6__AITI#5635__191-198 Advances in Technology Innovation, vol. 6, no. 3, 2021, pp. 191-198 Effects of Bi 3+ Ion-Doped on the Microstructure and Photoluminescence of La0.97Pr0.03VO4 Phosphor Hao-Long Chen1, Hung-Rung Shih2, Sean Wu3, Yee-Shin Chang4,* 1Department of Mechanical Engineering, National Pingtung University of Science and Technology, Pingtung, Taiwan 2Department of Mechanical and Computer-Aided Engineering, National Formosa University, Yunlin, Taiwan 3Department of Digital Game and Animation Design, Tungfang Design University, Kaohsiung, Taiwan 4Department of Electronic Engineering, National Formosa University, Yunlin, Taiwan Received 04 May 2020; received in revised form 04 May 2021; accepted 05 May 2021 DOI: https://doi.org/10.46604/aiti.2021.5635 Abstract The objective of this paper is to enhance the emission intensity of La0.97Pr0.03VO4 single-phased white light emitting phosphor. The Bi3+ ion-doped La0.97Pr0.03VO4 single-phased white light emitting phosphors are synthesized using a sol-gel method. The structure and photoluminescence properties of (La0.97-yBiy)Pr0.03VO4 (y = 0-0.05) phosphor are also examined. The XRD results show that the structure of La0.97Pr0.03VO4 phosphors with different concentrations of Bi3+ ion doping keeps the monoclinic structure. The SEM results show that the phosphor particles become smoother when the Bi3+ ion is doped. The excitation band for La0.97Pr0.03VO4 phosphor exhibits a blue shift from 320 nm to 308 nm as the Bi3+ ion contents are increased. The maximum emission intensity is achieved for a Bi3+ ion content of 0.5 mol%, which is about 30% greater than that with no Bi3+ ion doped. The CIE chromaticity coordinates are all located in the near white light region for different Bi3+ ion-doped La0.97Pr0.03VO4 phosphors. Keywords: sol-gel method, Pr3+ ion, sensitizer, phosphors, flux 1. Introduction Rare-earth ion-doped oxide-based phosphors have been the subject of many studies because of their excellent optical properties [1]. They are widely used in various optical devices, such as Plasma Display Panels (PDPs), Field Emission Displays (FEDs), and Light-Emitting Diodes (LEDs) [2]. White Light-Emitting Diodes (w-LED) perform better than traditional incandescent and fluorescent lamps [3-4]. A w-LED is produced using a blue InGaN chip with commercial Y3Al5O12:Ce3+ (YAG:Ce) yellow-emitting phosphors [5-6]. Unfortunately, this combination yields a low color-rendering index because there is no red-light-emitting component [7-8]. Recent advances in single-phased white light emitting phosphors, [9-10] have many applications for w-LEDs. For the vanadate groups, lanthanum orthovanadates (LaVO4) has many applications. Lanthanum orthovanadates have two types of crystal structure: monoclinic (m-) monazite and tetragonal (t-) zircon [11-12]. Monoclinic LaVO4 is the thermodynamically stable state, and m-monazite based materials are used in laser host materials [13], solar cells [14], and thin film phosphors [15]. A previous study results show that the CIE chromaticity coordinate is located in the white light region with x = 0.388 and y = 0.367 when the m-monazite LaVO4 is doped with Pr3+ ion at a concentration of 3 mol% produced using a sol-gel method [16]. * Corresponding author. E-mail address: yeeshin@nfu.edu.tw Advances in Technology Innovation, vol. 6, no. 3, 2021, pp. 191-198 The sol-gel method requires a low synthesis temperature, achieves higher purity, mixes activators well, and produces nano-scale particle powders [17-18]. Many studies enhance the photoluminescence properties of phosphors and improve the particle morphology by flux addition or by doping with different radius ions [19-20]. Doping with different radius ions is the simplest and most effective method. For example, the emission intensity for BaLa1.5Eu0.5ZnO5 phosphors with 70 mol% Ba2+ ion substituted by Sr2+ ion is 62% greater than that for phosphors with no Sr2+ ions, and 33% greater than that for commercial red sulfide phosphors [ZnS:(Mn2+, Te2+)] [20]. In order to increase the emission intensity of La0.97Pr0.03VO4 phosphor, the Bi3+ ion-doped La0.97Pr0.03VO4 phosphors are synthesized using a sol-gel method in this study. The crystal structure, surface morphologies, and the photoluminescence properties of Bi3+ ion-doped La0.97Pr0.03VO4 phosphors are also determined. 