Effect of dysprosium ion (Dy3+) doping on morphological, crystal growth and optical properties of TiO2 particles and thin films

Physica B: Condensed Matter 560 (2019) 67–74

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Effect of dysprosium ion (Dy3+) doping on morphological, crystal growth and optical properties of TiO2 particles and thin films

T

Chaima Ouled Amora, Kais elghnijia,∗, Constantin Virlanb, Aurel Puib, Elimame Elalouia a b

Materials, Environment and Energy Laboratory (UR14ES26), Sciences Faculty of Gafsa, University of Gafsa, 2112, Tunisia Faculty of Chemistry, “AlexandruIoanCuza” University of Iasi, Carol I Bd., no. 11, 700506, Iasi, Romania

A R T I C LE I N FO

A B S T R A C T

Keywords: Dy3+ doped TiO2 Anatase Morphology Photoluminescence

Dy3+ doped (0.5, 2.5, 5 and 7.5 wt %) TiO2 particles and thin films were obtained by modified alcoholysis-solgel route using tetrabutyl orthotitanate as a precurspor. By controlling the hydrolysis of this precursor through alcoholysis reaction, monodisperse and spherical TiO2 particles were obtained. X-ray diffraction and Raman results confirmed that the Dy3+ doped TiO2 particles are composed of only anatase phase. The Dy3+ doping inhibited any phase transformation and slowed down the particle growth of anatase TiO2. Scanning electron microscopy of bare TiO2 showed monodisperse, spherical and non-aggregated particles. In contrast, the Dy3+doped TiO2 samples exhibited poor dispersity. The luminescence spectra show three characteristic bands at 481, 577 and 683 nm, which are due to 4F9/2 → 6H15/2 (blue), 4F9/2 → 6H13/2 (yellow), and 4F9/2 → 6H11/2 (red) transitions of trivalent Dy3+ ions. The photoluminescence study revealed the dependence of the luminescent intensity on dopant concentration in TiO2 particles.

1. 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. [1–6]. 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 [7–10]. 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 gassensitive sensors and optoelectronics devices [11–16]. 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 [19–23]. 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

Corresponding author. Materials, Environment and Energy Laboratory (06/UR/12-01) Science Faculty of Gafsa University of Gafsa, 2112, Gafsa, Tunisia. E-mail address: [email protected] (Kais elghniji).

https://doi.org/10.1016/j.physb.2019.02.017 Received 19 December 2018; Received in revised form 9 February 2019; Accepted 11 February 2019 Available online 12 February 2019 0921-4526/ © 2019 Elsevier B.V. All rights reserved.

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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.

1381 1347

a

1443

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2. Experimental procedure 2.1. 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 resulting dysprosium solutions were added dropwise to solution (A) under continuous stirring, yielding transparent solutions (B). These solution were kept under constant stirring for 1 h. The doping levels of Dy3+ in the solution (B) were 0.5%, 2.5%, 5% and 7.5%. These solutions (B) were kept one month in a glovebox under nitrogen atmosphere to avoid contact with atmosphere. For hydrolysis study, varied amounts of water were added slowly to these solutions until formation of precipitates or translucent gel. After gelification, the resulting materials were dried at 80 °C for 12 h and then calcined at 450 °C for 1 h in static air with heating rate of 5 °C/min. The powders are labelled according to its Dy3+ content and calcination temperature; TiDyx(t). Where t means the calcination temperature and x= (0%, 0.5%, 2.5%, 5% and 7.5% Dy). For example, TiDy7.5 (450) TiDy7.5 (80) represent the Dy3+doped TiO2 samples with x = 7.5% treated at 450 °C and 80 °C.

1026

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wavenumber (cm ) Fig. 1. FT-IR spectra of (a) TBOT precursor, (b) ethanol, (c) hydrolyzed precursor solution (A), (d) hydrolyzed TBOT precursor.

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 ab). The band vibration observed at 1627 cm−1 is due to the water molecule. The hydrolyzed precursor solution (A) (Fig. 1c) showed two bands at 1443 cm−1 and 1347 cm−1, which are ascribed to eCH3 and eCH2 groups of the alkoxides, respectively [25]. The IR bands at 1026 cm−1 and 1113 cm−1 could be assigned to eCO groups stretching vibration and TieOeC groups, respectively [25]. This result clearly confirms that the alcoholysis can retard the hydrolysis of TBOT precursor. No apparent vibration bands associated with TieOeC groups were determined for the hydrolyzed precursor solution (TBOT) (Fig. 1d), indicating that the hydrolysis and condensation processes of TBOT precursor were completed.

