Effects of crystallization and dopant concentration on the emission behavior of TiO2:Eu nanophosphors
© Pal et al; licensee Springer. 2012
Received: 14 October 2011
Accepted: 3 January 2012
Published: 3 January 2012
Uniform, spherical-shaped TiO2:Eu nanoparticles with different doping concentrations have been synthesized through controlled hydrolysis of titanium tetrabutoxide under appropriate pH and temperature in the presence of EuCl3·6H2O. Through air annealing at 500°C for 2 h, the amorphous, as-grown nanoparticles could be converted to a pure anatase phase. The morphology, structural, and optical properties of the annealed nanostructures were studied using X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy [EDS], and UV-Visible diffuse reflectance spectroscopy techniques. Optoelectronic behaviors of the nanostructures were studied using micro-Raman and photoluminescence [PL] spectroscopies at room temperature. EDS results confirmed a systematic increase of Eu content in the as-prepared samples with the increase of nominal europium content in the reaction solution. With the increasing dopant concentration, crystallinity and crystallite size of the titania particles decreased gradually. Incorporation of europium in the titania particles induced a structural deformation and a blueshift of their absorption edge. While the room-temperature PL emission of the as-grown samples is dominated by the 5D0 - 7F j transition of Eu+3 ions, the emission intensity reduced drastically after thermal annealing due to outwards segregation of dopant ions.
Luminescent nanomaterials have gained considerable attention in recent years due to the breakthrough developments of technology in various areas such as electronics [1, 2], photonics , displays [4, 5], optical amplifications , lasers , fluorescent sensing , biomedical engineering,  and environmental control . The long emission lifetime and rich spectral properties of certain rare-earth [RE] ions are highly attractive in many ways. However, RE ions alone are weakly fluorescent due to the parity forbidden f-f transitions . Therefore, the use of host materials is crucial to excite the RE ions efficiently in a wide spectral range in order to utilize their full potential in optoelectronic devices. Oxide lattices have proved to be an excellent host material due to their good thermal, chemical, and mechanical stabilities [12, 13]. Among them, Y2O3 is a promising host for RE ions due to its low phonon frequencies, which make the nonradiative relaxation of the excited states inefficient . However, the high costs associated with synthesis have restricted its further use. As an alternative, TiO2, a well-known wide bandgap semiconductor, has demonstrated the possibility to be a good sensitizer to absorb light and transfer energy to RE ions. Moreover, the high refractive index and high transparency of TiO2 in the visible and infrared regions make it possible to use in optical devices. The additional advantages of using TiO2 are its low fabrication cost and good thermal and mechanical stabilities. However, due to the large mismatch of ionic radii (Eu+3 = 0.95 Å and Ti+4 = 0 0.68 Å) and charge imbalance between the Ti+4 and Eu+3 ions, successful incorporation of Eu ions into TiO2 nanocrystals through a soft, wet-chemical route still remains a great challenge. In most of the cases, Eu+3 ions either tend to locate on a crystal surface, causing an undesired Eu-Eu interaction, or form Eu2O3 aggregates, which act as quenching sites, resulting in a drastic decrease in the luminescent intensity . Numerous studies have been realized on the synthesis and optical characterization of Eu+3-doped TiO2 with the objective of improving the luminescence of the Eu+3 ions by energy transfer from TiO2. It has been reported that the mesoporous, semicrystalline TiO2 films are ideal matrices for incorporating Eu+3 ions in which the sensitized photoluminescence [PL] emission is due to the energy transfer from the TiO2 to Eu+3 ions in an amorphous TiO2 region . However, the emission intensity of Eu-doped TiO2 nanostructures has been found to reduce greatly or even disappear completely after annealing at high temperatures . In the literature, we can find several explanations for this behavior such as phase transition , segregation of Eu2O3 from TiO2 , or formation of a highly symmetric structure of Eu2Ti2O7 at high temperatures . Therefore, the fabrication of structurally pure, concentration-controlled, single-phase TiO2:Eu nanostructures with a controlled emission behavior is still a challenging task for their utilization in optoelectronics.
