Article pubs.acs.org/JPCC
Tailoring the Luminescence of Europium Ions in Mesoporous AlPO4 Monolithic Glass Jin He,†,‡ Yan Wang,†,‡ Yang Liu,†,‡ Kangpeng Wang,†,‡ Rihong Li,*,† Jintai Fan,† Shiqing Xu,§ and Long Zhang*,† †
Key Laboratory of Materials for High Power Lasers, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, 201800, Shanghai, China ‡ University of Chinese Academy of Sciences, 100049, Beijing, China § College of Materials Science and Engineering, China Jiliang University, 310018, Hangzhou, Zhejiang, China ABSTRACT: Controllable simultaneous luminescent monolithic mesoporous AlPO4 glasses doped with Eu ions were fabricated by dipping into a Eu-containing solution, followed by sintering in air. Reduction of partial Eu3+ ions to Eu2+ were achieved after annealing Eu3+ ions in AlPO4 meso-structure, which resulted in simultaneous luminescence of Eu3+ (590 and 613 nm) and Eu2+ (450 nm). Controllable luminescence and color tunability from blue to red were liable to obtain, which were characterized by emission spectra and described by CIE diagram. A deliberate tailoring of luminescence of Eu2+ and Eu3+ is achieved by controlling the Eu ions concentrations, sintering temperatures, and excitation wavelengths. Reduction and oxidation reactions of Eu ions were investigated by controlling the sintering atmospheres. XRD pattern indicated the typical amorphous glassy phase without obvious phase-separation even in high concentration (0.1 M). The presence of Eu2+ ions, which were reduced from Eu3+ by hole−electron pairs and diffused into mesoporous structure, was revealed by XPS spectra.
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INTRODUCTION Tunable light sources, especially white-light sources based on LEDs have a considerable impact on significant issues such as energy consumption, environment, and health of individuals.1−3 LEDs with wide tunability, even white-light emission have several key advantages over traditional LED, such as long lifetime, low energy consumption, small size, and design flexibility, which are bringing about a revolution in energyefficient lighting.4,5 Recent advances in LED lighting technology to generate tunable and white light emission have led to a new approach, which is utilizing near-ultraviolet or blue LED sources to excite bi- or tri-color phosphors.6−8 The key challenges for this approach are obtaining suitable phosphors and developing appropriate host matrixes. Phosphors containing Eu3+ ions as activators are widely used to emit red color due to the f−f transition. By comparison, the 4f−5d transition of the Eu2+ is more sensitive to the surroundings than Eu3+. It also exhibits a broad emission band from blue to red. Thereof, the fluorescent color can be tuned by controlling the ratio of Eu3+/Eu2+. However, orchestrating the coexistence of Eu3+/Eu2+ activators in a single host is still a formidable task, especially for the appearance of characteristic emission in order to generate white light.7,9 Considerable research has been pursuing the development of efficient Eu-phosphors based on melt glasses, porous sol gel silicate glasses or other organic/inorganic host materials.10−12 Due to the instability of Eu2+ ions in air, it is either difficult to obtain Eu2+ emission or impossible to achieve controllable simultaneous emission of Eu3+ and Eu2+. Few reports present investigations on simultaneous emission of Eu2+ and Eu3+ by single Eu3+-doped phosphors or materials.13 Much © XXXX American Chemical Society
effort has been made to seek suitable host materials to stabilize Eu2+ in air by their controlling of chemical modification.14−16 The porous glasses are proven to be attractive materials for Eudoped solid state lighting.17 Likely, The conventional sol−gel derived silicate porous glasses possess a neutral rigid silica network, thus, the reduction of Eu3+ to Eu2+ needs other metal ions codoped, for instance, Al ions, to form excessive negative charge in the silica host. Nevertheless, the reduction reaction should happen in high temperature (above 800 °C), and it cannot control the ratio of Eu3+/Eu2+.11 In this case, it is promising to develop an efficient approach to achieve tunable simultaneous emission of Eu3+ and Eu2+ ions in one single host matrix. We already reported a simple aqueous sol−gel route yielding transparent and colorless stoichiometric AlPO4 monolith glass. In contrast to silicate materials, it possesses mesoporous structure and more chemical activity to benefit the solidification of dopant.18−20 Furthermore, AlPO4 provides a suitable reductive atmosphere in the mesoporous network that dominates the stabilization of Eu2+ ions. Hereby, it emerges as a promising host for tunable light emission of Eu ions. In present work, we introduce an effective approach to control the tunable emission of Eu-doped AlPO4 mesoporous glasses from blue to red region across the (Commission Internationale de L’Eclairae; CIE; 1931) chromaticity diagram. To the best of our knowledge, there is no report on Eu ion-doped mesoporous AlPO4 glasses with controllable simultaneous fluorescence of Received: May 8, 2013 Revised: September 25, 2013
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dx.doi.org/10.1021/jp407125e | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
Eu2+/Eu3+. We deliberately tailor the fluorescence of Eu ions by controlling various parameters, including dipping concentrations, sintering temperatures, and excitation wavelengths, as well as atmosphere conditions. The essential reaction mechanism is revealed and studied by a series of optical and XPS spectra.