2.2. Preparation of Dy3+ doped TiO2 thin films The as prepared solutions were used to prepare of thin films by a dip-coating technique. Prior to deposition, the two solutions (A) and (B) were acidified by adding few drops of hydrochloric acid solution (0.1 M), until pH 3.61 and 3.54, respectively. The glass substrates were immersed in the acidified transparent solutions with a speed of 12 cm/ min and then dried at 75 °C during 15 min. The TiO2 thin films were calcined at 450 °C for an hour. Three-layered of TiO2 coatings were obtained by repeating the deposition and calcination procedures.

3.2. Optical aspect of gel

The morphologies of as prepared materials were examined by scanning electron microscopy (Bruker AXS Microanalysis GmbH). The structure phase of TiO2 materials was determined on X-ray diffractometer (“PANalytical X’ Pert High Score Plus”) equipped with a dual goniometer of Cu Kα radiation at 40 kV. Raman spectra were recorded with a LABRAM HR800 Raman Spectrometer at a wavelength of (633 nm). The Far-infrared (Far/FT-IR) was carried out by CsI disks and collected 70 scans in the range of 600–250 cm−1. The UV–Vis absorption spectra were determined with a Perkin Elmer Lamda 950 spectrophotometer using polytetrafluoroethylene as a reference. The spectra were recorded in 200–1500 nm wavelength range. fluorescence excitation and emission were performed at room temperature on a (Perkin Elmer LS-55) Luminescence Spectrophotometer under excitation light of a 500 W Xenon lamp.

After alcoholysis process between TBOT and ethanol, the transparent precursor solution (A) undergoes hydrolysis reaction with excess of water (50 mL), leading to a white polymer sol. The condensation reaction causes nucleation-growth of particulates to form homogeneous precipitate (Fig. 2a). After aging at 25 °C for one day, the white polymer formed a gelatinous precipitates (Fig. 2b). In contrast, The precursor solution (B) was to some extent turbid after adding water (∼100 mL) (Fig. 2c), indicating the presence of supersaturated hydrolyzed monomers from hydrolysis of Ti alkoxide [26]. After aging at 50 °C for one day until the formation of translucent monolithic gel (Fig. 2d). The optical aspect of gel has not changed with changing of water volume (∼500 mL). The high viscosity of gel is helpful for the inhibition of the grow-up of particles [27]. It is monolithic gel where high polymers rather than small colloidal particles. This phenomenon is similar to that reported in our previous works on the synthesis of transparent-monolithic TiO2 gels [28,29].

3. Results and discussion

3.3. Morphological analysis

3.1. Effect of alcoholysis process on the hydrolysis reaction

Fig. 3 shows the SEM images of dried and calcined samples. The dried TiDy0 (80) and TiDy0.5 (80) samples are spherical and non-aggregated. This result indicates that transparent precursor solution (A) undergoes a slow hydrolysis and the spherical titania particles are

2.3. Characterization

FT-IR spectroscopy in the range 900–2000 cm−1 was employed to investigate the effect of alcoholysis on the hydrolysis rate of TBOT 68

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a Hydrolysis (50 ml water)

pH=5.7

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Alcoholysis

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pH=4.1

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Dy(0.5-7.5%) then hydrolysis (100-500 mL water) Fig. 4. SEM images of bare TiO2 and Dy3+ doped TiO2 calcined at 450 °C. (a) TiDy0 (450), (b) TiDy0.5 (450), (c) TiDy5(450), (c) TiDy7.5 (450).