For the application in luminescent devices, small phosphor particles of a spherical morphology, narrow size distribution, and low dispersity are desired to improve their emission intensity and screen packing . To meet these demands, a variety of synthesis methods have been applied to fabricate RE-doped titania nanoparticles. Luo et al. could prepare Eu-doped TiO2 nanodots in the 50- to 70-nm size range by a phase-separation-induced self-assembly method . Yin et al. have studied the luminescence properties of spherical mesoporous Eu-doped TiO2 particles of 250 nm in diameter obtained through a nonionic surfactant-assisted soft chemistry method . Ningthoujam et al. could obtain Eu+3-doped TiO2 nanoparticles by urea hydrolysis in an ethylene glycol medium at a temperature of 150°C . Chi et al. have synthesized Eu-doped TiO2 nanotubes by a two-step hydrothermal treatment . On the other hand, Julian et al. could synthesize Eu+3-doped nanocrystalline TiO2 and ZrO2 by a one-pot sol-gel technique .
In the present work, we report the incorporation of Eu+3 ions in TiO2 nanoparticles by a simple and versatile sol-gel technique which could be extended to different lanthanide and transition metal ions in order to obtain multifunctional materials. The particles thus obtained have shown a perfectly spherical shape, improved size distribution, and excellent luminescent characteristics, elucidating the possibility of applying RE-doped titania nanoparticles as an efficient luminescent material. The dependence of the PL intensity of the nanophosphors on doping concentration and thermal annealing has been discussed.
Eu-doped TiO2 nanoparticles were prepared according to the following procedures: 2.5 ml of titanium tetrabutoxide (97%, Aldrich) was added slowly to 25 ml of anhydrous ethanol inside a glove box under nitrogen atmosphere and kept under magnetic stirring for 1 h at room temperature. Hydrolysis of the mixture was carried out by dropwise addition into 50 ml of deionized water inside a round-bottom flask under vigorous stirring. Prior to the addition, the pH of the water was adjusted to 3.0 by adding a nitric acid (0.1 M) solution in order to avoid the formation of europium hydroxide. The temperature of the mixture was maintained at 4°C to retard the hydrolysis rate.
Eu(III)-doped samples were prepared following the same procedure but dissolving the required amounts of Eu(NO3)2·6H2O corresponding to 0.5, 1, 2.5, and 5 mol% (nominal) in water before the addition of the Ti precursor. The white precipitate of TiO2 was separated through centrifugation, washed several times with water and ethanol, and finally dried at room temperature to obtain resulting materials. In order to induce crystallization, the as-grown samples (both the undoped and Eu-doped) were thermally treated at 500°C for 2 h in air atmosphere.
The crystalline phase of the nanoparticles was analyzed by X-ray diffraction [XRD] using a Bruker D8 DISCOVER X-ray diffractometer with a CuKα radiation (λ = 1.5406 Å) source. The size, morphology, and chemical composition of the nanostructures were examined in a JEOL JSM-6610LV field-emission scanning electron microscope [FE-SEM] with a Thermo Noran Super Dry II analytical system attached. The absorption characteristics of the synthesized samples in a UV-Visible [UV-Vis] spectral range were studied by diffuse reflectance spectroscopy (Varian Cary 500 UV-Vis spectrophotometer with DRA-CA-30I diffuse reflectance accessory). Micro-Raman spectra of the powder samples were acquired using an integrated micro-Raman system. The system includes a microspectrometer HORIBA Jobin Yvon HR800, an OLYMPUS BX41 microscope, and a thermoelectrically cooled CCD detector. The 332.6-nm emission of a He-Ne laser was used as the excitation source. PL measurements were performed at room temperature using a Jobin Yvon iHR320 spectrometer (HORIBA) with a 374-nm emitting diode laser as an excitation source.
