Hydrothermal Synthesis, Microstructure and Photoluminescence of Eu3+-Doped Mixed Rare Earth Nano-Orthophosphates
© The Author(s) 2010
Received: 19 April 2010
Accepted: 5 August 2010
Published: 18 August 2010
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© The Author(s) 2010
Received: 19 April 2010
Accepted: 5 August 2010
Published: 18 August 2010
Eu3+-doped mixed rare earth orthophosphates (rare earth = La, Y, Gd) have been prepared by hydrothermal technology, whose crystal phase and microstructure both vary with the molar ratio of the mixed rare earth ions. For La x Y1–x PO4: Eu3+, the ion radius distinction between the La3+ and Y3+ is so large that only La0.9Y0.1PO4: Eu3+ shows the pure monoclinic phase. For La x Gd1–x PO4: Eu3+ system, with the increase in the La content, the crystal phase structure of the product changes from the hexagonal phase to the monoclinic phase and the microstructure of them changes from the nanorods to nanowires. Similarly, Y x Gd1–x PO4: Eu3+, Y0.1Gd0.9PO4: Eu3+ and Y0.5Gd0.5PO4: Eu3+ samples present the pure hexagonal phase and nanorods microstructure, while Y0.9Gd0.1PO4: Eu3+ exhibits the tetragonal phase and nanocubic micromorphology. The photoluminescence behaviors of Eu3+ in these hosts are strongly related to the nature of the host (composition, crystal phase and microstructure).
Nanostructure materials with controlled chemical composition, crystal phase structure, morphology and particle size have been extensively investigated during the past few decades because of their high surface/volume ratio and the special quantum confinement effect [1, 2]. Nanomaterials can show remarkable tunable properties and play an important role as active components in the preparation of nanoscale electronic, optical, optoelectronic, electrochemical and electromechanical devices [3–7]. Herein, the fabrication of nanomaterials with well-controlled dimensionality, morphologies, phase purity, chemical composition and desired properties remains one of the most challenging issues . One simple solution to control the particle size and morphology is soft chemistry routes and in particular the hydrothermal process, which is extensively employed in the synthesis of rare earth ions activated inorganic compounds, such as yttrium vanadate, lanthanum fluoride, lanthanum phosphate and yttrium oxide [9–11].
Because of their excellent luminescent properties, rare earth orthophosphates have been extensively applied as phosphors, laser hosts, heat resistant materials and moisture sensors, whose crystal structure and synthesis technology have been studied long time ago [12, 13]. For example, LaPO4: Ce, Tb phosphors have been used as green emission component of tri-chromatic luminescent lamp [14, 15]. Presently, it is important to synthesize rare earth orthophosphate phosphors with regular morphology, composition and size. Ever since Meyssamy et al. has fabricated LaPO4: Eu and LaPO4: Tb nanocrystals by a simple hydrothermal method, lots of works have been focused on the study of rare earth phosphate nanocrystals [16–27]. The crystal structure of pure LnPO4 compounds can be changed with the decrease in Ln ionic radius: i.e., the orthophosphates structure from Ho to Lu as well as Y only exist in the tetragonal zircon (xenotime) structure, while the lanthanide orthophosphates structure (Ln = La~Dy) exist in the hexagonal structure under hydrothermal treatment . Mixed orthophosphates composed of two rare earth elements have also been investigated, indicating that these phosphates can be used as host lattices for spectroscopic investigations [29–35].
For REPO4 phosphor of light RE3+ with larger ion radius, the monoclinic crystal phase structure is preferred. For REPO4 phosphor of middle RE3+ with intermediate radius, a partly hydrated hexagonal structure is favorable. For REPO4 phosphor of heavy RE3+ with smaller radius, a tetragonal crystal phase is adopted. Therefore, it is very interesting that what will happen when rare earth ions with different radii are introduced into one REPO4 systems with PO4 3−. In this text, we have investigated the crystal phase structures, microstructure (morphology and particle size) of the mixed orthophosphates REPO4 (RE = La, Gd, Y) prepared by a facile hydrothermal technology. Because of the difference in ion radii between these rare earth ions, the crystal phase and microstructure of the products show obvious differences. At the same time, Eu3+ ions have been doped in the mixed rare earth phosphates in order to examine the influence of the hosts on the luminescence of Eu3+, whose photoluminescent behaviors are studied in detail.
