Optical absorption intensity analysis using Judd-Ofelt theory and photoluminescence investigation for red-emitting Eu3+: TiO2 nanoparticles
Graphical abstract
Introduction
TiO2 is a semiconducting material existing in three different phases such as brookite, anatase and rutile which has the band-gap of order 3.2 eV. Among them, rutile and anatase modifications of TiO2 are thermodynamically stable. The nanosized TiO2 have large scale application in solar cells, electrochemical gas sensor, photocatalysts, microelectronics, and electronic gadgets [[1], [2], [3], [4], [5], [6]]. At the encompassing temperature and pressure, the nano sized material offers a decreased number of electron-hole recombination and improves the active exterior region that enriches the photocatalytic performances of the material. TiO2 is proven to be an excellent host for doping metal ions to improve the luminescence properties due to the modified recombination rate of the electron-hole pair [[7], [8], [9]], specifically for rare-earth ions.
The addition of rare earth (RE) component decreases the recombination of the electrons and holes by trapping them and enabling their quick development along the surface of TiO2. RE ion doping does not have any impact on the band gap of TiO2 but upgrades the light retention of colloids [8]. By the RE ion doping, the crystallite size of TiO2 reduces due to the higher adsorption capacity [[10], [11], [12]]. From the literature, it is known that the luminescence of the samples often depends on the preparation process. The rare-earth doped metal oxides can be prepared using sol-gel, Chemical Vapour Deposition (CVD), solution combustion, hydrothermal and chemical co-precipitation method [13].
The hydrothermal process has been perceptible among all other processes; due to it is the simple way to synthesis nanoparticles. It requires low temperature, undergoes quick reaction, and produce nano-sized particles It has the potential to provide products with good morphology, which is one of the main criteria for developing a material with better emissions [14]. Among the different RE activators, Eu3+ ions are popular for its spectroscopic properties and arrangement of their bonds in the luminescent materials [[15], [16], [17]]. The Eu3+ ions are of greater significance in different hosts because of its high color purity of red emission. However, the luminescent characteristics and efficiency of europium ion are fragile in terms of their dependence on crystallographic environment. The europium doped TiO2 was reported by few scientists and they noticed that luminescence of europium ion was enhanced due to the energy transfer from the host lattice.
Also, the luminescence of Eu3+ ion doped nano TiO2 revealed that the intense emission occurs only for materials calcined at 400 °C, while in the samples calcined above 400 °C, the luminescence started to diminish and get vanished at 900 °C [18]. Even at various excitation wavelengths, the color coordinates for Eu3+ phosphors were established to be in the red color region of the CIE diagram. Hence, the Eu3+ phosphors can be suitable for the advance of artificial white light and red component in white LEDs.
Additionally, one can obtain some evidence of the polarization activities and coordination environment of the europium ion by Judd−Ofelt factors. Even though the significant mismatch in the ionic radii of Eu3+ is 0.98 Å and Ti is 0.61 Å makes it difficult to replace Ti4+ by Eu3+ in the TiO2 lattice [19]. Here in this work, we establish that the low RE ion concentration dilution will certainly be replacing the host matrix and enable the perfect system for energy transfer. We report on the synthesis of pure TiO2 and 1–11 mol% of Eu3+ doped TiO2 nanophosphors via the hydrothermal method and the luminescence properties of the synthesized materials is studied in detail. The Judd− Ofelt strength parameters (Ω2, Ω4) and specific radiative parameters were calculated by the Judd− Ofelt theory, which is useful in the TiO2 matrix for considering the coordination state of europium ion.