Results and discussion
EDS estimated quantitative composition analysis of undoped and Eu-doped TiO2 nanoparticles
Nominal Eu concentration in the sample (mol%)
Lattice parameters and cell volume of different samples calculated from XRD results
Cell volume (Å3)
The position and FWHM of the Eg mode in the undoped and Eu-doped TiO2 nanoparticles
Position of the Eg mode (cm-1)
In conclusion, highly uniform, spherical-shaped Eu-doped TiO2 phosphor particles could be synthesized through a simple sol-gel technique at a large scale. The low-cost phosphor particles are about 50 nm in average diameter and have about 10% size dispersion. With the increasing nominal doping concentration up to 5.0 mol%, the average diameter of the particles reduces to 38 nm. Under ultraviolet excitation, the phosphor particles show the characteristic emission corresponding to the 5 D 0 - 7 F j transition of Eu+3 ions along with a broad band in the 400- to 500-nm range belonging to anatase TiO2. Thermal annealing-induced crystallization of the nanoparticles causes a drastic reduction of PL emission intensity, suggesting amorphous TiO2 as an ideal framework for an efficient energy transfer between the titania host and incorporated Eu+3 ions. The low fabrication cost, high yield, controlled morphology, and good luminescent performance of the as-grown TiO2:Eu+3 nanoparticles provide the possibility of using them as efficient red-emitting phosphors.
The work was supported by CONACYT, Mexico and VIEP-BUAP through the Red Temática de Nanociencia y Nanotecnología and VIEP/EXC/2011 projects, respectively. MP thanks Cuerpo Académico de Materiales Avanzados (BUAP-CA-250) for the partial financial support. The authors are sincerely thankful to Dr. Rutilo Silva (Institute of Physics) and Dr. Efrain (Centro Universitario de Vinculación) of the Autonomous University of Puebla for facilitating the EDS and XRD, respectively.
- Jianhua H, Guogen H, Xiongwu H, Rui W: Blue-light emission from undoped and rare-earth doped wide bandgap oxides. J Rare Earths 2006, 24: 728–731.View ArticleGoogle Scholar
- Linh NH, Trang NT, Cuong NT, Thao PH, Cong BT: Influence of doped rare earth elements on electronic properties of the R0.25Ca0.75MnO3systems. Comput Mater Sci 2010, 50: 2–5.View ArticleGoogle Scholar
- Joannopoulos JD, Villeneuve PR, Fan S: Photonic crystals: putting a new twist on light. Nature 1997, 386: 143–149.View ArticleGoogle Scholar
- Ghis A, Meyer R, Rambaud P, Levy F, Leroux T: Sealed vacuum devices: fluorescent microtip displays. IEEF Trans Electron Devices 1991, 38: 2320–2322.View ArticleGoogle Scholar
- Du YP, Zhang YW, Sun LD, Yan CH: Efficient energy transfer in monodisperse Eu-doped ZnO nanocrystals synthesized from metal acetylacetonates in high-boiling solvents. J Phys Chem C 2008, 112: 12234–12241.View ArticleGoogle Scholar
- Tanabe S, Sugimoto N, Ito S, Hanada T: Broad-band 1.5 μm emission of Er3+ions in bismuth-based oxide glasses for potential WDM amplifier. J Luminesc 2000, 87: 670–672.View ArticleGoogle Scholar
- DeLoach LD, Payne SA, Chase LL, Smith LK, Kway WL, Krupke WF: Evaluation of absorption and emission properties of Yb3+doped crystals for laser applications. IEEE J Quantum Electron 1993, 29: 1179–1191.View ArticleGoogle Scholar
- Soukka T, Kuningas K, Rantanen T, Haaslahti V, Lovgren T: Photochemical characterization of upconverting inorganic lanthanide phosphors as potential labels. J Fluoresc 2005, 15: 513–528.View ArticleGoogle Scholar
