Effect of dysprosium ion (Dy3+) doping on morphological, crystal growth and optical properties of TiO2 particles and thin films
Introduction
Over the past few decades, a large number of research works have been focused on the synthesis of Trivalent rare earth (RE) ions-doped glasses due to their potential application in the optical and electrical sciences such as, laser emitters, sensors, fluorescent markers, and light emitting diodes, etc. [[1], [2], [3], [4], [5], [6]]. The RE ions exhibit a strong emission associated with the 4f–4f transition from the excited level to the ground level. The 4f-4f transitions have sharp luminescence peaks from the ultraviolet (UV) to the infrared (IR) region. Thus, the Dy3+ (4f9) is one of the attractive ion among RE for the luminescence efficiency [[7], [8], [9], [10]].
In nanomaterials science, Dy3+-doped semiconductor oxides (TiO2, SnO2, ZnO) have also attracted extensive attention to meet the pressing demands for the high efficiency and easy large-scale production of gas-sensitive sensors and optoelectronics devices [[11], [12], [13], [14], [15], [16]]. Doping the TiO2 with Dy3+ ions could create some impurity energy levels in band structure and facilitate the energy transfer between TiO2 and Dy3+ dopants [17,18]. However, it is still a challenge to optimize the location of (RE) dopant i.e., either within the TiO2 lattice (insertion or substitution) or loaded on its surface (both external and internal), due to a significant mismatch in ionic radius between Dy3+ ions into Ti4+. There are several methods to synthesize TiO2:RE particles and thin films such as sol-gel, hydrothermal, co-precipitation, solid-state reaction and atomic layer deposition [[19], [20], [21], [22], [23]]. The sol gel process is predominant as compared to the others methods due to the simplicity of operation and low cost.
Although number of papers on the sol-gel synthesis of Dy3+ doped TiO2 anatase is undergoing an exponential increase, however, so far, no work has been devoted to the effect of the Dy3+ dopants on the nucleation-growth process and stabilization of TiO2 colloidal sol. This is understandable, because the hydrolysis reaction of metal alkoxides by conventional sol-gel occurs so rapidly that uniform and fine particles are difficult to obtain. Therefore, the effect of Dy3+ ions on the hydrolysis-condensation rate and optical aspect of TiO2 gel in sol-gel conventional route is rather difficult.
A modified precursor solution was made by modified sol-gel through the intermediate of alcoholysis reaction between ethanol and titanium alkoxide precursor (TBOT). Ethanol is able to react with the metal alkoxide and modify this precursor at a molecular level. The alcoholysis can significantly inhibit the fast hydrolysis reaction and favors the homogeneous nucleation and growth process [24]. After alcoholysis, various concentrations of Dy3+ ions were added to the modified precursor solution. The influence of Dy3+ doping content on the nucleation-growth process, morphology, crystalline and structure UV–Visible response of TiO2 anatase particles was evaluated. With literature proposing no luminescence from Dy3+ ion in TiO2 anatase framework, our results show that nanocrystalline anatase powders can actually host this ion that can successfully be excited and luminescence response can be obtained. The molar ratios Ti/Dy3+ adopted in this work are quite different from those reported in the literature. For application use, Dy3+ doped TiO2 thin films were made by dipping glass substrates into the transparent precursor solution.
Section snippets
Preparation of Dy3+doped TiO2 powders
Dy3+doped TiO2 (TiDyx) powders were prepared by modified alcoholysis-sol-gel method using Tetrabutyl orthotitanate (TBOT) as precursor. In a typical synthesis, 9.9 g of TBOT was diluted in 90 ml absolute ethanol with molar ratio ethanol/TBOT = 9 in a glovebox under nitrogen atmosphere to ensure alcoholysis reactions (transparent precursor solution A). Subsequently different amounts (0.04, 0.019, 0.39 and 0.6 g) of dysprosium salt (DyCl3.6H2O) were dissolved in 2 ml of absolute ethanol. The
Effect of alcoholysis process on the hydrolysis reaction
FT-IR spectroscopy in the range 900–2000 cm−1 was employed to investigate the effect of alcoholysis on the hydrolysis rate of TBOT precursor. Prior to analysis, (i) transparent precursor solution (A) and (ii) TBOT solution were hydrolyzed using water (50 mL). The hydrolyzed solutions were collected and then dried at 80 °C. The IR spectra of TBOT solution and ethanol were used for comparative purpose (Fig. 2 a-b). The band vibration observed at 1627 cm−1 is due to the water molecule. The
Conclusion
Bare and Dy3+doped TiO2 particles were prepared by modified alcoholysis sol-gel route. We successfully confirmed that the co-existence of dysprosium dopant in the TiO2 precursor solution inhibits the growth and agglomeration of TiO2 particles, avoiding the formation of a precipitate. The SEM images of bare TiO2 by modified sol-gel show a monodispersed and spherical TiO2. The obtained Dy3+ doped TiO2 powders were mainly composed of irregular aggregates of various sizes and shapes. This result
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2022, Thin Solid FilmsCitation Excerpt :Fig. 6 displays the PL excitation spectra of 1% Dy3+ doped TiO2 film monitored at emission wavelengths of 482 nm and 577 nm. The deconvolution of the Dy3+ excitation spectra fitted properly with Gaussian and showed several peaks at 332, 351, 362, 372, 388, 426, 454, and 474 nm corresponding to the Dy3+ electronic transitions 6H15/2→ 4P5/2, 4P7/2, 4P9/2, 4I11/2, 4I13/2 + 4F7/2, 4G11/2, 4I15/2 and 4F9/2, respectively [31,37,38]. Among all these transitions, the 6H15/2→ 4I13/2 + 4F7/2 transition has more intensity.
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2022, Applied Surface Science AdvancesCitation Excerpt :A white light could be produced by varying the yellow to blue intensity ratio (Y/B). The yellow emission is usually predominant in the emission spectra when the Dy3+ cation is hosted at a low-symmetry local site and the blue turns stronger when the Dy3+ ion is hosted at a high-symmetry local site [8]. The chemical environment around the dopant, on the other hand, has a significant impact on these sensible transitions.
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2022, Journal of LuminescenceCitation Excerpt :Fig. 8a shows the deconvolution of the Dy3+ excitation spectrum of 1% Dy3+ doped TiO2 film monitored at an emission wavelength of 577 nm. The excitation spectrum could be fitted properly with Gaussian bands positioned at 351, 362, 388, 426, 454, and 474 nm corresponding to the transitions 6H15/2 → 4P7/2, 4P9/2, 4I13/2 +4F7/2, 4G11/2, 4I15/2 and 4F9/2, respectively [38,46,47]. Among all these transitions, the 6H15/2 → 4I13/2 +4F7/2 have more intensity compared to the other transitions.