Dazzling red emission from TiO2 nanoparticles impregnated co-doped Gd3++Eu3+: PVA polymer nanocomposites for photonic applications

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JIEC 3114 1–11 Journal of Industrial and Engineering Chemistry xxx (2016) xxx–xxx

Contents lists available at ScienceDirect

Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec

Dazzling red emission from TiO2 nanoparticles impregnated co-doped Gd3++Eu3+: PVA polymer nanocomposites for photonic applications

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K. Naveen Kumara,* , R. Padmab , L. Vijayalakshmic, Misook Kanga,*

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a b c

Department of Chemistry, College of Science, Yeungnam University, Gyeongsan, Gyeongbuk 38541, Republic of Korea Department of Physics, Sri Venkateswara University, Tirupati 517502, AP, India NRI Institute of Technology, Pothavarappadu, Agiripalli, Vijayawada 521212, AP, India

A R T I C L E I N F O

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Article history: Received 12 May 2016 Received in revised form 27 September 2016 Accepted 1 October 2016 Available online xxx Keywords: (Gd3+ + Eu3+ + TiO2NPs): PVA composites Red emission Energy transfer

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A B S T R A C T

Red emission was obtained from rare earth doped polymer nanocomposites, namely, composites of polyvinyl-alcohol (PVA) co-doped with Gd3+ and Eu3+ and embedded with TiO2 nanoparticles (TiO2NP), under ultraviolet (UV) excitation. We successfully synthesized Eu3+: PVA, Gd3+: PVA, (Gd3+ + Eu3+): PVA, and (Gd3+ + Eu3+ + TiO2NP): PVA films by solution casting method. From X-ray diffraction patterns and Fourier-transform infrared spectral profiles, the structural details of the films and the ion–polymer interaction mechanism responsible for their formation were systematically analyzed. The thermal stability and decomposition dynamics of the prepared samples were evaluated by thermogravimetry and differential thermal analysis. Pertinent optical absorption bands related to Eu3+ and Gd3+ ions in the polymer composites were observed and assigned to the corresponding electronic transitions. The PVA film containing different concentrations of the Eu3+ dopant displayed red emission at 618 nm (5D0 ! 7F2) under UV excitation at 396 nm (7F0 ! 5L6). Upon co-doping with Gd3+ to form the (Gd3+ + Eu3+): PVA film, it exhibited red emission that was stronger than that from the singly doped Eu3+: PVA film under 270 nm excitation because of the energy transfer from Gd3+ to Eu3+ ions. After the TiO2 nanoparticles were evenly dispersed in the co-doped (Gd3+ + Eu3+): PVA films, the photoluminescence properties were remarkably enhanced and prominent red emission was observed under 274 nm excitation. The red emission of Eu3+ was significantly enhanced through an efficient energy-transfer process from the Gd3+ ions to Eu3+ ions and from the TiO2 nanoparticles to Eu3+ ions. A possible energy-transfer mechanism was clearly demonstrated by several fluorescent methods and lifetime decay dynamics. Based on the above results, these polymer composite films are promising candidates for red luminescent photonic devices. ã 2016 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

Introduction Over the past few decades, researchers have devoted considerable attention to the synthesis of polymer-based materials because of their use in novel photonic systems. Rare-earth doped polymerbased devices have several advantages over inorganic devices in design and fabrication, such as light weight, design flexibility, shape, and ease of fabrication [1]. Much attention has been concentrated on complexes doped with rare earth ions for their

Abbreviations: FTIR, Fourier-transform infrared; LCMD, light-converting molecular devices; PVA, polyvinyl alcohol; TG/DTA, thermogravimetry–differential thermal analysis; XRD, X-ray diffraction. * Corresponding authors. E-mail addresses: [email protected] (K. N. Kumar), [email protected] (M. Kang).