- Das GK, Yang-Tan TT: Rare-earth-doped and codoped Y2O3nanomaterials as potential bioimaging probes. J Phys Chem C 2008, 112: 11211–11217.View ArticleGoogle Scholar
- Xiao Q, Si Z, Yu Z, Qiu G: Sol-gel auto-combustion of samarium-doped TiO2nanoparticles and their photocatalytic activity under visible light irradiation. Mater Sci Eng B 2007, 137: 189–194.View ArticleGoogle Scholar
- Liu Y, Luo W, Li R, Chen X: Optical properties of Nd3+ion-doped ZnO nanocrystals. J Nanosci Nanotechnol 2010, 10: 1871–1876.View ArticleGoogle Scholar
- Wakefield G, Holland E, Dobson PJ, Hutchison JL: Luminescence properties of nanocrystalline Y2O3:Eu. Adv Mater 2001, 13: 1557–1560.View ArticleGoogle Scholar
- Wang L, Li Y: Na(Y1.5Na0.5)F6single-crystal nanorods as multicolor luminescent materials. Nano Lett 2006, 6: 1645–1649.View ArticleGoogle Scholar
- Wang H, Lin CK, Liu XM, Lin J, Yu M: Monodisperse spherical core-shell-structured phosphors obtained by functionalization of silica spheres with Y2O3:Eu3+layers for field emission displays. Appl Phys Lett 2005, 87: 181907–181909.View ArticleGoogle Scholar
- Luo M, Cheng K, Weng W, Song C, Du P, Shen G, Xu G, Han G: Enhanced luminescence of Eu-doped TiO2nanodots. Nanoscale Res Lett 2009, 4: 809–813.View ArticleGoogle Scholar
- Yin J, Xiang L, Zhao X: Monodisperse spherical mesoporous Eu-doped TiO2phosphor particles and the luminescence properties. Appl Phys Lett 2007, 90: 113112–113114.View ArticleGoogle Scholar
- Ningthoujama RS, Sudarsana V, Vatsaa RK, Kadam RM, Jagannath , Gupta A: Photoluminescence studies on Eu doped TiO2nanoparticles. J Alloys Compounds 2009, 486: 864–870.View ArticleGoogle Scholar
- Rocha LA, Avila RL, Caetano BL, Molina EF, Sacco HC, Ciuffi KJ, Calefim PS, Nassar E: Europium incorporated into titanium oxide by the sol-gel method. Mater Res 2005, 8: 361–364.View ArticleGoogle Scholar
- Zhao J, Duan H, Ma Z, Wang T, Chen C, Xie E: Temperature and TiO2content effects on the photoluminescence properties of Eu3+doped TiO2-SiO2powders. J Appl Phys 2008, 104: 053515–053515–5.View ArticleGoogle Scholar
- Li J-G, Wang X, Watanabe K, Ishigaki T: Phase structure and luminescence properties of Eu+3doped TiO2nanocrystals synthesized by Ar/O2radio frequency thermal plasma oxidation of liquid precursor mists. J Phys Chem B 2006, 110: 1121–1127.View ArticleGoogle Scholar
- Rubio MI, Ireland TG, Fern GR, Silver J, Snowden MJ: A new application for microgels: novel methods for the synthesis of spherical particles of Y2O3:Eu phosphor using a copolymer microgel of NIPAM and acrylic acid. Langmuir 2001, 147: 7145–7149.View ArticleGoogle Scholar
- Chi B, Victorio ES, Jin T: Synthesis of Eu-doped, photoluminescent titania nanotubes via a two-step hydrothermal treatment. Nanotechnology 2006, 17: 2234–2241.View ArticleGoogle Scholar
- Julian B, Corberan R, Cordoncillo E, Escribano P, Viana B, Sanchez C: One-pot synthesis and optical properties of Eu3+-doped nanocrystalline TiO2and ZrO2. Nanotechnology 2005, 16: 2707–2713.View ArticleGoogle Scholar
- Xu CY, Zhang PX, Yan L: Blue shift of Raman peak from coated TiO2nanoparticles. J Raman Spectroscopy 2001, 32: 862–865.View ArticleGoogle Scholar
- Popa M, Diamandescu L, Vasiliu F, Teodorescu CM, Cosoveanu V, Baia M, Feder M, Baia L, Danciu V: Synthesis, structural characterization, and photocatalytic properties of iron-doped TiO2aerogels. J Mater Sci 2009, 44: 358–364.View ArticleGoogle Scholar
