Optical Materials 85 (2018) 275–280 Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat Synthesis of yttrium aluminum garnet nanoparticles in confined environment III: Cerium doping effect T Francesco Armettaa, Dariusz Hreniakb, Łukasz Marciniakb, Eugenio Caponettia, ∗ Maria Luisa Saladinoa, a Dipartimento Scienze e Tecnologie Biologiche, Chimiche e Farmaceutiche - STEBICEF and INSTM UdR - Palermo, Università di Palermo, Viale delle Scienze Bld.17, Palermo I, 90128, Italy b Department of Spectroscopy of Excited States, Institute of Low Temperature and Structure Research, Polish Academy of Sciences, ul Okólna 2, Wrocław, Poland ARTICLE INFO ABSTRACT Keywords: Cerium yttrium aluminum garnet (Ce:YAG, CexY3-xAl5O12) nanoparticles doped with different cerium amount were Ce:YAG nanoparticles obtained by calcining the precursors synthesized via co-precipitation in w/o microemulsion for 1 h at 900 °C. The Yttria structural and morphological properties were investigated by using X-ray Diffraction (XRD), Infrared Ce(III) Spectroscopy (IR) and Transmission Electron Microscopy (TEM) coupled with Energy Dispersive X-ray Confined environment Spectroscopy (EDS) in order to investigate the effect of doping level on formation and microstructure of obtained w/o microemulsion nanoparticles. It was found that the composition of the final products strongly depends on the concentration of Phase composition cerium. The formation of single YAG phase was observed only at 0.5% cerium. For other concentrations, a mixture of YAG and yttria phases was obtained indicating an effect of the cerium on stabilization of sesquioxide phase. Optical properties of the single phase powder were also investigated in details. 1. Introduction mechanism of YAG nanoparticles was discussed on the basis of observed particle microstructure and morphology after several thermal treat- For decades, the synthesis of nanoparticles in confined environ- ments [8]. The results indicated that the YAG growth process could be ments such as microemulsions or liquid crystals have been considered influenced by a not complete oxidation of the surfactant (Cetyl- one of the best way to tailor the particle size and shape [1–3]. In trimethylammonium bromide, CTAB). Moreover, it cannot be excluded general, the particle size is influenced by the water/surfactant molar the formation of a complex between aluminum ions and the degrada- ratio [4], by the water/oil molar ratio and by the temperature, which tion products of surfactant, which could affect the availability of alu- affect the microstructure environment in the phase diagram [5]. If the minum to participate in the crystallization process. crystalline phase of the required material is obtained at high tem- YAG, whose cubic structure is in the space group Ia-3d, doped with perature, the precursors prepared in microemulsions can be thermally lanthanides ions, is an important material with interesting technolo- treated. Experimental data demonstrated that the systems maintain a gical applications in displays, X-ray scintillators, lasers, and white light memory of the microstructure used as nanoreactor influencing the final LED which are nowadays well consolidated [9–13]. Notwithstanding, features of the material [6]. the interest of researcher is still growing because the optical properties This kind of studies has been performed to obtain YAG (yttrium of nanocrystals are expected to be dependent not only on the kind of aluminum garnet, Y3Al5O12) nanoparticles via co-precipitation of hy- dopant but also on the synthesis route that may influence particle size, droxides in a bicontinuos microemulsions [7,8]. The preparation in its distribution and morphology [14–17]. In this work, the effect of the microemulsion allowed to obtain nanoparticles with well-defined cerium amount on structure of YAG nanoparticles prepared in micro- shape, not agglomerated and of smaller size (10–45 nm) respect to emulsion has been investigated. those obtained by using other synthetic procedures. Furthermore, the The samples have been prepared in the bicontinuous microemulsion effect of the thermal treatment on the evolution of precursors toward constituted by water/CTAB/1-butanol/n-heptane and calcining the the formation of YAG phase has been also investigated demonstrating precursors at high temperature. The crystalline habit of the obtained the role of the surfactant in the growth mechanism. The possible growth materials has been investigated by using X-ray Diffraction (XRD) and ∗ Corresponding author. E-mail address: [email protected] (M.L. Saladino). https://doi.org/10.1016/j.optmat.2018.08.060 Received 17 July 2018; Received in revised form 21 August 2018; Accepted 23 August 2018 0925-3467/ © 2018 Elsevier B.V. All rights reserved. F. Armetta et al. Optical Materials 85 (2018) 275–280 Infrared Spectroscopy (IR). The morphology and particles size as well as the elemental composition have been investigated by using Transmission Electron Microscopy (TEM) coupled with an Energy Dispersive X-ray Spectroscopy (EDS). Emission and Excitation spectra, lifetimes and quantum efficiency measurements of the single phase powder have been also carried out. 2. Materials and methods 2.1. Materials Cetyltrimethylammonium bromide (CTAB, Aldrich ≥98%), 1-bu- tanol (Aldrich, 99.8%), n-heptane (Sigma-Aldrich, 99%), Y(NO3)3·6H2O (Sigma-Aldrich, 99.8%), Al(NO3)3·9H2O (Sigma-Aldrich, 99.8%), Ce (NO3)3·6H2O (Sigma-Aldrich, 99.99%), ammonia solution (Sigma- Aldrich 28%) and deionized water were used as received. The con- ductance of deionized water was < 1.5 μS/m. 2.2. Preparation of the nanoparticles The precursor synthesis of Ce:YAG nanoparticles at different cerium amount has been performed following the procedure already reported [7,8]. The samples have been prepared in the bicontinuous micro- emulsion R70, resulted the more suitable to obtain separate nano- particles in the previous studies. The yttrium, aluminum, cerium nitrate − solutions (30 - x, 50 and x mmol L 1, respectively) were prepared by dissolving proper amounts of Y(NO3)3·6H2O, Al(NO3)3·6H2O and Ce (NO3)3·6H2O in water in order to obtain 0, 0.1, 0.2, 0.5, 1.0 and 2.0% of cerium atoms with respect to total yttrium plus cerium atoms, ac- cording to the formula CexY3-xAl5O12. For each batch, a white hue sol was instantaneously observed as a Fig. 1. XRD patterns of the obtained powders. * YAG phase peaks, ° Y O phase result of mixing with ammonia microemulsion. A complete precipita- 2 3 peaks. tion occurred in 12 h. The precipitates were thus filtered and repeatedly washed with water to remove residual ammonia, nitrate ions, and surfactant molecules. The obtained white precipitates were over-night emission 300 mm focal length monochromators were in Czerny Turner fi dried at 100 °C, then treated at 900 °C for 1 h. After calcination, solid con guration. Excitation arm was supplied with holographic grating of pellets were formed. The obtained pellets had a yellowish coloration 1800 lines/mm, blazed at 250 nm, while the emission spectra was with tonality influenced by the cerium amount. supplied with ruled grating, 1800 lines/mm blazed at 500 nm. The spectral resolution was 0.1 nm. The R928P side window photo- 2.3. Characterization techniques multiplier tube from Hamamatsu was used as a detector. Quantum efficiency of luminescence was measured using the same XRD patterns were acquired by a Philips PW 1050/39 diffractometer equipment supplied with 30 cm in diameter integrating sphere. in the Bragg-Brentano geometry using Ni-filtered Cu Kα radiation The lifetimes measurements were carried on a femtosecond laser setup (λ = 1.54056 Å) in the 2θ range 5–90° with a step of 0.05° and a time composed of a Coherent Libra-S all-in-one ultrafast oscillator and re- fi for step of 5 s. The X-ray generator worked at power of 40 kV and generative ampli er laser system, with pulse duration less than 100 fs at 30 mA, and the resolution of the instrument (divergent and antiscatter 1 kHz repetition rate, a Coherent OPerA-Solo optical parametric am- plifier and a Hamamatsu C5677 streak camera with time resolution of slits of 0.5°) was determined using R-SiO2 and R-Al2O3 standards free from the effect of reduced crystallite size and lattice defects. The phase 14 ps. ® identification has been performed by using the X'pert HighScore Software. 3. Results and discussion − The ATR spectra were recorded in the 40-4000 cm 1 range, with a − step of 2 cm 1, by using a Bruker Vertex 70 Advanced Research FT-IR Structural and morphological investigation. The XRD patterns of the Spectrometer equipped with Platinum ATR. The measurements have obtained powders are reported in Fig. 1. been performed at 2 hPa. Data have been corrected with a scattering- The undoped powder is constituted by single garnet phase [7,8]. The type baseline. XRD patterns of the samples doped with 0.1 and 0.2% of cerium can be Transmission Electron Microscopy (TEM) micrographs were acquired described in terms of two crystalline phases, the garnet phase and the by using a JEM-2100 (JEOL, Japan) operating at 200 kV accelerating cubic yttrium oxide (Y2O3, yttria) phase, plus a small amorphous voltage, equipped with an energy dispersive X-ray spectrometer (EDS, component probably constituted by aluminum oxide, usually re- Oxford, UK) suitable for element identification. Each powder was presented by a typical diffuse profile centred at about 30° [18], evi- homogeneously dispersed in isopropanol by sonication. A small drop denced in this case by an increased background, (see Fig. S1 of the was deposited on a lacey-carbon grid of 300 mesh, and after complete Support Information, S.I). Reflections corresponding to the garnet solvent evaporation the nickel-copper grid was introduced into the TEM phase are dominant and evident peaks broadening indicates that crys- chamber analysis. tallites are nanometric sizes. Emission and excitation spectra of the nanoparticles were measured On the contrary, the powders doped with higher cerium amount are using the FLS980 Fluorescence Spectrometer from Edinburgh completely crystalline.