Fig. 2. An image of modified precursor solution (alcoholysis), (B) precipitate obtained after adding water (50 mL), (B) gelatinous precipitates, (C) transparent sol containing Dy3+ with 100–500 mL of water, (D) translucent gel with obtained within three after aging at 50 °C.

the rate of hydrolysis reaction. Thus, the as prepared microparticles are monodisperse, spherical and non-aggregated. However, obvious change in TiO2 particle morphology with increasing Dy content in the precursor solution. The dried TiDy5 (80) sample consists of irregular aggregates with flat faces adhering to the smooth relief surface of large aggregate. The dried TiDy7.5 (80) sample has amorphous structure and the detection of discrete particles is difficult. This result indicating that the homogeneous nucleation-growth process was disrupted by Dy doping. The morphology of samples was essentially preserved even at calcination temperature 450 °C (Fig. 4). At this temperature, spherical TiDy0(450)) and TiDy0.5 (450) particles do not undergo any sintering process. Similarly, the TiDy5 (450) and TiDy7.5 (450) samples show a small shrinkage without changing the morphology of particles. Thus, we can conclude that the change in structure is due exclusively to Dy doping rather than the heat treatment. Our results contradict with those observed by El-Bahy et al. [31] for conventional sol–gel Lanthanide ions (La3+, Nd3+, Sm3+, Eu3+, Gd3+, and Yb3+)/doped TiO2 materials. The authors argued that the doping lanthanide ions did not affect the morphology of TiO2 particles. 3.4. X-ray diffraction studies Fig. 5 shows the evolution of the titania XRD powder spectra as function of the Dy3+ doping (calcination T = 450 °C). The examination of the diffractograms of the prepared samples indicates that, with increasing Dy3+ doping, there is a parallel decrease of intensities of the TiO2 anatase. This phenomenon, more evident for TiDy7.5 (450) sample, could arise from the increased surface disorder and/or for the presence of defect sites induced by the Dy3+ ions. By using the width of the anatase diffraction peak and the Scherrer equation, the crystallite sizes (XRD) of the TiDy0(450), TiDy0.5 (450), TiDy2.5 (450) and TiDy5(450) samples were estimated to have values: 14.5 nm, 14 nm, 7.5 nm and 7 nm, respectively. The particle size TiDy7.5 (450) sample is expected to be very fine (3–5 nm). The reduction of the particle size is predisposed by formation of DyeOeTi bonds, which lead to amorphization of TiO2 anatase [32]. From Rietveld analysis of TiDy0 (450) and TiDy7.5 (450), lattice parameters were estimated and summarized in Table 1. As clearly shown, Dy3+ doping produces a decreasing on the tetragonality (c/a) of anatase structure of TiDy7.5 (450). This reduction is likely

Fig. 3. SEM images of undoped TiO2 and Dy3+ doped TiO2 dried at 80 °C. (a) TiDy0 (80), (b) TiDy0.5 (80), (c) TiDy5(80), (c) TiDy7.5 (80).

formed through homogeneous nucleation-growth process. Previous work reported that the solvation of Titanium ethoxide Ti(OC2H5)4 in ethanol can lead to dimeric complex Ti2(OC2H5)8·2C2H5OH, while Titanium isopropoxide Ti(OC3H7i)4 in isopropyl alcohol is monomeric. Slower rates of hydrolysis and condensation reactions are expected for oligomeric alkoxide because there is a high steric hindrance, leading to monodisperse and spherical TiO2 particles [30]. In our case, TBOT precursor Ti (OC4H9)4 possesses a high steric hindrance than Titanium ethoxide due to the long alkyl chains (-C4H9)4. During nucleophilic substitution, the alkyl chains exert steric hindrance of water, decreasing 69

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TiDy7.5(450)

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Fig. 6. Raman spectra of bare TiO2 and Dy3+ doped TiO2 calcined at 450 °C.

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Table 1 Cell volume and lattice parameters obtained from the Rietveld refinement of Xray diffraction data.

3.6. Far-FT-IR spectroscopy The raman spectroscopy was a privileged technique for characterizing nanometer-sized TiO2 nanoparticles. However, this method reaches its limits for too small particles size (7 nm) due to the broading of TieO bond modes. Nevertheless, Far infrared spectroscopy, sensitive to short-range orders, could characterize TiO2 powders with particles size of 3–5 nm. This would make this technique complementary to Raman. The anatase structure of the TiO2 samples were characterized by Far infrared in the range 200–650 cm−1, as shown in Fig. 7. The anatase TiO2 is tetragonal with four formula molecular units per unit cell. This structure has a space group D194h (I4/amd). According to group theoretical analysis there are three infrared active modes

