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.
Keywordstitania nanoparticles europium doping optical properties photoluminescence
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.
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