3. RESULTS AND DISCUSSION 3.1. XRD. It is well-known that rare earth ions incorporated in porous glasses have a tendency toward crystallization and result in phase separation of porous glasses easily after being heated in high temperature. Thereof it is challenging to immobilize rare earth ions in high concentration without serious aggregates. Much effort was made to seek a homogeneous nanoscale porous host which could probably solve above problems.10,11 XRD patterns were employed to assess the crystallization of Eu ions and phase separation of glasses. An identical broad band between 18 and 40° (2θ) ascribed to amorphous glass presents in Figure 1.
2. EXPERIMENTAL SECTION 2.1. Samples Preparation. Transparent and colorless AlPO4 mesoporous glasses were prepared via a simple aqueous sol−gel route in our previous work and characterized by BET surface area measurements obtained from a Micromeritics ASAP 2010 volumetric adsorption analyzer with N2 as adsorbent at 77 K.18 Mesopore size distributions were calculated according to the BJH (Barrett−Joyner−Halenda) method. The obtained monolithic AlPO4 mesoporous glass possesses a surface area as high as 464 m2/g and an average pore diameter of 5.0 nm. A total of 200 mg of the mesoporous glasses were immersed into 10 mL of ethanolic solution with different concentrations (C) of Eu(NO3)3·6H2O from 1.0 × 10−1 to 1.0 × 10−3 M for 16.0 h (Table 1), followed by washing with absolute ethanol to Table 1. AlPO4 Mesoporous Glasses Eu Ion-Doped in Eu(NO3)3 Concentrations (C) sample ID glass-a glass-b glass-c glass-d glass-e
C (M) 1.0 5.0 1.0 2.0 1.0
× × × × ×
10−3 10−3 10−2 10−2 10−1
Figure 1. XRD patterns of Eu-doped (glass-e) and undoped AlPO4 glass.
It demonstrates that the AlPO4 glasses doped with and without Eu ions are an amorphous state, as there are no visible diffraction peaks present. Eu ions were thereby homogeneously separated by a mesoporous framework of AlPO4 without crystallization. It also suggests no apparent aggregates of Eu ions in the glass. 3.2. Optical Properties. Simultaneous luminescence of Eu2+ and Eu3+ in one single host encounters grave difficulties in obtaining stable and controllable Eu2+ ions. One possible approach to overcome such problems is the incorporation of Eu ions in mesoporous glasses with a reductive environment.21−24 The emission spectra of samples with different Eu(NO3)3 concentrations were displayed in Figure 2. Obviously simultaneous luminescence of Eu2+ and Eu3+ ions present at their characteristic emission with the maximum around 450, 590, and 613 nm, due to 4f65d1 → 4f 7 transition of
remove the absorbed ions on the surface of glasses, subsequently dried at 50 °C in air for 24 h. Then these samples were annealed at different temperatures, as requested, for 8 h and stored in a vacuum desiccator. 2.2. Characterization. The emission and excitation spectra were performed utilizing a HORIBA JobinYvon IBH FL-322 Fluorolog 3 spectrometer equipped with a 450 W xenon arc lamp, double grating excitation, and emission monochromators (2.1 nm/mm dispersion, 1200 grooves/mm), and a Hamamatsu R928 photomultiplier tube or a TBX-4-X single-photoncounting detector. Spectroscopic properties were measured by reflection on monolithic AlPO4 glasses with a thickness of 0.5 mm. All emission spectra have been normalized and arbitrarily shifted vertically for clarity. A front-face holder for samples was used to clip and oriented at 90° to minimize the specular reflection from the excitation beam. Suitable cutoff filters were employed, and the acquired spectra were corrected for the optical transfer function of the systems. All the emission and excitation spectra bandwidths were set to 2 nm, and the emission spectra were normalized at 450 nm for comparison. The lifetime was measured by FLS920 spectrometer. The absorption spectra were recorded with Perkin-Elmer Lambda 750 UV/vis/NIR spectrometer with a spectral resolution of 1 nm. The high resolution XPS measurements were operated on Kα (Thermo Fisher Scientific, U.S.A.) in ultrahigh vacuum (pressure
