Evidence of Trivalent Europium Incorporated in Anatase TiO 2 Nanocrystals with Multiple Sites Wenqin Luo, †,‡ Renfu Li, †,‡ Guokui Liu, § Mark R. Antonio, § and Xueyuan Chen* ,†,‡ Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China, State Key Laboratory of Structural Chemistry, Fuzhou, Fujian 350002, China, and Chemical Sciences and Engineering DiVision, Argonne National Laboratory, Argonne, Illinois 60439 ReceiVed: February 21, 2008; ReVised Manuscript ReceiVed: April 17, 2008 Trivalent europium ions have been successfully incorporated in anatase TiO 2 nanocrystals via a sol-gel solvothermal synthesis, in spite of a large mismatch in ionic radius between Eu 3+ and Ti 4+ . The photoluminescence intensity of Eu 3+ :TiO 2 nanocrystals has been significantly improved, which is comparable to that of common red phosphors. Multiple sites of Eu 3+ in TiO 2 nanocrystals have been identified by the technique of site-selective spectroscopy. Eu 3+ ions at two lattice sites exhibit sharp emission and excitation peaks with site symmetries descending from D 2d to approximate C 2v and D 2 as a result of the lattice distortion, whereas Eu 3+ ions at disordered site near the surface are analogous to Eu 3+ ions located in a glasslike environment. Extended X-ray absorption fine structure is utilized for identifying the local structure and Eu 3+ coordination. The luminescence dynamics and crystal-field levels of Eu 3+ at different sites have been analyzed. A growth mechanism for the incorporation of Eu 3+ in the anatase lattice is also suggested. 1. Introduction Recently, rare-earth (RE) ions doped semiconductor nano- phosphors have attracted extensive attention due to their unique optical properties and prospective applications in optoelectronic devices, flat plane displays, and biosensors. 1–7 It is expected that RE luminescence in this new class of nanomaterials can be efficiently sensitized by exciton recombination in nanocrys- tals. The exciton Bohr radius of semiconductors is much larger than that of insulators, 8 which could result in pronounced quantum confinement effect for small nanoparticles (e.g., 2-10 nm for CdS, ZnO, and TiO 2 ). As a result, the optical properties of RE incorporated in semiconductor nanocrystals could be tailored via size control or band gap engineering, which is very attractive in fabricating a nanodevice for technological applica- tions. Titania is a well-known wide band gap semiconductor (band gap of 3.2 eV for anatase 9 ) and a good candidate to be used as the host material of RE ions because of its good mechanical, optical, and thermal properties. 10 The luminescence properties of RE in nanocrystals depend critically on their locations in the host. If RE ions have similar chemical properties (viz., the same oxidation state and similar ionic radius) to the host metallic ions, it is easy to incorporate RE ions into the host lattice to replace the cations. Such is the case for most of the insulating nanocrystals like Y 2 O 3 and NaYF 4 . However, a key issue for the doped semiconductor nanocrystals is whether RE ions are really being incorporated into the lattice since their chemical properties usually differ from those of the host cations. For example, Bol et al. argued that RE ions may not reside on the lattice of the II-VI (ZnS and CdS) semiconductor nanocrystals but probably locate on the surface site. 2 Similarly, due to a large mismatch in ionic radius between RE 3+ and Ti 4+ (0.086-0.103 nm versus 0.061 nm, respectively, for coordination number VI) 11 and their charge imbalance, it is difficult to incorporate RE 3+ ions into the TiO 2 lattice through chemical methods. Various routes were devel- oped in an effort to incorporate RE 3+ into TiO 2 nanoparticles and improve the efficiency of RE 3+ luminescence. 10,12–20 Sharp emission lines were reported in Eu 3+ (0.5 at %) doped TiO 2 nanomixture of anatase and rutile synthesized by Ar/O 2 radio frequency thermal plasma. 15 The origin of these emissions, photoluminescence (PL) from Eu 3+ in anatase or rutile phase, remains unclear. The Stark level splittings were observed in the PL spectra of Sm 3+ doped anatase titania powders. 21 However, to date, little spectroscopic evidence for the incor- poration of Eu 3+ into anatase nanocrystals has been presented. How to synthesize pure phase TiO 2 nanophosphors with Eu 3+ ions incorporated into the TiO 2 lattice still remains a challenge. In this article, we report a facile method to effectively incorporate Eu 3+ ions into the anatase nanocrystals. We explicitly prove the existence of multiple sites of Eu 3+ in TiO 2 nanocrystals by means of site-selective spectroscopy. Very sharp and intense excitation and emission lines due to the crystal- field (CF) splitting of Eu 3+ in TiO 2 nanocrystals are observed in the low-temperature PL experiments. Finally a growth mechanism is proposed to illustrate how Eu 3+ ions are incor- porated into the TiO 2 lattice. 2. Experimental Section 2.1. Nanoparticle Synthesis. Rare earth ions doped TiO 2 nanoparticles are generally prepared by sol-gel method, however, in this method, the morphology of the samples can hardly be controlled. 21,22 In this work, a facile sol-gel solvo- thermal method was introduced to prepare nearly spherical Eu 3+ : TiO 2 nanoparticles with nominal dopant concentration of 2 atom %. In detail, 1 mL of tetra(n-butyl)titanate (Sinopharm, 98.0%) was dissolved in 20 mL of ethanol (Sinopharm, 99.7%), to * Corresponding author. Phone and fax: +86-591-8764-2575. E-mail: xchen@fjirsm.ac.cn. † Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences. ‡ State Key Laboratory of Structural Chemistry. § Argonne National Laboratory. J. Phys. Chem. C 2008, 112, 10370–10377 10370 10.1021/jp801563k CCC: $40.75  2008 American Chemical Society Published on Web 06/20/2008