Investigation on the physicochemical properties of La-doped Er0.05Y1.95O3 nanopowders
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A series of high-purity Er0.05Y1.95O3 nanopowders with different lanthanum content was prepared by modification of the Pechini sol–gel method using citric acid and ethylene glycol as the chelating agent. The microstructure of the powders was studied by means of X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy. In order to evaluate the structural characteristics of the obtained gel, XRD measurements were carried out with calcination gels in selected temperatures. Simultaneous differential thermal analysis with thermal gravimeters indicates a decrease of calcination temperature with an increasing content of lanthanum ions. Morphological properties of the nano-sized powders were examined by scanning electron microscopy. Strong luminescence in near IR region was observed under 980 nm excitation at room temperature. By varying the concentration of La3+ ion, various intensities of upconversion luminescence can be easily achieved.
KeywordsPowders Citrate process Yttrium oxide Luminescence
Recently, new types of optical devices, such as high-energy laser, optical fiber amplifiers, and optical rotators, have attracted significant attention due to their unique physical properties. Y2O3, either pure or often doped with rare-earth ions, has been widely investigated as a host material for potential applications in lasers, three dimensional volumetric displays, fluorescent labels, and luminous pipes for high-intensity discharge lamps [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13]. The performance of these devices is mainly dependent on the physicochemical properties of the powders that were sintered. The powders are useful due to their good chemical and physical stability, high corrosion resistance, and broad transparency range (0.2–10 μm) [1, 2, 5]. The recent research and reports have shown the relatively low phonon energy (430–550 cm−1) [1, 2, 3] and high thermal conductivity [13.6 W(mK)−1] [1, 2, 3, 4]. Pure yttrium oxide exhibits C-bixbyite, cubic, hexagonal, or monoclinic crystal structure [5, 6, 7, 8, 9, 10]. Many different chemical methods, such as sol–gel EDTA, hydrothermal, co-precipitation method, spray pyrolysis, thermochemical reactions, and solid state reaction, were widely used to synthesize nano-sized powders of pure and doped Y2O3 [10, 11, 12, 13, 14, 15]. Furthermore, citrate sol–gel process is one of wet chemical methods that receives attention due to its high efficiency and possibility to prepare high clean and nano-sized powders [4, 6, 7, 8, 9, 13, 15]. Numerous studies conducted thus far indicate that the choice of preparation method influences the size, shape, and crystalline structure of powders, which determine optical, transmittance, and luminescence properties of sinters [1, 2, 3, 4, 5]. The rare-earth metal ions, e.g.: Nd, Er, Eu, and Tm, have been successfully used as an additive material in order to improve the performances of Y2O3 materials [3, 4, 5, 6, 7, 10, 11, 12]. Many papers have indicated that the doping of Y2O3 with erbium ions allows for luminescence emission in the infrared range [3, 14]. Additional substitution of La ions can also enhance luminescence in Er:Y2O3 by tuning the symmetry of the crystal field around the rare-earth ions . Notably, doped with rare-earth ions, Y2O3 may be used as a magneto-optic ceramic potential to build a magneto-optic device [4, 16]. This device is based on Faraday phenomena, and it uses external magnetic fields. Criteria for magneto-optical material are high transparence, chemical stability, and high Verdet constant. Nano-sized yttria meets the first two criteria. The Verdet constant V may be improved by doping with diamagnetic or/and paramagnetic ions, because it is the sum of the two types of component ions: V = Vdiamagnetic (λ) + Vparamagnetic (λ; T), where λ is wavelength and T is temperature. [4, 16, 17, 18, 19, 20, 21].
This paper focuses on the development of high clean Lax:Er0.5Y1.95−xO3 nanopowders with doping concentration varied from 0 to 0.1 mol. The amount of La ions was varied to improve the sintering process and luminescence properties. In addition, lanthanum (paramagnetic) and erbium (paramagnetic) ions were used to improve the Verdet constant. The presented results show that varying the amount of lanthanum ions added to the powders has a significant effect on both microstructure and luminescence properties. The structure evolution and microstructure characterization of the powders were carried out by X-ray diffraction (XRD), Fourier-transform infrared (FT-IR) spectroscopy, and scanning electron microscopy (SEM). The surface of powders was analyzed by the Brunauer–Emmett–Teller (BET) method. Strong dependence of luminescence due to the La3+ concentration was observed. High yield upconversion emissions in Er-doped Y2O3 powders were measured under the excitation of a 980 nm continuous wave diode laser.
In this paper, the synthesis of polycrystalline Er0.05Y1.95O3, La0.01Er0.05Y1.94O3, La0.05Er0.05Y1.9O3, and La0.1Er0.05Y1.85O3 nano-sized powders is reported. The powders were obtained using a modified citrate sol–gel process with ethylene glycol and citric acid as fuel. Thermal analysis and evolved gas analysis of the abovementioned precursor gels were carried out in the temperature range 25–700 °C in a simultaneous thermogravimetry DTA/TG/EGA with a fast Quadrupole Mass Spectrometer (QMS) setup (Netzsch STA 449 F3 with a SDT 2960) with a heating rate 10 °C min−1 in an ambient gas atmosphere and alumina as reference.
Structural characterization and phase identification of the obtained powders were carried out using a X’Pert PANalytical X-ray diffractometer, with CuKα (1.5405 Å) radiation. XRD patterns of the gels and calcined powder were recorded over the angular range 10°–90° with step size 0.01°. Qualitative phase analysis was conducted by use of Highscore Plus software and database PCPDFWIN v.2.3.
Fourier-transform infrared (FT-IR) spectra of the samples were conducted using an FT-IR spectrometer (Brucker 70 V) with the KBr pellet method in the wave number range 400–4000 cm−1 with a step 3 cm−1.