unique properties such as high color purity, narrow emission bands, long lifetime, large Stokes shift, high quantum efficiency, and ease of processing. Polymer composites doped with rare earth ions are used as light-converting molecular devices (LCMDs). This application has opened a new class of versatile materials for photonic applications such as displays, bioimaging, and biolabeling [2], and more specifically, for integrated functional photonic devices such as waveguide amplifiers, optical sensors, electroluminescent displays, light-emitting diodes, polymer fiber lasers, and compact lasers [3]. In particular, polyvinyl alcohol (PVA) films have been identified as potential hosts of rare earth ions, nanoparticles, and dyes that are widely used in holography, image storage, laser applications, display applications, optical sensors, and photovoltaic cells [4]. Among the rare earth elements, europium (Eu3+) is characterized by strong red emission, which is attributed to the electronic

http://dx.doi.org/10.1016/j.jiec.2016.10.002 1226-086X/ã 2016 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

Please cite this article in press as: K.N. Kumar, et al., Dazzling red emission from TiO2 nanoparticles impregnated co-doped Gd3++Eu3+: PVA polymer nanocomposites for photonic applications, J. Ind. Eng. Chem. (2016), http://dx.doi.org/10.1016/j.jiec.2016.10.002

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transition of 5D0 ! 7F2 under ultraviolet (UV) excitation, and it exhibits high luminescence efficiency. Europium doped materials such as glasses, ceramics, and polymer composites show prominent red emission, and their use is preferred in solid-state lighting and display devices. Generally, light emission from rare earth ions depends on the local chemical environment or co-doping ions in different host matrices [5]. Co-doped polymer systems have shown predominant enhancements in photoluminescence (PL) over that of a singly doped polymer matrix. The fluorescence efficiency of rare earth complexes plays a pivotal role in photonic devices, and it can be enriched by different methods. Several techniques have been employed to improve the efficiency of luminescence of rare earth complexes by increasing the dopant concentration, but they falter at higher concentrations because of ionic aggregation and the aggregates act as quenching centers [6]. To overcome this quenching effect for higher luminescence efficiency, appropriately selected secondary rare earth ions are added to the polymer complex. The resulting luminescence efficiency of the primary rare earth ions is significantly enhanced as energy migrates from the secondary rare earth ions in the polymer system. In this co-doped system, the primary rare earth ions act as activators while and the secondary rare earth ions acts as sensitizers. The migration of energy from a sensitizer to an activator occurs, wholly or partially, through a radiative or non-radiative process, or a combination of both radiative and non-radiative processes. This energy-transfer (ET) process has become an attractive domain in several photonic applications [7]. In order to enhance the fluorescence of activator ions, the sensitizer ion concentration in the polymer matrix must be increased; the fluorescence intensities will also be quenched at a higher sensitizer concentration. Nevertheless, the sensitizer ion concentration will not affect the quenching centers, but the sensitizer (secondary) ions must be monitored as aggregation centers. Here, these aggregates act as quenching centers [8]. In the present study, Eu3+ ions serve as the active rare earth ions, and initially, they were activated by Gd3+ ions. The sensitizing ability will be saturated at a particular optimized concentration of the sensitizer ions, as indicated by the emission performances of activator ions in the co-doped system. To further enhance the luminescence efficiency of the activator, tertiary rare earth ions or nanofillers can be added as another sensitizer to the co-doped system. The performance of nanofillers in the co-doped system is critical as it enhances the photoluminescence [9], and homogeneous dispersion of nanofillers in a polymer matrix has become a new field of research for several advanced applications. For example, several researchers have concentrated on the luminescence efficiency enhancement by the incorporation of nanofillers [10]. In particular, the incorporation of titanium oxide (TiO2) nanoparticles (NPs) into the polymer matrix enhances the optical properties of the polymer complex. Based on this property, we impregnated the polymer matrix with TiO2 NPs in the present study, and the TiO2 nanofillers were dispersed in co-doped (Gd3 + + Eu3+): PVA films to obtain improved emission intensities of Eu3+. Experimental studies 3+