related to a crystal size decrease in the TiO2 lattice short order, so particle size decreases from 14 nm for TiDy0(450) to 4 nm for TiDy7.5 (450). 3.5. Raman spectroscopy In order to analyze the influence of the dopants on the ordering in the TiO2 crystal, Raman spectra from TiDy0.5 (450), TiDy2.5 (450), TiDy5(450) and TiDy7.5 (450) as well as TiDy0(450) have been acquired, as shown in Fig. 6. All Raman spectra reveal the anatase phase purity of synthesized materials. The Raman peaks at 143 cm−1 (Eg), 198 cm−1 (Eg), 395 cm−1 (B1g), 512 cm−1 (B1g/A1g), and 639 cm−1 (Eg) are assigned to TiO2 anatase. The Eg modes at 143 cm−1 and 198 cm−1 are mostly related to the Ti4+ vibration inside the TiO6 octahedra TiO6. The Eg mode at 639 cm−1 is connected to the movement of the O2− anions between two octahedral TiO6 [33]. The mode B1g and A1g are assigned to υsym and υanti-sym of OeTieO in anatase structure. It is important to note that the peaks at 143 cm−1 and 639 cm−1 gradually decreased with increasing Dy3+ content. An overall broadening of the Raman modes of anatase was observed for the highest Dy3+ doped sample (TiDy7.5 (450)). This phenomenon is connected to the shrinkage of particle size and loss of long range periodicity in nanostructured TiO2 anatase [34–40]. Only B1g and A1g + B1g modes still remained in the raman spectrum of TiDy7.5 (450) sample with the formation of overtone scattering (B1g) centered at 800 cm−1. The origin of this signal is not fully clear but it is commonly assigned to a B1g overtone scattering and frequently observed in nanoparticles TiO2. Similar results have been reported by CristianVasquez et al. [41], who reported the synthesis of anatase Al/Fe doped TiO2 nanoparticles. The band at 800-851 cm−1 is originated from overtone B1g and stretching modes of short apical TieO bonds at the surface of TiO2. The evolutions in raman intensity suggest that the Dy3+ dopant interacts strongly with the TiO2 framework decreasing the particles size and the crystallization degree.

Fig. 7. Far-FTIR spectra of TiDy7.5 (450) (a), TiDy5(450) (b), TiDy2.5 (450) (c), TiDy0(450) (d). 70

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Wavelength (nm)

reduction in particle size with increasing Dy3+ content or to energy band-widening of TiO2 semiconductor [45]. However, the Dy3+ doped TiO2 samples display several absorptions in the UV–Vis–IR spectral regions which originate due to the f-f induced electric dipole transitions of Dy3+ ion from its ground state (6H15/2) to various higher excited states (4G11/2, 4I15/2, 4F9/2 6F3/2, 6F5/2, 6F7/2 +6H5/2, 6H7/2+6F9/2, 6H9/ 6 2+ F11/2) at wavelenghts of 425, 454, 471, 746, 795, 893, 1080 and 1260 nm, respectively [46,47]. Fig. 8b shows the UV–vis absorption of Dy3+ doped TiO2 (5 wt% 3+ Dy ) samples dried and calcined at 450 °C and 750 °C. A blue-shifted of absorption edges with the increase of the calcinations temperature is also evident. Furthermore, all the samples have same absorption bands in the UV–Vis–IR spectral regions. Hence, we can conclude that Dy3+ exists mostly on the surface or/and at the grain boundary of TiO2. 3.8. Photoluminescence analysis Fig. 9 (left) shows the excitation spectrum of TiDy5 (450) sample, monitored at emission wavelength 577 nm. This spectrum displays six excitation peaks characteristics of electronic transitions of Dy3+, i.e. 6 H15/2 → 4M15/2 + 6P7/2 (353 nm),6H15/2 → 4I11/2 (363 nm),6H15/2 → 4 I13/2 +4F7/2 (387 nm), 6H15/2 → 4G11/2 (425 nm),6H15/2 → 4I15/2 (454 nm) and 6H15/2 → 4F9/2 (471 nm) [48]. Nota that the intense transition at 454 nm has been chosen for the measurement of emission spectra of doped samples. Fig. 9 (right) shows the emission spectra of doped samples as a function of Dy3+ concentration, under excitation by 454 nm. Three peaks are observed at 481, 577 and 683 nm and assigned to 4F9/2 → 6H15/2 (blue), 4F9/2 → 6H13/2 (yellow), and 4F9/2 → 6H11/2 (red), respectively [49]. The two emissions 4F9/2 → 6H15/2 and 4F9/2 → 6 H13/2 were used to determine the local host site of the Dy3+ dopant. When Dy3+ cation is hosted at a low-symmetry local site, the yellow emission is often dominant in the emission spectrum. If Dy3+ ion is located at a high-symmetry local site, the blue emission become stronger than the yellow emission. In the present case, the yellow emission is the dominant emission, confirming that Dy3+ ions occupy low symmetry sites in TiO2 anatase [49,50]. However, the examination of PL spectra indicates that the luminescence intensity of the 4F9/2 → 6 H13/2 emission of Dy3+ increases with increasing Dy3+ ion, reaching a maximum value for 2.5% and then a quenching is observed. For concentrations above 2.5%, high interaction is expected between Dy3+ ions, leading to an increase of non-radiative process originated of PL quenching. As is well known, when the RE ions concentration is high, the probability of cross relaxation (CR) between close ions Dy3+ is very

