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Microwave synthesis and studies room temperature optical properties of LaF3: Ce3+, Pr3+, Nd3+ nanocrystals
Sidheshwar G Gaurkhede
Department of Physics, Bhavan's College of Science, Andheri (W) Mumbai - 400058, India
PACS 61.05.cp, 77.22.d DOI 10.17586/2220-8054-2020-11-1-117-122
Lanthanum fluoride (LaF3:Ce3+, Pr3+, Nd3+) was synthesized by water soluble LaCl3 + CeCl3+ PrCl3 + NdCl3 and NH4F as starting materials in de-ionized water as solvent using microwave assisted technique. The structure of LaF3:Ce3+, Pr3+, Nd3+ nanocrystals analyzed by XRD and TEM analysis is found to be in hexagonal structure and average crystalline particle size is 20 nm (JCPDS standard card (32-0483) of pure hexagonal LaF3 crystals). The absorption edge in UV spectra is found at 250 nm corresponding to energy of 4.9 eV. It further shows a wide transparent window lying between 200 nm-800 nm. ForLaF3; Ce3+, Pr3+, Nd3+ nanocrystals emission of blue color (458 nm)has been observed with at an excitation wavelength of 254 nm. The measured relative second harmonic generation (SHG) efficiency of LaF3: Ce3+, Pr3+, Nd3+ in de-ionized water with respect to KDP crystal is 0.186.
Keywords: Microwave radiation, Hexagonal shape, X-ray diffraction, Luminescent, SHG. Received: 23 January 2020
1. Introduction
LaF3 is an ideal host material for various phosphors to its low phonon energy and the consequent minimal multi phonon relaxation of its excited states [1,2]. Lanthanum fluoride is an excellent F- ionic conductor among other rare earth fluorides [3]. Lanthanum fluoride based chemical sensors widely used in potential application in sensing the fluorine, oxygen, and carbon monoxide due to its high chemical stability and ionic conductivity [4]. Ion conducting nature of the rare earth fluorides (solid electrolyte) is exploited as sensor materials to construct various electrochemical sensors like gas sensor, biosensor, and ion selective electrode [5]. Miura et al. reported the use of lanthanum fluoride film in biosensor and room-temperature oxygen sensor based on its high F- ion-conducting property. The working principle of LaF3 based biosensor and oxygen sensors is explained as the movement of F-ion conduction [6,7]. Fe-dorov P.P. [8] et al. review the major aspects of inorganic chemistry of nanofluorides, methods of synthesis including nanochemical effects, preparation of 1D, 2D, and 3D nanostructures, surface modification of the nanoparticles, fluoride nanocomposites and applications of nanofluorides. The orthorhombic ,0-YF3 structure and ionic conductivity of rare earth fluorides and of tysonite-structured were investigated by Trnovcova et al. [9,10].
HaiGuoetal. [11] have reported water-soluble LaF3; Ce3+, Tb3+ nanodiskettes having particle size of 25 nm synthesized by ionic liquid-based hydrothermal process. The luminescent properties of LaF3; Ce3+, Tb3+ nanodiskettes show intense green emission (541 nm) at 254 nm excited wavelength both in solid state and dispersed in solution. A simple and straightforward method was developed by Yong Zhang et al. [12,13] to produce water-dispersible LaF3; Ce3+, Tb3+ nanocrystals and to grow these nanocrystals on silica microspheres which show a raspberry-like structure with LaF3 nanocrystals. The nanocrystals were 25 nm in size and exhibited strong green fluorescence for excited wavelength of 254 nm. There is a report by Cong-Cong Mi et al. [14] on Polyethyleneimine (PEI) functionalized multicolor luminescent LaF3; Ce3+, Tb3+ nanoparticles which were synthesized via a novel microwave-assisted method and nanoparticles possessed a pure hexagonal structure with an average size of was 12 nm and green fluorescence was observed when sample was excited with 252 nm wavelength. The lanthanide series of trivalent ions is Ce3+, Pr3+, Nd3+ having ultra violet (UV) and visible luminescence spectra consisting of many narrow lines whose half-widths reach only several cm-1. It was found that the in Nd3+ UV and visible luminescence depend on the excitation wavelength [15].
The synthesized LaF3:Ce3+, Pr3+, Nd3+ nanoparticles exhibit hexagonal shape and exhibit blue luminescence. In the present case LaF3:Ce3+, Pr3+, Nd3+ nanoparticles have been synthesized using a conventional microwave radiation technique for first time.