The starting materials La2O3, Y2O3, Gd2O3 and Eu2O3 are firstly dissolved into concentrated nitric acid, and the appropriate volume of deionized water was added to form the 0.2 mol l−1 RE(NO3)3 (RE = Y, La; La, Gd; Y, Gd) and 0.02 mol l−1 Eu(NO3)3, respectively. Then, the mixed orthophosphates doped with Eu3+ nanophosphors are synthesized by the hydrothermal process, which are described in the following: the different volume of Y(NO3)3, La(NO3)3 (Gd(NO3)3, La(NO3)3; Y(NO3)3, Gd(NO3)3) and Eu(NO3)3 (1:0.05 in molar ratio) solutions are mixed with appropriate amounts of NH4H2PO4 to form the emulsion. The final pH value is adjusted to 3.0 with HNO3 solution (1 M). After being stirred, the milky colloid precursor is obtained, suggesting that the nanoscale particle formation already occurred. In order to make the products to crystallize well, the milky colloid precursor is poured into closed Teflon-lined autoclaves to be treated by hydrothermal process (pressure 2.8 MPaG, 0Cr18Ni9Ti stainless steel outdoor shell, 25 mL, safe temperature 200°C, Peking University Qingniao Company, China) at 160°C for 3 days. The resulting product is filtered, washed with deionized water and absolute alcohol to remove ions possibly remaining in the final products Y x La1–x PO4: 5% Eu3+, LaxGd1-xPO4: 5% Eu3+, Y x Gd1–x PO4: 5% Eu3+, respectively, (x = 0.1, 0.5, 0.9) and finally dried at 60°C in air for further characterization.
The X-rays powder diffraction (XRD) patterns of all samples are performed on a Bruke/D8-Advance with CuK α radiation (λ = 1.540 Å), whose operation voltage and current are maintained at 40 kV and 40 mA, respectively. Transmission electron microscopic (TEM) images are obtained on a JEOL 2010 microscope with an accelerating voltage of 200 kV. The excitation and emission spectra are recorded with RF-5301 spectrophotometer (resolution used in the excitation and emission spectra measurement is 1 nm). All spectra are normalized to a constant intensity at the maximum. Luminescence lifetime measurements are carried out on an Edinburgh FLS920 phosphorimeter using a 450 W xenon lamp as excitation source. A Netzsch thermoanalyzer, STA 409, is used for simultaneous thermal analysis combining the thermogravimetry (TG) and differential scanning calorimetry (DSC) with a heating rate of 10°C min−1.
Generally speaking, the inherent crystal structure determines the crystal growth habitual behavior and final morphology. The hexagonal phase crystal of Y x La1–x PO4 commonly appears the anisotropic growth, in which exists apparent layer-like structure along C axle while not along other axles. So it prefers to grow along c axle to release more energy and form the more stable system than other two directions [18, 37, 38]. Finally, Y x La1–x PO4 with hexagonal phase will grow to form nanowire or nanorod along  direction. On the other hand, the tetragonal phase of mixed orthophosphate does not possess apparent layer-like structure and cannot show the dominated growth direction, resulting in the irregular nanoparticles. For pure monoclinic La0.1Y0.9PO4: Eu3+, crystal structure consists of isolated PO4 tetrahedron and REO9-PO4 chain parallel with c axle. So the crystal still prefers to grow along the  direction to make crystal system more stable in spite of that the existence of the chain is not so apparent as layer-like structure of hexagonal phase [18, 37, 38].
Photoluminescence lifetimes for La x Gd1–x PO4 Eu3+ and Y x Gd1–x PO4: Eu3+ Nanophosphors
In summary, the Eu3+ activated rare earth phosphate (Y x Gd1–x PO4, La x Gd1–x PO4 and Y x La1–x PO4) nanophosphors (x = 0.1, 0.5, 0.9) have been synthesized by hydrothermal technology. The crystal phase and microstructure of the products are strongly depended on the difference in the ion radii of rare earth elements. For Y 3+ and La3+ ions, the difference in the radii is so large that Y x La1–x PO4 cannot be favorable for the formation of the pure phase except that Y0.1La0.9PO4 powders present the pure monoclinic phase and nanowires. As for Y x Gd1–x PO4 and La x Gd1–x PO4, the radii difference between two rare earth ions cannot make a big influence on the crystal structure and the morphology. With the increase in the Y content in Y x Gd1–x PO4, the structure of the product has been changed from the hexagonal phase to the tetragonal phase and the morphology from nanorods to nanowires. Similarly, La x Gd1–x PO4 (x = 0.1, 0.5) powders have the hexagonal phase and La0.9Gd0.1PO4 belongs to the monoclinic phase. With the increase in the La3+ content, the ratio of the length to width has been changed. Y0.1Gd0.9PO4: Eu3+ and La0.5Gd0.5PO4: Eu3+ nanophosphors present the longest lifetime in the corresponding series. These lanthanide phosphates can be expected to have some potential applications in such fields as fluorescent lamps, plasma display panels and luminescent probes or labels for biomolecule system.
The work is supported by the Science Fund of Shanghai University for Excellent Youth Scientists and National Natural Science Foundation of China (20971100).
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