2. Experimental Method (La0.97-yBiy)Pr0.03VO4 (y = 0-0.05) phosphors are synthesized using a sol-gel method with the raw materials of ammonium metavanadate (NH4VO3), lanthanum acetate [La(CH3CO2)3], and praseodymium acetate [Pr(CH3CO2)3]. These starting materials with a purity of 99.99% are supplied by Aldrich Chemical Company, INC. A solution of ammonia (NH4OH) is added to the NH4VO3 solution to accelerate dissolution, and the lanthanum acetate and praseodymium acetate are separately dissolved in deionized water. Citric Acid is used as the raw material and mixed using DI water as a solvent. After mixing, the solution is stirred and heated at 120°C for 5 h, and then is placed in an oven at 120°C for drying. Finally, the powders are calcined in a furnace at 950°C for 6 h in air using a heating rate of 4°C /min. The structural characterization of these samples is analyzed by X-ray powder diffraction (XRD, Bruker axs), using CuKα radiation with a source power of 30 kV and a current of 20 mA. The morphologies of the phosphor powder are determined using the scanning electron microscopy (FE-SEM, Hitachi S4800-I). A Hitachi U-3010 UV visible spectrophotometer is used to measure the optical absorption behavior of the (La0.97-yBiy)Pr0.03VO4 phosphor. The samples are placed inside a closed quartz glass and measured from 200 to 700 nm at room temperature. Both of the excitation and the emission spectra for the phosphors are measured at room temperature using a Hitachi F-7000 fluorescence spectrophotometer with a 150 W xenon arc lamp as the excitation source. 3. Results and Discussion The surface morphologies of the phosphor particles must be as smooth as possible, with a high degree of crystallization, to ensure good photoluminescence efficiency. 3.1. XRD and FE-SEM characterization The X-ray powder diffraction patterns for (La0.97-yBiy)Pr0.03VO4 (y = 0-0.05) phosphors calcined at 950°C in air for 6 h using a sol-gel method are shown in Fig. 1. The crystal structure of different concentrations of Bi3+ ion-doped La0.97Pr0.03VO4 phosphors is monoclinic for LaVO4 (JCPDS No. 70-2392). There is no secondary phase if the Bi3+ ion concentration is increased because the Bi3+ ion (1.03Å) and La3+ ion have similar radii (1.032Å) [21] and the same valence. Therefore, a solid solution is formed when the La3+ ion is substituted by Bi3+ ion in the host material. Fig. 2 shows the FE-SEM surface morphologies of (La0.97-yBiy)Pr0.03VO4 (y = 0-0.05) phosphors calcined at 950°C for 6 h in air. The particle shapes are irregular and there are aggregations. For the Bi3+ ion concentration is 0.5 mol%, the particles become smooth, granular, and more uniform in size. A liquid-phase sintering behavior is observed when the Bi3+ ion concentration is more than 0.5 mol% because the Bi3+ ion oxidizes with O2- ion to form Bi2O3 during the calcination process in air. Bi2O3 has a lower melting point of 815°C and acts as a flux in the calcination process, so the phosphor particles coagulate 192 Advances in Technology Innovation, vol. 6, no. 3, 2021, pp. 191-198 due to the surface tension of the liquid when the fluxes melt. The melted fluxes also enable the phosphor particles to slide and rotate easier, so there is greater particle-particle contact and particle growth increases [22]. When the fluxes melt sufficiently, the liquid-phase sintering leads metal oxide particles to aggregate. Fig. 1 The X-ray diffraction patterns of (La0.97-yBiy)Pr0.03VO4 phosphors calcined at 950°C for 6 h in air (a) 0 mol% (b) 0.2 mol% (c) 0.5 mol% (d) 1 mol% Fig. 2 FE-SEM micrographs of La0.97Pr0.03VO4 doped with various Bi3+ ion concentrations calcined at 950°C for 6 h in air 3.2. Luminescence properties The absorption spectra of (La0.97-yBiy)Pr0.03VO4 phosphors calcined at 950°C for 6 h in air are shown in Fig. 3. For La0.97Pr0.03VO4 phosphor, there are two broad absorption peaks in the absorption spectrum from 200 to 350 nm, which correspond to the O2- and V5+ charge transfer for the VO4 3- internal anions [23-25]. The absorption band from 280 to 350 nm centered at 315 nm is attributed to the 4f-5d characteristic transition absorption of a Pr3+ ion because typical Pr3+-activated oxide phosphors always demonstrate strong 4f-5d transition band absorption at approximately 200-330 nm [26-27]. There are small absorption peaks from 440 to 500 nm and 