2. Experimental
2.1. Synthesis of nanocrystals
Synthesis of LaF3:Ce3+, Pr3+, Nd3+ nanocrystals follows an aqueous route and uses a microwave heating at low power range. The method is simple and cost effective. Water soluble LaCl3 + CeCl3+ PrCl3 + NdCl3 and NH4F are mixed to obtain a solution in 1:3 molar proportions [16]. A 10 ml homogenous mixture (in the ionized water) in a 100 ml beaker using 0.064 mol of each LaCl3 + CeCl3 + PrCl3 + NdCl3 allowed a 10 ml solution of 0.768 mol NH4F to drip into this solution uniformly through a funnel attached with a stopper to facilitate control of dripping, and placed the whole set up inside a conventional microwave set at low power range (in on-off mode set at 30 sec) for around 30 min. The low power range setting largely helped us avoid spill off of the solution. A white crystalline precipitate identified as doped LaF3 nanocrystals appears almost instantly having settled down to the bottom of the beaker. White precipitate is then washed several times with de-ionized water and then drying it in microwave oven for about 15 min. The dried sample was then stored in sealed ampoules for further characterization and analysis.
3. Characterization
Powder X-ray diffraction (XRD) measurements have been performed using a PANALYTICAL X'PERT PROMPD diffractometer model using CuKa radiation A = 1.5405 A.U with a scanning rate of 20 per min in the 2° range from 0° to 80°. Transmission electron microscope (TEM) analysis has been carried out for different magnification by PHILIPS (CM 200) 0.24 nm resolution, operating at 200 kV. The UV-visible spectrum of the samples was recorded in the spectral range of 200 nm-800 nm using a double beam (Perkin Elmer Corp.) spectrophotometer. The fluorescence spectrum was measured on LS 45 luminescence spectrometer (Perkin Elmer Corp.) using a high energy pulsed Xenon source for excitation and FL Win Lab software. NLO studies for the measurements of SHG efficiency are obtained through the crystalline powder sample by using Kurtz and Perry technique.
4. Result and discussion
The XRD pattern obtained from the LaF3:Ce3+, Pr3+, Nd3+ nanocrystals shown in Fig. 1. The results of the XRD are in good agreement with the hexagonal LaF3 structure as described in the reports LaF3 (JCPDS card No. 32-0483) [17]. The average crystallite size estimated from the Scherrer equation, D = 0.90A/fi cos 6, where D is the average crystallite size, A is the x-ray wavelength (0.15405 nm), 6 and fi being the diffraction angle and full width at half maximum of an observed peak, respectively. The strongest peak (111) at 26=27.84° for LaF3 :Ce3+, Pr3+, Nd3+ samples have been used to calculate the average crystallite size (D) of the nanoparticles [18].The average crystallite sizes of LaF3:Ce3+, Pr3+, Nd3+ nanoparticles are in the range of 15 nm-20 nm, which is in agreement with the TEM and SEM results. The XRD pattern of the LaF3 nanoparticles is nearly similar to that of LaF3: Ce3+, Tb3+ [19]. No XRD signals are observed for impurity phases.
700-
600-
500-
0
20 30 40 SO 60 70 28(degrce)
80
Fig. 1. X-ray diffraction pattern of LaF3:Ce3+, Pr3+, Nd3+ nanocrystals
Figure 2 shows the transmission electron microscopy (TEM) image of LaF3:Ce3+, Pr3+, Nd3+ nanocrystals. It is seen that most of the nanocrystals are in the form of hexagon, sphere and nanorods. Most of the nanorods of LaF3: Ce3+, Pr3+, Nd3+ are found to be well separated with some instances of agglomeration with average particle size of 20 nm are found. Fig. 3 shows the selected area electron diffraction (SAED) pattern. Three strong diffraction rings corresponding to the (002), (111) and (300) reflections, have been observed which is in close agreement with the hexagonal LaF3 structure [20]. This shows that the original structure of LaF3 may be retained even after modification.
FIG. 2. TEM image of LaF3:Ce3+, Pr3+, Nd3+ nanocrystals
Fig. 3. Selected area electron diffraction (SAED) pattern image of LaF3:Ce3+, Pr3+, Nd3+ nanocrystals
Figure 4 shows the optical absorption spectrum of the nanocrystallites with an absorption edge at 250 nm in the UV region, with corresponding energy band gap lying at 4.9 eV. Absorption against wavelength values was used to determine the band gap energy. The band gap energy was calculated by determining the 'hv' value. A wide transparent window is present between 200 - 800 nm suggesting its use in optoelectronics devices.