- Ohsaka T: Temperature dependence of the Raman spectrum in anatase TiO2. J Phys Soc Jpn 1980, 48: 1661–1668.View ArticleGoogle Scholar
- Choi HC, Jung YM, Kim SB: Size effects in the Raman spectra of TiO2nanoparticles. Vibrational Spectrosc 2005, 37: 33–38.View ArticleGoogle Scholar
- Wendlandt W, Hecht HG: Reflectance Spectroscopy. New York: Wiley Interscience; 1966.Google Scholar
- Asahi R, Taga Y, Mannstadt W, Freeman AJ: Electronic and optical properties of anatase TiO2. Phys Rev B 2000, 61: 7459–7465.View ArticleGoogle Scholar
- Kumar S, Jindal Z, Kumari N, Verma NK: Solvothermally synthesized europium-doped CdS nanorods: applications as phosphors. J Nanopart Res 2011, 13: 5465–5471.View ArticleGoogle Scholar
- Yu J, Xiang Q, Zhou M: Preparation, characterization and visible-light-driven photocatalytic activity of Fe-doped titania nanorods and first-principles study for electronic structures. Appl Catalysis B: Environmetal 2009, 90: 595–602.View ArticleGoogle Scholar
- Coronado D, Gattorno G, Pesqueira ME, Cab C, Coss R, Oskam G: Phase-pure TiO2nanoparticles: anatase, brukite and rutile. Nanotechnology 2008, 19: 145605–145615.View ArticleGoogle Scholar
- Frindell KL, Bartl MH, Popitsch A, Stucky GD: Sensitized luminescence of trivalent europium by three-dimensionally arranged anatase nanocrystals in mesostructured titania thin films. Angew Chem Int Ed 2002, 41: 959–962.View ArticleGoogle Scholar
- Lei Y, Zhang LD: Fabrication, characterization and photoluminescence properties of highly ordered TiO2nanowire arrays. J Mater Res 2001, 16: 1138–1144.View ArticleGoogle Scholar
- Saraf LV, Patil SI, Ogale SB, Sainkar SR, Kshirsager ST: Synthesis of nanophase TiO2by ion beam sputtering and cold condensation technique. Int J Mod Phys B 1998, 12: 2635–2647.View ArticleGoogle Scholar
- Serpone N, Lawless D, Khairutdinov R: Size effects on the photophysical properties of colloidal anatase TiO2particles: size quantization versus direct transitions in this indirect semiconductor. J Phys Chem 1995, 99: 16646–16654.View ArticleGoogle Scholar
- Liu Y, Claus RO: Blue light emitting nanosized TiO2colloids. J Am Chem Soc 1997, 119: 5273–5274.View ArticleGoogle Scholar
- Zhang WF, Zhang MS, Yin Z: Microstructures and visible photoluminescence of TiO2nanocrystals. Phys Stat Sol 2000, 179: 319–327.View ArticleGoogle Scholar
- Ningthoujam RS, Sudarsan V, Godbole SV, Kienle L, Kulshreshtha SK, Tyagi AK: SnO2:Eu3+nanoparticles dispersed in TiO2matrix: improved energy transfer between semiconductor host and Eu3+ions for the low temperature synthesized samples. Appl Phys Lett 2007, 90: 173113–17115.View ArticleGoogle Scholar
- Li L, Tsung CK, Yang Z, Stucky GD, Sun LD, Wang JF, Yan CH: Rare-earth-doped nanocrystalline titania microspheres via energy transfer. Adv Mater 2008, 20: 903–908.View ArticleGoogle Scholar
- Frindell KL, Bartl MH, Robinson MR, Bazan GC, Popitsch A, Stucky GD: Visible and near IR luminescence via energy transfer in rare earth doped mesoporous titania thin films with nanocrystalline walls. J Solid State Chem 2003, 172: 81–88.View ArticleGoogle Scholar
- Alejandre E, Torres M, Hipolito M, Frutis M, Flores G, Mendoza J, Falcony C: Structural and luminescent properties of europium doped TiO2thick films synthesized by the ultrasonic spray pyrolysis technique. J Phys D: Appl Phys 2009, 42: 095102–095109.View ArticleGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.