580 to 620 nm, respectively, which are corresponding to the inner 4f orbital characteristic transition of the Pr3+ ion. The different concentrations of Bi3+ ion do not affect the curves shape but do affect the intensities of the excitation and emission peaks. Fig. 4 shows the excitation spectra for (La0.97-yBiy)Pr0.03VO4 (y = 0-0.05) phosphor calcined at 950°C for 6 h in air. The signals are detected at 489 nm. There are two excitation bands in the excitation spectra. The first one from 200 to 350 nm centered at 315 nm is attributed to the charge transfer from the oxygen ligands to the central vanadium atom inside the VO4 3− 193 Advances in Technology Innovation, vol. 6, no. 3, 2021, pp. 191-198 anionic group, which overlaps the 4f-5d characteristic transition of the Pr3+ ion at the absorption band in (La0.97-yBiy)Pr0.03VO4 phosphor. The other one centered at 448 nm is attributed to the 3H4→ 3P2 electronic transition of the Pr3+ ion inner 4f orbital [28]. Fig. 3 Absorption spectra for (La0.97-yBiy)Pr0.03VO4 phosphors that are calcined at 950°C for 6 h Fig. 4 Excitation spectra for (La0.97-yBiy)Pr0.03VO4 phosphors that are calcined at 950°C for 6 h (The signals are detected at 489 nm) The intensity of the excitation peak increases significantly if La0.97Pr0.03VO4 is doped with 0.5 mol% of Bi3+ ion and then decreases as the Bi3+ ion concentrations increases further. It is due to the extra absorption involving the Bi-O component in addition to the V-O charge transfer bands. In this study, the excitation wavelength is 315 nm, which is in good accordance with the 1S0→ 3P1 transition (A-band) for the electron level transition of Bi3+ ion [29]. Therefore, the role for Bi3+ ion-doped La0.97Pr0.03VO4 phosphor in this study is to be a sensitizer. The excitation peak intensities are decreased when the Bi3+ ion concentration is higher than 0.5 mol%, because an increase in the Bi3+ ion concentration causes the phosphor particles to aggregate. It can dissipate the absorbed energy in the form of non-radiation rather than transfer it to the Pr3+ ions [30]. However, it can be observed that there is a blue shift for the excitation band from 320 nm to 308 nm as the Bi3+ ion contents increased. This may be the reason for the special empirical rule, that is substitutions of the dodecahedral La3+ by larger ions leading to spectral red shift and smaller ions leading to blue shift for most of the phosphor modification system [31]. Fig. 5 shows the emission spectra for (La0.97-yBiy)Pr0.03VO4 phosphors calcined at 950°C for 6 h in air under an excitation of 315 nm. At 315 nm excitation, there is no emission peak of the Bi3+ ion observed in the emission spectra, but there are emission peaks in the visible light region at 480-520, 530-570, 580-610, 610-620, and 625-650 nm, which respectively correspond to the 3P0→ 3H4, 3P0→ 3H5, 1D2→ 3H4, 3P0→ 3H6, and 3P0→ 3F2 electron transitions of Pr3+ ions. The 1D2→ 3H4 transition is more intense than that for 3P0→ 3H4 at a low doping concentrations of Pr3+ ion (x = 0.005-0.02), but the intensity decreases if the Pr3+ ion concentration increases further (x = 0.03-0.1) [32]. It is due to the difference in the ionic radii of La3+ 194 Advances in Technology Innovation, vol. 6, no. 3, 2021, pp. 191-198 ion and Pr3+ ion, which may compress the Pr-O bond in the host lattice when a slight Pr3+ ion concentration doped. This results in a strong crystal field effect causing the Stark splitting of the multiplet structure which leads the 4f-5d state of the Pr3+ ion to shifts to a lower energy state that is closer to the 1D2 state, causing the emission intensities of the 1D2→ 3H4 transition to be higher than that of 3P0→ 3H4 transition. The results for this study show that the 3P0→ 3H4 transition is greater than the intensity of the 1D2→ 3H4 transition because the Pr3+ ion concentration is fixed of 3 mol%. Therefore, the CIE chromaticity coordinates are located in the white light region (x = 0.388, y = 0.367). Fig. 5 Emission spectra for (La0.97-yBiy)Pr0.03VO4 phosphors calcined at 950°C for 6 h under an excitation of 315 nm According to the results in the emission spectra, the intensity of emission peaks increases as the concentrations of Bi3+ ion in La0.97Pr0.03VO4 phosphor increase. The co-doped with Bi3+ ions can increase an absorption in the ultraviolet region (315 nm) because