The emission spectra of synthesized LaF3: Ce3+, Pr3+, and Nd3+ ion is shown in Fig. 5. The emission spectra was obtained by monitoring (254 nm) 4f to 5d transition of Ce3+ ions. The broadband emission is located at 458 nm due to the electronic transitions from 5d to 4f state of Ce3+ ions [21]. The sharp emission peaks originates from the 4f5d - 4f2 transitions of Pr3+ ions [3H4 ^3P2 (458 nm), 3H4 ^3P0 (497 nm), 3H4 ^1D2 (608 nm)]. The quenching of Ce3+ emissions and the enhancement of Pr3+ emissions is strong evidence of efficient energy transfer from Ce3+ to Pr3+ and Nd3+. The emission spectrum is mainly located in the region corresponding to blue colour. Here, the doping Ce3+ ions act as sensitizers and the doping ions Pr3+, Nd3+ can be considered as luminescent centers. It is well known fact that the luminescent spectra of trivalent lanthanide ions in crystals come mainly from two types of electronic transitions: 4f-4f transition and 5d-4f transition. The excited electronic configuration of Ce3+ is 5D1. The
Fig. 4. UV-VIS absorption spectra of LaF3 doped Ce3+, Pr3+ Nd3+ nanocrystals in ionized water
Fig. 5. Excitation (Aem=458 nm) and Emission (Aex=254 nm) spectra of LaF3 nanocrystals doped Ce3+, Pr3+, Nd3+ ions in deionized water
5d electron has a strong interaction with the neighboring anion ligands in the compounds and results in broadband emissions. The 4f orbital is shielded from the surroundings by the filled 5s2 and 5p6 orbital. Therefore, the influence of the host lattice on the optical transitions within the 4P configuration is small [15,22].
Figure 6 shows the energy level scheme of LaF3; Ce3+, Pr3+ Nd3+, with optical transitions and energy transfer processes. The Ce3+ ion excited at 254 nm absorbs one photon and is pumped to the 5d level. Then, it relaxes to the ground state by radiative process with emission of photons; and transfers its energy to a nearby Pr3+ ion in the ground state, promoting this Pr3+ ion to excited state. Then, the excited Pr3+ ion relaxes to the 3P2, 3P0, and *D2 levels by non-radiative process. The Pr3+ ion excited by 254 nm is pumped to the 3P2, 3P0, and *D2 states. As mentioned above, 3H4 ^3P2 (458 nm), 3H4 ^3P0 (497 nm), 3H4 (608 nm) transitions have been observed only for
low Pr3+ doped samples. Blue fluorescence from the higher energy 3P2 level has been observed for high Pr3+ doped samples [23].
Fig. 6. Energy level scheme for LaF3: Ce3+, Pr3+, Nd3+ nanocrystals
The nonlinear optical property of LaF3:Ce3+, Pr3+, Nd3+ in de-ionized water was tested by passing the output of Nd: YAG Quanta ray laser emitting 1064 nm, generating about 6mJ / pulse through the samples. SHG is a key technology as frequency doublers of laser light. The SHG efficiency of LaF3: Ce3+, Pr3+, Nd3+ nanocrystals were determined in de-ionized water by modified Kurtz and Perry technique [24]. Crystalline powder of Potassium dihy-drogen phosphate (KDP) taken as the reference material. The measured relative SHG efficiency of LaF3: Ce3+, Pr3+, Nd3+ in de-ionized water with respect to KDP crystal is 0.186.
5. Conclusions
LaF3 nanocrystals successfully have been synthesized using LaQ3+CeCl3+PrCl3+NdCl3 and NH4F in deionized water. Elongated and assorted size hexagonal geometry of LaF3 nanocrystals has been observed. XRD and TEM studies indicate that the average particle size is 20 nm. The conductivity at room temperature for LaF3 sample prepared in deionized water is found to be in close agreement with reported values. The absorption edge in UV spectra is found at 250 nm corresponding to energy of 4.9 eV. It further shows a wide transparent window lying between 200 nm-800 nm. For LaF3; Ce3+, Pr3+, Nd3+ nanocrystals emission of blue color (458 nm) has been observed with at an excitation wavelength of 254 nm. The SHG property was tested by using Nd: YAG laser. The second harmonic efficiency of LaF3; Ce3+, Pr3+, Nd3+ nanocrystals is found to be 0.186 in deionized water using KDP as standard material.
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