Photoluminescent Layered Y(III) and Tb(III) Silicates Doped with Ce(III) Mariya H. Kostova, ² Rute A. Sa ´ Ferreira, ‡ Duarte Ananias, ²,§ Luı ´s D. Carlos,* ,‡ and Joa ˜ o Rocha* ,² Department of Chemistry, and Department of Physics, CICECO, UniVersity of AVeiro, 3810-193 AVeiro, Portugal, and Department of Biochemistry, NMR Centre and Centre of Neurosciences and Cell Biology, UniVersity of Coimbra, 3001-401 Coimbra, Portugal ReceiVed: May 5, 2006; In Final Form: June 6, 2006 The synthesis and structural characterization of new layered rare-earth silicates K 3 [M 1-a Ce a Si 3 O 8 (OH) 2 ], M ) Y 3+ , Tb 3+ , a , 1 (AV-22 materials), have been reported. These materials combine the properties of layered silicates, such as intercalation chemistry, and photoluminescence and may find applications in new types of sensor devices. For mixed Tb/Ce-AV-22, evidence has been found for the energy transfer from the large Ce 3+ 4f 1 f 5d 1 broad band to the sharp Tb 3+ 4f 8 lines. This energy transfer allows the fine-tuning of the color emission in the blue-green region of the chromaticity diagram. Upon Ce 3+ excitation (342 nm), the radiance of Tb/Ce-AV-22 is approximately 2 times higher than that measured under direct Tb 3+ excitation, which reinforces the existence of effective room-temperature Ce 3+ -to-Tb 3+ energy transfer. 1. Introduction Layered 1 microporous silicates are host-guest systems suit- able for engineering multifunctional materials with tuneable properties. 2,3 In particular, it has been shown that solids combining the properties of layered silicates (intercalation chemistry) or zeolites (such as ion exchange and molecular sieving) and photoluminescence (PL) may be obtained by inserting Ln 3+ cations into the frameworks, layers, micropores, and interlayer spaces. 4 While research into zeolites made photoluminescent by Ln-doping via ion-exchange is not new, the preparation of layered and zeolite-type stoichiometric Ln silicates is an emerging field. 5 Because the 4f 1 f 5d 1 transition of Ce 3+ is parity allowed, its absorption cross section is much larger than that of the intra- 4f lines, and thus this ion may act as an Ln 3+ sensitizer. The Ce 3+ -to-Ln 3+ (mainly Tb 3+ ) energy transfer has been much studied in the past decades and shown to depend on the host lattice and the extent of the overlap between the Ce 3+ emission and the Tb 3+ excitation spectrum. 6-10 In this context, one of the main motivations has been the development of new, highly efficient, green-emitting phosphors to be used in the low- pressure mercury vapor lamps. Ce 3+ -containing materials also find potential applications in optoelectronics. Ce 3+ in a LiY 6 matrix has been shown to lase, and this is particularly important because Ce 3+ lasers emit in the blue or ultraviolet and exhibit tunability, due to the inherently broad emission lines. 11 A microporous, stoichiometric, photoluminescent cerium silicate has also been described. 3 Recently, we have reported the synthesis, structure, and PL properties of mixed layered Ln silicates K 3 [M 1-a Ln a Si 3 O 8 (OH) 2 ], M ) Y 3+ , Tb 3+ , Ln ) Eu 3+ , Er 3+ , Tb 3+ , and Gd 3+ , named AV- 22 materials. 1 Building upon this work, we now wish to report on the related system K 3 [M 1-a Ce a Si 3 O 8 (OH) 2 ], M ) Y 3+ , Tb 3+ , a , 1. The emission spectra, the (x,y) chromaticity coordinates, radiance, and time-decay measurements clearly show that the incorporation of Ce 3+ and Tb 3+ ions into the same layered silicate induces an effective Ce 3+ to Tb 3+ energy transfer channel. 2. Experimental Methods 2.1. Synthesis. The syntheses of AV-22 materials have been described. 1 In a typical K 3 [TbSi 3 O 8 (OH) 2 ] synthesis, an alkaline solution was made by mixing 1.24 g of precipitate SiO 2 (93% m/m SiO 2 , Riedel-de Hae ¨n), 20.32 g of H 2 O, and 9.12 g of KOH (Merck). An amount of 0.82 g of TbCl 3 ‚6H 2 O (Aldrich) was added to this solution, and the mixture was stirred thoroughly. The gel, with composition 4.23 K 2 O:1.0 SiO 2 :0.06 Tb 2 O 3 :58 H 2 O was transferred to Teflon-line autoclaves, and the hydrothermal synthesis was carried out under autogenous pressure for 7 days at 230 °C. The obtained microcrystalline powders were filtered, washed at room temperature with distilled water, and dried at 100 °C. Mixed Y/Ce and Tb/Ce were prepared by introducing in the parent gel 5% Ce. 2.2. Characterization. Within experimental error, chemical analysis by EDS confirmed the K:Ln:Si (Ln ) Y or Tb) molar ratios obtained by powder XRD, ca. 3:1:3. ICP-AES chemical analysis yielded ca. 3.2 and 2.1 mol % Ce for, respectively, the Tb/Ce and Y/Ce samples, despite the fact that larger Ce amounts were present in the parent synthesis gel. All AV-22 materials were characterized by powder X-ray diffraction (XRD) on an X’Pert MPD Philips diffractometer (Cu KR radiation) fitted with a curved graphite monochromator and scanning electron mi- croscopy (SEM) performed on a Hitachi S-4100 microscope. Y-AV-22 was also characterized by 29 Si magic-angle spinning (MAS) NMR. The photoluminescence (PL) and lifetimes measurements were recorded on a Fluorolog-3 (FL3-2T model) with double excitation spectrometer, fitted with a 1200 grooves/mm grating blazed at 330 nm, and a single emission spectrometer (TRIAX 320), fitted with a 1200 grooves/mm grating blazed at 500 nm, coupled to a R928P photomultiplier. The excitation sources were * Corresponding author. Phone: +351234370946. Fax: +351 234424965. E-mail: lcarlos@fis.ua.pt (L.D.C.); rocha@dq.ua.pt (J.C.R.). ² Department of Chemistry, University of Aveiro. ‡ Department of Physics, University of Aveiro. § University of Coimbra. 15312 J. Phys. Chem. B 2006, 110, 15312-15316 10.1021/jp062758w CCC: $33.50 © 2006 American Chemical Society Published on Web 07/14/2006