Fig. 8. (a) UV–vis–NIR absorption spectra of bare TiO2 and Dy3+ doped TiO2 particles with different doping concentration; (b) UV–vis–NIR absorption spectra of Dy3+ doped TiO2 treated at various temperatures.

associated with this structure (2Eu + A2u) [42,43]. These spectral bands appear at approximately ∼265–285 cm−1 (Eu), 345 cm−1 (A2u) and 440 cm−1 (Eu). In addition, the presence of various vibrations in the spectral range of 550–650 cm−1 can be assigned to the stretching vibrations of TieO bonds. The intensity of these bands decreased and shifted to low wavenumbers, suggesting that the Dy dopant interacts strongly with the TiO2 surface. The Dy3+ ions at the grain boundaries may slow the growth of titania grains and hinder the anatase crystallization. Unfortunately, a comparative discussion with the literature is difficult here since, to our knowledge, there is no relevant bibliographic data. 3.7. UV–visible study Fig. 8a presents the UV–vis absorption spectra of as prepared materials and calcined at 450 °C. For the undoped TiO2, the typical absorption edge around 400 nm due to the intrinsic band-gap excitation of anatase is clearly visible [44]. With respect to the absorption edge of TiDy0(450) particles, absorption edges of Dy3+ doped samples are blueshifted (Fig. 8a, inset). This phenomenon is connected either to 71

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Fig. 10. XRD patterns of glass, TiDy0 (450) and TiDy5(450) thin films. Inset; (b TiDy0 (450) and TiDy5(450) powders scratched off from the glass substrate.

high [51].

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3.9. Characterization of TiO2 films 80

3.9.1. X-ray diffraction patterns of Dy doped TiO2 thin films Fig. 10 shows the XRD patterns of glass, TiDy0(450) and TiDy5 (450) thin films. No XRD peaks TiDy0(450) and TiDy5 (450) thin films were detected even after three depositions. The sharp diffraction band was produced by the amorphous structure of glass substrate. This was attributed to that the TiO2 films were so thin that the X-ray diffraction peaks of TiO2 anatase were beyond the detection limit. However, after scratching of TiDy0(450) and TiDy5 (450) powders from the film on glass substrate, XRD spectra of two samples (inset) exhibited the typical peaks ascribed to the anatase TiO2 structure.

Transmittance (%)

3+

3.9.2. Transparency of thin films To evaluate the properties of the acidified precursor solutions for the production of thin film, the transparent precursor solutions (B) containing 5% and 7.5% Dy3+ were transformed into transparent TiDy5 (450) and TiDy7.5 (450) thin films coated on glass substrate (Fig. 11). As shown, the glass substrate is visually transparent, appearing light brownish yellow due to Dy3+ doping. The TiDy0(450) thin film has a low transparency than doped TiDy5 (450), TiDy7.5 (450) thin films and glass substrate. These results suggest that the transparency of doped thin films are related not only to the acidity but also due to Dy3+ dopant. Moreover, the TiDy5 (450) and TiDy7.5 (450) thin films exhibit high transmittance (≥85%) and interference fringes in visible region, revealing the highly transparency and uniform TiO2 thin films coated on glass substrate. Herein, it should be noted that the 4f-4f transitions of the Dy3+ ions are not observed even after three TiO2 thin films deposition. This result clearly indicated the fineness of the films and low absorption cross section of the dopant. In order to confirm the presence of Dy3+ dopant in thin films, the TiDy5 (450) and TiDy7.5 (450) were carefully scratched off from the glass substrate and diffuse reflectance spectra were recorded in the range of 200–800 nm (Fig. 12). The obtained powders display several absorptions in the UV–Vis–IR which are related to the Dy3+ dopants.