Preparation of TiO2 NPs embedded (Gd films

3+

+ Eu ): PVA nanocomposite

The polymer composite films investigated in this study were developed by solution casting method. The chemicals used were PVA (molecular weight, MW = 186  103), Eu(NO3)35H2O, Gd (NO3)36H2O, and TiO2 NPs, which were all obtained from Sigma –Aldrich Company, USA. The PVA films (thickness: 100 mm) doped with Eu(NO3)35H2O in different concentrations of 0.05, 0.1, 0.15, and 0.2 wt%. The Eu3+ salt was first dissolved in triple-distilled

water as a solvent, and this Eu3+-containing solution was added to the PVA matrix. The optimum concentration of the Eu3+ salt was determined from the luminescence characteristics. Along with the optimum concentration of Eu3+ salt, Gd(NO3)36H2O was also added in different concentrations (0.05, 0.075, 0.1, 0.15, and 0.2 wt %) to the PVA matrix, and the optimum concentrations of Eu3+ and Gd3+ salts in the co-doped system was determined. Finally, PVA and the optimum amounts of these rare earth nitrates were dissolved in triple-distilled water containing a homogeneous dispersion of TiO2 NPs in very low concentration (0.005, 0.01, 0.015, 0.02, or 0.025 wt%); the mixture was stirred at ambient temperature (30  C) for 11–13 h to obtain a solution. The solution was cast onto polypropylene dishes and allowed to evaporate slowly at the room temperature for 72 h. In order to removal of all traces of the solvent, the final product was dried upon warming. The freshly dried polymer nanocomposite films were collected from the polypropylene dishes for further characterization.

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Characterization

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The prepared rare-earth-doped PVA composites with and without TiO2 NPs were examined by an X-ray diffractometer (XRD; MPD, PANalytical, The Netherlands) with 2u ranging from 10 to 90 at a scan speed of 10 /min; a nickel-filtered Cu Ka radiation source was used. The diffractometer was operated at 40 kV and an anode current of 30 mA. The optical absorption spectra of the RE3+: PVA and (RE3+ + TiO2NP): PVA films were recorded on a spectrophotometer (S-3100, Scinco, Korea) in the wavelength range of 200–600 nm. The resolution, accuracy, and reproducibility of the spectrometer were 0.95, 0.5, and <0.02 nm, respectively. The FTIR spectra of the RE3+: PVA and (RE3+ + TiO2NP )—PVA films were obtained on Thermo Nicolet IR200 spectrometer in the range of 4000–500 cm1. Thermogravimetry and differential thermal analysis (TG–DTA) profiles of the prepared PVA composites were recorded with a TG analyzer and differential scanning calorimeter (SDT Q600TA, TA Instruments, USA). The samples were scanned from room temperature to 600  C at a heating rate of 10  C/min in a nitrogen atmosphere. The fluorescence spectral data of the RE3+: PVA and (RE3+ + TiO2NP): PVA films were collected on visible fluorescence spectrometer (FluoroMate FS-2, Scinco, Korea ). A 150-W Xe arc lamp was used as the excitation source in this fluorescence spectrometer for the state emission measurements. The evaluation of lifetime from the decay curves profiles was carried out with the accompanying computer-controlled phosphorimeter with a Xe flash lamp.

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Results and discussion

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XRD and TEM analysis

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Structural analysis was carried out through XRD characterization of the pure TiO2 NPs, and the results are shown in Fig. 1(b). There are 11 distinct XRD peaks at 2u of 25.35 , 36.87, 37.82 , 38.53 , 48.03 , 53.92 , 55.09 , 62.60 , 68.90 , 70.28 , and 75.13 , which can be indexed to the (1 0 1), (1 0 3), (0 0 4), (11 2), (2 0 0), (1 0 5), (2 11), (2 0 4), (11 6), (2 2 0), and (2 1 5) planes, respectively, of face-centered cubic titanium oxide (JCPDS card No. 21-1272) suggest that microstructural changes of the polymer composites took place. The XRD patterns of pure PVA, PVA doped with single and dual rare earth ions (Gd3+, Eu3+), and co-doped (Gd3+ + Eu3+): PVA with and without TiO2 NPs are shown in Fig. 1(a). The pure PVA film exhibited a strong crystalline peak at 19.2 , a weak shoulder at 22.1, and a low peak at 40.2 , representing reflections from the (1 0 1), (2 0 0), and (111) planes, respectively, of the monoclinic unit cell of PVA [12,13]. Upon addition of the dopant (Gd3+, Eu3+) ions into the PVA matrix, the characteristic XRD peaks’ intensities were