Section snippets
Materials and method
Europium oxide (Eu2O3), ammonia (NH3), concentrated sulphuric acid (H2SO4) and Potassium persulphate (K2S2O8), all are of 99.99% purity or spectroscopic grade, were purchased from Sigma Aldrich. DDW (Double distilled water) was taken from the purification unit in our research laboratory. In this study, Illmenite was condensed to metatitanic acid through the sulfate method. As a source material, Metatitanic acid [TiO(OH)2] was used for the synthesis of titanium dioxide. The flow chart of the
Equipment
The collected powder samples are characterized by various methods to analyze the structural, surface morphology, absorption and emission properties. The crystallinity, phase composition and structure of the samples are examined by using PXRD (Powder X-Ray Diffraction) with the help of a Rigaku diffractometer, Cu target Kα radiation. The diffraction patterns were recorded in a continuous mode at a scanning speed of 1°/min over the angular range of 20°≤ 2θ ≤ 80°. The surface morphology of the
PXRD analysis (powder X-ray diffraction)
The PXRD patterns of pure TiO2 and Eu3+ doped TiO2 (1–11 mol%) nanophosphors are shown in Fig. 2. The PXRD patterns of Eu3+ doped TiO2 are well matched with JCPDS 21–1272 and it confirms the tetragonal structure of the samples. As Eu3+ concentration increases (above 9 mol%), an additional peak is observed and it indicates that not all Eu3+ ions in the matrix sites. Instead, they are in interstitial sites for 9 and 11 mol% Eu3+ doped TiO2 [21]. The PXRD patterns of Eu3+ doped TiO2
show the (101),
UV–visible spectroscopy analysis
The optical properties of Eu3+ doped TiO2 nanophosphor are examined using UV–Visible diffused reflectance measurements. The absorption spectra of pure and Eu3+ doped TiO2 are shown in Fig. 7. From the DRS spectra, it was found that appropriate excitation wavelength for all samples is 376 nm. The absorption maximum marginally moves to higher wavelengths when the concentration of Eu3+ increases. A strong band at 376 nm was due to the 5 d-4f absorption transition of Eu3+ [30].
The Kubelka-Munk
Photoluminescence study
Fig. 9 shows photoluminescence excitation spectra of 5 mol% Eu3+ doped TiO2 nanophosphors. The excitation spectrum monitored at a wavelength of 612 nm is recorded in the range 340–550 nm. The spectrum showed a series of sharp peaks ranging from 340 to 550 nm are attributed to f-f transitions of Eu3+ ions. The peaks in the spectra exist at 376 nm (7F0→5D4), 397 nm (7F0→5L7), 409 nm (7F0→5L6), 431 nm (7F0→5D3) and 479 nm (7F0→5D2) [36].
The emission spectra of TiO2: Eu3+ (1–11 mol% concentration)
Color chromaticity coordinate
The widely accepted mathematical description of colors is the Commission Internationale de I'Eclairage 1931 (CIE-1931) color scheme [58]. The color coordinate values were therefore calculated using the Osram Sylvania CIE co-ordinates software. This system is composed of three main color stimuli symbolized by integrals X, Y, and Z tristimulus. Here we used photoluminescence results to make a chart of coordinates for each dopant.where,
Conclusion
The undoped and Eu3+ doped TiO2 nanoparticles were effectively synthesized by the hydrothermal technique. The XRD analysis confirmed the pure anatase phase of Eu3+ doped TiO2 nanoparticles. The average crystallite size of the synthesized samples is in the range of 9–22 nm. The HRTEM and SAED patterns also confirms that the particles size are in nanometric range. The absorption studies showed the band gap widening in the for Eu3+ doped TiO2 nanophosphors. The JO analysis for the nanophosphor was
Declaration of competing interest
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: The authors are grateful to the DST (SERB) New Delhi for providing financial support through major project No.SR/S2/CMP/-0069/2012. Authors acknowledge DST-FIST, PURSE for providing XRD facility at the Department of Physics, Bangalore University, Bengaluru and also thanks to the Minority Welfare Department, Directorate of Minorities, Government of Karnataka, for
Acknowledgment
The authors are grateful to the DST (SERB) New Delhi for providing financial support through major project No. SR/S2/CMP/-0069/2012. Authors acknowledge DST-FIST, PURSE for providing XRD facility at the Department of Physics, Bangalore University, Bengaluru and also thanks to the Minority Welfare Department, Directorate of Minorities, Government of Karnataka, for providing fellowship under the scheme, M. Phil & Ph.D. Fellowship.
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