the Bi3+ ion acts as a sensitizer in the La0.97Pr0.03VO4 phosphor. A good sensitizer absorbs the excitation energy and transfers energy to a luminescent center (activator), but does not play a role as a luminescent or quenching center. More energy is absorbed and transferred to the Pr3+ ions via the Bi3+ ion, so the emission intensity of the La0.97Pr0.03VO4 phosphor increases. There is a maximum intensity of emission peak when the Bi3+ ion concentration is 0.5 mol%. These results show that the sensitization effect of Bi3+ ion on the Pr3+ emission behavior varies with the Bi3+ ion concentrations. As can be seen in the emission spectra, the emission peak appearances are attributing to the characteristic electronic transition of Pr3+ ion. There is no Bi3+ ion emission peak observed, because the Bi3+ acts as a sensitizer for doping in the La0.97Pr0.03VO4 phosphor. The excitation wavelength, 315 nm, is not only absorbed by LaVO4 host, but also absorbed by Bi3+ sensitizer and Pr3+ ion, respectively. The 4f-5d characteristics transition absorption of Pr3+ ion usually overlaps with the oxygen ligands to the central vanadium atom inside the VO4 3− anionic group. Therefore, the energy (315 nm) is supposed firstly to be absorbed by the LaVO4 host, Bi3+ ion, and the Pr3+ ion to the conduction band, the 3P1 level, and the 4f-5d state, respectively. The energy in the 4f-5d state of Pr3+ ion relaxes to a lower state of 3P0, and both of the energies in the conduction band and the 3P1 level are all transferred to the 4f-5d state of the Pr3+ ion. Simultaneously, these energies from 4f-5d state relaxes rapidly to the lowest emission level, 3P0 and 1D2, via non-radiative transition, and finally transits from 3P0 to the 3HJ, J=4, 5, 6 and the 3F2 state, respectively, and from 1D2 to 3H4 state. The mechanism for energy absorption and transfer for La0.97Pr0.03VO4 phosphor doped with Bi3+ ion is shown in Fig. 6. Fig. 7 shows the CIE color coordinate diagrams for (La0.97-yBiy)Pr0.03VO4 (y = 0-0.05) phosphors, y = 0.005, 0.01, 0.02, 0.03, 0.05, and 0.1. For the La0.97Pr0.03VO4 phosphor with no Bi3+ ion doped, the emission color is in the near white light region with the CIE chromaticity coordinates of (x = 0.388, y = 0.367). For phosphors that are doped with Bi3+ ions, different concentrations of Bi3+ ion do not affect the shape of curves, but the intensity of the emission spectra changes. Therefore, the CIE color coordinates for La0.97-yBiyPr0.03VO4 phosphor are all located in the near white light region. 195 Advances in Technology Innovation, vol. 6, no. 3, 2021, pp. 191-198 Fig. 6 The mechanism for the absorption of energy and transfer for La0.97Pr0.03VO4 phosphors doped with Bi3+ ion under an excitation wavelength of 315 nm Fig. 7 CIE color coordinate diagrams for (La0.97-yBiy)Pr0.03VO4 (y = 0-0.05) phosphor 4. Conclusions The (La0.97-yBiy)Pr0.03VO4 (y = 0-0.05) phosphors were synthesized using a sol-gel method at a calcination temperature of 950°C for 6 h in air. When doped with Bi3+ ions, the crystal structure of (La0.97-yBiy)Pr0.03VO4 is monoclinic structure of LaVO4, and there are no secondary phases. The surface morphologies of (La0.97-yBiy)Pr0.03VO4 phosphors become smoother and more granular as the Bi3+ ion concentration increases. The role for a Bi3+ ion in the La0.97Pr0.03VO4 phosphor system not only can be a flux, but also acts as a sensitizer. Under excitation at 315 nm, the emission intensity increases as the concentration of the Bi3+ ion increases. The phosphor emission has a maximum intensity at a Bi3+ ion concentration of 0.5 mol%. All of the CIE chromaticity coordinates of (La0.97-yBiy)Pr0.03VO4 (y = 0-0.05) are located in the near white light region. Acknowledgements The authors would like to thank the National Science Council of the Republic of China for financially supporting this project with grant no: MOST 105-2221-E-150-055-MY3. Conflicts of Interest The authors declare no conflict of interest. 196 Advances in Technology Innovation, vol. 6, no. 3, 2021, pp. 191-198 References [1] H. Yie, S. Kim, Y. Kim, and H. S. Kim, “Modifying Optical Properties of Phosphor-in-Glass by Varying Phosphor Size and Content,” Journal of Non-Crystalline Solids, vol. 463, pp. 19-24, May 2017. [2] W. Dai, J. Hu, S. Shi, J. Zhou, K. Huang, S. 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