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Please cite this article in press as: K.N. Kumar, et al., Dazzling red emission from TiO2 nanoparticles impregnated co-doped Gd3++Eu3+: PVA polymer nanocomposites for photonic applications, J. Ind. Eng. Chem. (2016), http://dx.doi.org/10.1016/j.jiec.2016.10.002

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Fig. 1. XRD profiles of (a) pure PVA and composite films of Gd3+: PVA, Eu3+: PVA, (Gd3+ + Eu3+): PVA, and (Gd3+ + Eu3+ + 0.015TiO2NPs): PVA and (b) TiO2 NPs.

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reduced and the peaks broadened. This change in intensity suggests that there were strong interactions between the polymer chains and dopant ions and also a disturbance in the semicrystalline phase of the PVA bonding structure [14]. The size of the nanoparticles that were dispersed in the polymer matrix was determined by transmission electron microscopy (TEM). The TEM image in Fig. 2 shows that the TiO2 nanoparticles were well dispersed in the polymer composites and had sizes of 30–40 nm.

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FTIR analysis

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FTIR spectroscopy is a versatile tool for confirming the chemical structure of the composite films, the occurrence of possible interactions between the host and dopant ions, and for obtaining conformational information on the complex formation. The FTIR spectra of the pure PVA, Gd3+: PVA, Eu3+: PVA, (Gd3++ Eu3+): PVA, and (Gd3+ + Eu3+ + TiO2NP): PVA matrix were obtained in the wavelength range of 4000–530 cm1, and they are shown in Fig. 3. A broad peak from 3660 to 3010 cm1, centered at 3284 cm1, was observed. This spectral peak is ascribed to symmetric stretching vibrations of O H from the intramolecular and intermolecular hydrogen bonds in PVA, which means the peak position was more sensitive to hydrogen bonding. Compared to the spectrum of pure PVA, the OH stretching peak position is shifted toward higher wavenumbers and the stretching peak of C¼O (1664 cm1) is stronger in the spectrum of the rare-earth doped PVA matrix while comparing with pure PVA. This suggests that hydrogen bonding interactions occurred between the hydroxyl groups in the PVA molecular chains. The bands at 2849 and 2916 cm1 are assigned to symmetric and antisymmetric stretching vibrations of C H alkyl groups [15], while the band at 1664 cm1 is assigned to the acetyl C¼C group in the PVA film. The bands at 1585 and 1431 cm1 are assigned to the aromatic C¼C stretching group and CH2 bending vibrations, respectively. A band corresponding to the CH2 wagging vibrational mode is visible at 1358 cm1. The combination of the C H wagging vibration and CO stretching of acetyl groups is represented by the band at 1256 cm1. The band at 1094 cm1 is ascribed to C O stretching vibrations [16]. The vibrational modes of C C stretching and CH2 stretching are represented by the bands at 947 and 849 cm1. The band assignment of the FTIR profiles of PVA and other synthesized polymer composite materials are summarized in Table 1. Upon addition of the rare earth ions (Gd3+ and Eu3+) with and without TiO2 nanoparticles, we observed some structural

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Fig. 2. TEM images of pure TiO2 NPs (top row) and the (Gd3+ + Eu3+ + 4TiO2NPs): PVA nanocomposite film (bottom row).

Fig. 3. FTIR spectra of pure PVA, Gd3+: PVA, Eu3+: PVA, (Gd3++ Eu3+): PVA, co-doped (Gd3+ + Eu3+ + 0.015TiO2NP): PVA films.