The distribution of the particle (agglomerate) size of the yttria powders was determined by the laser light diffraction method (Mastersizer 2000S, Malvern Instruments) with measurement error 5%.
The specific surface area of the prepared powders was estimated by the BET (Brunauer–Emmett–Teller) method using an ASAP 2010 v4.00 G instrument with tolerance under 10%.
Powders were milled in rotary mill (600 rpm) for 1 h in an ethanol medium with ϕ = 1 mm zirconium balls and jars.
The luminescence spectroscopy of the powders was performed using a Hamamatsu NIR (0.1 nm resolution) spectrometer with continuous wave 980 nm laser diode (Spectra-Laser) as a light source.
The chemicals used were lanthanum (III) nitrate hexahydrate La(NO3)3 × 6H2O (Sigma-Aldrich; 99.9%), erbium (III) nitrate hexahydrate Er(NO3)3 × 6H2O (Sigma-Aldrich; ≥ 99.9%), and yttrium nitrate hexahydrate Y(NO3)3 × 6H2O (Sigma-Aldrich; 99.9%). Glycerin alcohol (PEG, Sigma-Aldrich) and citric acid (CA, Sigma-Aldrich) were used as starting materials. First, metal hydrates were dissolved separately in minimum quantities of distilled water. The solutions of erbium hydroxide and lanthanum hydroxide were mixed homogeneously with PEG. At the same time, a yttrium hydroxide solution was mixed with CA using a magnetic stirrer maintained at a constant rotation speed of 300 rpm for 2 h at 80 °C. After that, both solutions were mixed together by the magnetic sitter at a constant rotation speed of 300 rpm and heated at 80 °C. The stoichiometric proportion of CA to PEG was 3:7, respectively. The solution was heated at 180 °C for several hours until formation of gel took place. The precursor gel of La0.05Er0.05Y1.9O3 was calcined at different temperatures, which were selected by differential thermal analysis with thermal gravimeters (DTA/TG) results: 180, 280, 350, 500 °C, and 700 °C for 2 h in ambient air. On the basis of studies of the decomposition of the gel precursors, the optimum conditions for production of yttrium oxide doped with erbium and lanthanum powder were determined. The optimum conditions for calcination were 700 °C for 10 h in air.
The type of the gaseous decomposition products confirms the oxidation reactions of a gel, and consequently, the total decomposition of the materials studied. The QMS analysis allows a more precise description of the type of the volatile decomposition products emitted under oxidation. The QMS spectra of the volatile decomposition products emitted under heating of selected gel are presented Fig. 2. According to the QMS results, one can clearly see the presence of difference components: OH (m/z = 17), H2O (m/z = 18), CO (m/z = 28), NO (m/z = 30), O2 (m/z = 32), CO2 (m/z = 44), and NO2 (m/z = 46) when the gel was calcined from room temperature to 700 °C in ambient air. At the beginning, there was a release of fragments of the water molecules (H2O, OH) and NO2; next, below 200 °C, there is an indication that desorption of chemically and physically absorbed water also existed at temperatures lower than 470 °C. However as the temperature grows above 250 °C, all signals drastically increased, reaching a maximum at 290 °C. Further heating of the gel above 290 °C decreased emission of CO and NO gases and other signals were not observed. In the second temperature range above 300 °C, there was a release of all gases with a maximum at 374 °C, which can also be confirmed by the first endothermic peaks at 374 °C. At this temperature, the TG measurement indicates a strong of mass loss, as shown Fig. 1b. The temperature at the beginning of crystallization varies according to the La ion content. Finally at temperatures higher than 400 °C, the gel residue undergoes oxidation processes and thus the emission of OH, O2, H2O, CO2, and rest of the CO. Above temperatures higher than 500 °C, no gaseous decomposition products were detected.
The lattice parameters (dhkl, a, V, Dxrd, and ρ) of Er0.05Y1.95O3, La0.01Er0.05Y1.94O3, La0.05Er0.05Y1.9O3, and La0.1Er0.05Y1.84O3 powders after milled
Bragg position peaks of Er0.05Y1.95O3, La0.01Er0.05Y1.94O3, La0.05Er0.05Y1.9O3, and La0.1Er0.05Y1.84O3 powders
BET parameter (m2 g−1) of Er0.05Y1.95O3, La0.01Er0.05Y1.94O3, La0.05Er0.05Y1.9O3, and La0.1Er0.05Y1.84O3 powders before and after milled
The novel ceramic powders: Er0.05Y1.95O3, La0.01Er0.05Y1.94O3, La0.05Er0.05Y1.9O3, and La0.1Er0.05Y1.85O3, were synthesized by the citric sol gel reaction at 700 °C for 10 h. The samples were confirmed to be single phase by XRD. FT-IR spectroscopy presents a weak detected displacement of the bands. The increasing lanthanum content leads to both a decrease of temperature crystallization of precursors and make to possible to display the Bragg peaks at lower temperature. Intensity of all emission band in NIR region could be increased by La3+ doping of the powders. Typical luminescence spectra of the powders were a strong NIR emission of Er3+ at 1.53 μm due to the (4I13/2 → 4I15/2) transition under the 980 nm excitation. The main mechanism for such enhancement was the increase of GSA cross section of Er3+ ions by the tailoring effect of La3+ ions. High content of lanthanum ions is recommended because of its ability to decrease the calcination temperature and improving the luminescence of optical ceramics. Presented doped yttria ceramic powders were full characterized and could be sintering in order to obtain a transparence bulk sample and exanimate magneto-optical properties.
This work was supported by the National Science Center of Poland under Grant Number 2016/23/D/ST8/00014.
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