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Fig. 11. TiO2 thin films, (a) glass substrate, (b) TiDy0 (450), (c) TiDy5 (450), (d) TiDy7.5 (450) thin films. Transmittance spectra of the TiDy5 (450) and TiDy7.5 (450) thin films.

3+

Bare and Dy doped TiO2 particles were prepared by modified alcoholysis sol-gel route. We successfully confirmed that the co-existence 72

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99 4 6 4

Reflectance (%)

98

G11/2

I15/2

F9/2

6

F3/2 6

F5/2

97

96

TiDy0(450) particles TiDy5(450) particles TiDy7.5(450) particles

95

94 400

500

600

700

800

wavelength (nm) Fig. 12. Reflectance spectra of TiDy0 (450), TiDy5 (450) and TiDy7.5 (450) powders scratched off from the glass substrate.

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 further confirms that the Dy3+ doping can significantly affect the hydrolysis reaction and the nucleation-growth process of TiO2 particles. The structural properties of TiO2 particles as evaluated in XRD were in corroboration with the Raman spectra and Far-FT-IR spectroscopies. It was found that the doping of Dy3+ could efficiently inhibit the crystal transformation and reduce crystallite size. Further addition of Dy3+ ions (≥5%) leads to a decrease in the short lattice order and amorphization of crystalline structure of TiO2. The absorption spectra of Dy3+ doped TiO2 ions show various absorption bands in the UV–VIS and IR spectral regions which are due to the f-f induced electric dipole transitions of Dy3+ ion. The strongest emission band is observed at 577 nm (yellow). Futhermore, an increase in concentration of Dy3+ enhances the luminescence intensity and the concentration quenching effect was observed after 2.5 wt% Dy3+. The application of modified sol–gel/dip-coating method allows preparation of the transparent thin films of Dy3+doped TiO2 on the glass surface. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. References [1] V.H. Rao, P.S. Prasad, M.M. Babu, P.V. Rao, T. Satyanarayana, L.F. Santos, N. Veeraiah, Spectroscopic studies of Dy3+ ion doped tellurite glasses for solid state lasers and white LEDs, Spectrochim. Acta A. 188 (2018) 516–524 https://doi.org/ 10.1016/j.saa.2017.07.013. [2] C. Madhukar Reddy, B. Deva Prasad Raju, N. JohnSushma, N.S. Dhoble, S.J. Dhoble, A review on optical and photoluminescence studies of RE3+ (RE =Sm, Dy, Eu, Tb and Nd) ions doped LCZSFB glasses, Renew. Sustain. Energy Rev. 51 (2015) 566–584 https://doi.org/10.1016/j.rser.2015.06.025. [3] S.A. Azizan, S. Hashim, N.A. Razak, M.H.A. Mhareb, Y.S.M. Alajerami, N. Tamchek, Physical and optical properties of Dy3+: Li2O–K2O–B2O3 glasses, J. Mol. Struct. 1076 (2014) 20–25 https://doi.org/10.1016/j.molstruc.2014.07.032. [4] G.V. Honnavar, K.P. Ramesh, Optical spectroscopy of rare earth-doped oxyfluorotellurite glasses to probe local environment, Bull. Mater. Sci. 40 (2017) 991–997 https://doi.org/10.1007/s12034-017-1454-5. [5] Q. Jiao, G. Li, D. Zhou, J. Qiu, Effect of the glass structure on emission of rare-earth doped borate glasses, J. Am. Ceram. Soc. 98 (2015) 4102–4106 https://doi.org/10. 1111/jace.13832. [6] P. Devangad, M. Tamboli, K.M. Muhammed Shameem, R. Nayak, A. Patil, V.K. Unnikrishnan, C. Santhosh, G.A. Kumar, Spectroscopic identification of rare earth elements in phosphate glass, Laser Phys. 28 (2018) 015703https://doi.org/ 10.1088/1555-6611/aa86da.

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