Please cite this article in press as: K.N. Kumar, et al., Dazzling red emission from TiO2 nanoparticles impregnated co-doped Gd3++Eu3+: PVA polymer nanocomposites for photonic applications, J. Ind. Eng. Chem. (2016), http://dx.doi.org/10.1016/j.jiec.2016.10.002

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Table 1 FTIR band assignment of (a) PVA (b) 0.1Gd3+: PVA, (c) 0.1Eu3+: PVA, (d) (0.1Gd3+ + 0.1Eu3+): PVA, and (e) (0.1Gd3+ + 0.1Eu3+ + 0.015TiO2NP): PVA. The concentrations of the dopants are in wt%. Assignment of the bands

OH stretching vibrations CH symmetric stretching CH anti symmetric stretching Acetyl C¼C group Aromatic C¼C stretching group CH2 bending vibrations CH2 wagging vibration CH wagging mode, CO stretching of acetyl groups CC stretching vibrational mode CH2 stretching vibration

197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222

223 224 225 226 227 228 229

Wavenumber (cm1) b

c

d

e

3284 2849 2916 1664 1585 1431 1358 1256 947 847

3289 2849 2919 1665 1592 1432 1360 1257 948 848

3296 2854 2919 1672 1601 1432 1362 1257 948 848

3308 2856 2925 1684 1607 1432 1364 1258 948 848

3326 2857 2932 1689 1610 1432 1367 1258 948 850

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Thermal analysis

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The TG–DTA profiles were systematically analyzed to understand the effect of incorporation of rare earth ions and TiO2 nanoparticles in the PVA matrix on the thermal propertiesof the composite films. Thermograms were recorded for films of PVA, Gd3 + : PVA, Eu3+: PVA, (d) (Gd3+ + Eu3+): PVA, and (Gd3+ + Eu3+): PVA with embedded TiO2 NPs in the temperature range from 27 to 700  C, and they are shown in Fig. 4. Four stages of decomposition of PVA matrix were observed, and the weight loss at each stage is given in Table 2, along with corresponding temperature of all the samples. The initial weight loss (around 7%) at 95  C could have resulted from the degradation of large polymer chains into small fragments through primary decomposition and the removal of absorbed water [20]. The second stage of decomposition at 289  C

232 233 234 235

236 237 238 239 240 241 242 243

Ref.

a

modifications. The investigation of the FTIR spectral peaks and intensity parameters upon doping yielded information regarding Q3 the formation of the polymer complex. A new absorption band due to the stretching vibration of the carbonyl group (C¼O)[17] was observed at 1743 cm1 which is. This band can be ascribed to the absorption of hydrogen ions from the OH groups of PVA. More importantly, the intensity of the peak at 1142 cm1, which is sensitive to the PVA crystallinity, decreased in the doped systems, indicating that the crystallinity of the PVA matrix was reduced by the addition of dopant ions. The intensity of the peak corresponding to the C¼C band decreased upon addition of the rare earth ions, indicating a decrease in the number of C¼C groups along the PVA main chain. This can be ascribed to a possible interaction between the C¼C groups and the hydrogen of OH groups in the PVA structure, altering the C¼C p groups to C C s groups. The band at 2849 cm1 related to C H symmetric vibrations shifted to a higher wavenumber (nearly 2857 cm1) upon addition of the rare earth ions and TiO2 nanoparticles. The absorption of the hydrogen ions from the free OH groups played a pivotal role in the formation of the complexes. Moreover, the OH stretching vibrational band at 3660–3010 cm1 clearly decreased in intensity and shifted toward higher wavenumbers as the dopant ions and TiO2 nanoparticles were incorporated. This rather encouraging result suggests that non-covalent bonds were formed between Gd3+, Eu3+, and OH groups of the PVA skeleton [18]. The FTIR spectra of the (Gd3+ + Eu3 + + TiO2NP): PVA nanocomposite suggest that the polymer was probably connected to nanoparticles by a weak physical force instead of a strong chemical bonding force. The transparency decreased with the addition of dopant ions and TiO2 nanoparticles, which might have been due to the presence of TiO2 nanoparticles within the polymer [19]. The decrease in the FTIR peak intensity confirms that a complex was formed between rare earth ions and PVA matrix, which is in good agreement with the XRD analysis.

