatures below 650ºC and hexagonal symmetry that it is stable above this temperature. Monoclinic phase shows the luminescent properties; however, there is not a clear agreement about the emission efficiency of the hexagonal structure in this system. Page 3 of 20 RSC Advances In the combustion method, the role of the fuel in the fulfillment of the synthesis has been investigated. Generally urea, CO(NH2)2, is the fuel used and some authors concluded that an excess urea should be required. The optimal quantity of urea is 1.5 times the theoretical one [10] considering that with low concentration of urea a complete reaction of raw materials it is not attained, while if the quantity is too high, the SrAl2O4 monoclinic structure does not synthesize. The maximum amount of urea it is limited to 2.5 times the theoretical one because the hexagonal phase becomes to appear when a higher quantity of urea are added [11]. The theoretical or stoichiometric amount of urea is calculated from the combustion reaction equation following this equation: 3Sr(NO3) 2 +6Al(NO3)3.9H20+ 20CO(NH2)2 →3SrAl 2O4 +94H2O +20CO2+32N2 (1) The incorporation of an excess of urea excess implies the contribution of other terms in the above equation Manuscript 3Sr(NO3)2 +6Al(NO3)3.9H20+ 20·m CO(NH2)2 +30(m-1)O2 → 3SrAl2O4 +(40·m +54) H2O (2) +20·m CO2+(20·m +12)N2 where m denotes the urea ratio. An excess of urea implies therefore a higher consumption of oxygen in order to complete the reaction. To optimize the combustion synthesis an oxidizing agent as nitric acid could be Accepted particularly relevant. 2+ 3+ Therefore, in this work different urea amount in the combustion synthesis of SrAl2O4: Eu Dy are used and it carried out a study related the effect of nitric acid incorporation. The use of oxidizing agent could improve the reaction completion as well to tailor phosphorescent properties. The correlation between combustion process, morphology and optical properties of strontium aluminate based particles is discussed. Advances RSC 2. Experimental details 2+ 3+ SrAl2O4: Eu , Dy particles were synthesized via solution combustion synthesis. The solution was prepared by dissolving Al(NO3)3⋅9H20 (Merck, 98.5%) in deionized water and SrCO3 ( Merck, 99.9%), Eu2O3 (Metal Rare Earth Limited, 99.5%) , Dy2O3 (Rare Earth Limited, 99.5%) in nitric acid HNO3 (Sigma-Aldrich, 65%) to form nitrate solution. The above materials were mixed according to the chemical formula Sr1-x- yEuxDyyAl2O4, where x = 0.02 and y= 0.01. During the 30 min under heating at 80 ºC the solution was stirred, RSC Advances Page 4 of 20 after that, the quantity of urea was added. The full precursor solution dried at 100ºC in a porcelain crucible with a glass-watch lid. The solution was introduced into a furnace maintained at 400ºC and heated up to 600°C; where the ignition took place in a few minutes (CA. 10 min). A white powder with an extremely porous and foam structure was obtained. Selected synthesized powders were also thermally treated at 600 or 900ºC in a furnace under argon atmosphere to improve the luminescent properties. Phase identification of the powders was determined by X-ray diffraction analysis using a Bruker diffractometer (D8, Bruker, Germany) using Cu Kα. The microstructure of the samples was evaluated using secondary electron images of field emission scanning electron microscopy (FESEM, Hitachi S-4700). Image analysis of FESEM micrographs was used to determine the grain evolution by using an Image analysis program (Leica Qwin, Leica Microsystems Ltd, Cambridge, England) considering more than 100 grains in each measurement. Optical properties of these materials by measuring emission and excitation spectra were Manuscript investigated. The photoluminescence spectra of the phosphor particles were recorded with a spectrofluorometer (Fluorolog®-3, HORIBA Jobin Yvon) at room temperature. The luminescence intensity was measured over the wavelength 425–650 nm, a Xenon arc lamp was used as excitation source (λexc= 380nm and 464nm). The decay profiles were also recorded using the same instrument after the samples were sufficiently excited for about 5 min. Accepted 3. Results and discussion 3.1. Influence of the ratio of urea on the synthesis Structural characterization Concerning the combustion synthesis, the urea content and the oxidizing agent ratio are studied. X -ray Advances diffraction patterns of the combustion powders by using the stoichiometric amount of urea, m=1 and excess, m=2 and 3, are shown in Fig. 1. The combustion reaction for m=1 produces a powder which XRD patterns show major crystalline phase as unreacted raw materials and minor presence of Sr (NO3) 2. The main RSC appearance of SrCO3 indicates the carbonation of the nitrate species owing to the reaction medium created by carbon dioxide during the combustion. The peaks of Sr(NO3)2 are related to starting Sr(NO3)2, indicating that the reaction is uncompleted in agreement with previous reported result [10-13].