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[15] [15] [15] [15] [16] [16] [16] [16] [17] [17]

was accompanied by a weight loss of 68% resulting from the decomposition of acetate and side chains of PVA. The third decomposition stage occurred at 420  C, with a weight loss of 17% resulting from oxidation combustion of the PVA main chain. The complete decomposition of the pure PVA film occurred at 465  C [21]. For the films of Gd3+: PVA, Eu3+: PVA, (Gd3+ + Eu3+): PVA, and (Gd3+ + Eu3+ + 0.015TiO2NP): PVA, the first weight loss was observed at 92, 89, 87, and 90  C, with a corresponding weight loss of 5%, 4%, 5%, and 6%, respectively. This weight loss was likely caused by volatilization of small molecules like residual absorbed water in the polymer composites, which might have resulted from microscale Brownian motion of the main chain of PVA of the second weight loss was much lower and occurred in the doped samples at 168, 181, 171, and 164  C, with corresponding weight loss of 7%, 2%, 2%, and 4%, respectively. This lower value of the weight loss suggests that phase transition took place upon the addition of dopant ions. The third degradation temperature of the Gd3+: PVA, (c) Eu3+: PVA, (d) (Gd3+ + Eu3+): PVA, (e) (Gd3+ + Eu3 + + 0.015TiO2NP): PVA films occurred at 211, 244, 238, and 208  C, with a corresponding weight loss of 37%, 49%, 64%, and 44%, respectively. This thermal degradation occurred because of the heating of the polyene structure to form a polyaromatic structure. As a result, the polyenes decomposed to form macro-radicals, which in turn further decomposed to form cis and trans derivatives. In addition, the macro-radicals could form polyconjugated aromatic structures as a result of intramolecular cyclization and condensation reactions according to the Diels–Alder mechanism. The final stage of degradation was observed in all doped samples at a temperature above and near 420  C, possibly owing to thermooxidation of carbonized residues [22]. The highest temperature a sample could withstand, an indication of its thermal stability, was 298, 220, 246, 246, and 216  C for PVA, Gd3+: PVA, Eu3+: PVA, (Gd3 + + Eu3+): PVA and (Gd3+ + Eu3+ + 0.015TiO2NP): PVA, respectively. All the thermal degradation temperatures of the polymer samples were verified by the endothermic and exothermic peaks of their DTA profiles. The glass-transition (Tg) and melting temperatures (Tm) were determined from the DTA profiles of all the polymer composite samples, and their values are listed in Table 2. The Tg values of the rare-earth-doped PVA samples were observed to vary slightly. This variation in thermal properties was primarily a result of changes in the polymers’ interaction with dopant ions through hydrogen and covalent bonding. The interactions between the polymer molecules and dopant salts (or nanofillers) were crucial factors of the film properties because many other parameters such as crystallinity, molecular packing, and extent of dispersion of nanofillers also depend on them [23]. The thermal stability and melting temperatures of the rare-earth-doped PVA composites and nanocomposites were reduced from those of pure PVA, possibly owing to the reduction in intra- and inter-hydrogen bonds of PVA

Please cite this article in press as: K.N. Kumar, et al., Dazzling red emission from TiO2 nanoparticles impregnated co-doped Gd3++Eu3+: PVA polymer nanocomposites for photonic applications, J. Ind. Eng. Chem. (2016), http://dx.doi.org/10.1016/j.jiec.2016.10.002

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Fig. 4. TG/DTA profiles of (a) PVA, (b) 0.1Gd3+: PVA, (c) 0.1Eu3+: PVA, (d) (0.1Gd3+ + 0.1Eu3+): PVA, and (e) (0.1Gd3+ + 0.1Eu3+ + 0.015TiO2NPs): PVA composite films. 293 295

after the addition of dopant ions or dispersion of nanofillers and the resulting decrease in the crystallinity, as proved by XRD in an earlier study [24].

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Optical absorption studies

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The absorption spectra of pure PVA; Eu3+: PVA, Gd3+: PVA, and (Gd3++ Eu3+): PVA composites; and the (Gd3++ Eu3+ + TiO2NP): PVA nanocomposite were recorded in the wavelength range of 240– 650 nm; the results are shown in Fig. 5. The un-doped PVA

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298 299 300

exhibited a broad low-intensity absorption peak at 280 nm, which was actually a combination of two peaks at 280 and 264 nm. These bands are mainly ascribed to carbonyl segments of the general form of (CH¼CH)nCO , where n = 1, 2, . . . , possibly because of the existence of acetaldehyde and dissolved air in the vinyl acetate monomer during polymerization. The absorption band of simple carbonyl groups along the polymer chain was represented by the small shoulder at 264. The absorption band at 280 nm is ascribed to carbonyl groups associated with ethylene unsaturation of the type  (CH¼CH)2CO , which indicates the presence of conjugative

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Table 2 TG/DTA results of (a) PVA, (b) 0.1Gd3+: PVA, (c) 0.1Eu3+: PVA, (d) (0.1Gd3+ + 0.1Eu3+): PVA, (e) (0.1Gd3+ + 0.1Eu3+ + 0.015TiO2): PVA films. The concentrations of the dopants are in wt%. Sample

TG (degradation temperature with weight loss) ( C) (%)  2 Ist

(a) (b) (c) (d) (e)

95 92 89 87 90

(7%) (5%) (4%) (5%) (6%)

IIIrd

IVth

Tg ( C)

Tm ( C)

– 168 (7%) 181 (2%) 171 (2%) 164(4%)

289 (68%) 211 (37%) 244 (49%) 238 (64%) 208 (44%)

420 (17%) 419 (11%) 428 (13%) 423 (12%) 417 (9%)

113 116 107 105 116

339 236 293 291 231

Fig. 5. Absorption spectra of pure (a) PVA, (b) Gd3+: PVA, (c) Eu3+: PVA, (d) (Gd3 + + Eu3+): PVA, (e) (Gd3+ + Eu3+ + 0.015TiO2NPs): PVA composite films. 310 311

DTA

IInd

double bonds of polyenes. The weak signal of this particular band was due to the isolated carbonyl groups [25]. The rare earth ions

are characterized by partially filled 4f shells, and they are shielded by the 5s2 and 5p6 electrons. The Gd3+–PVA composite film exhibited two absorption f–f bands at 280 and 312 nm assigned to the electronic transitions 6IJ ! 8S7/2 and 6PJ ! 8S7/2, respectively [26]. Moreover, the broad absorption band at 273 nm could be a charge-transfer transition band (CTB) resulting from the electron transfer from the ligand O2 (2p6) orbital to the empty state of 4f6 of Eu3+ ions [27]. For the Eu3+–PVA composite film, a strong absorption band at 374 nm was due to the electronic transition 7F0 ! 5G4, which represents f–f optical excitations between the 7F0 ground state of Eu3+ ions [28]. For the (Gd3++ Eu3+ + TiO2NP): PVA nanocomposite film, the absorption bands at 278 and 320 nm are assigned to amorphous titanium species and non-framework anatase TiO2. The charge-transfer band of Eu3+ ions at 278 nm overlaps the absorption band of TiO2 nanoparticles; these two bands indicate the presence of TiO2 nanoparticles in the co-doped polymer matrix [29].

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Photoluminescence analysis

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The excitation and emission spectral profiles of the Gd3+: PVA film are shown in Fig. 6. The Gd3+ ion has a 4f7 electron configuration and the energy band is between the ground singlet (8S7/2) and first excited (6P7/2) electronic states. It was theoretically demonstrated that the 4f7 electron levels of gadolinium extend to

330

Fig. 6. (a) Excitation and (b) emission spectra of Gd3+: PVA composite film.

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150,000 cm1, but only the states up to 67,000 cm1 have been revealed experimentally [30]. We observed three prominent excitation bands at 253, 266, and 270 nm in the excitation spectrum of the Gd3+–PVA film, as shown in Fig. 6(a), and they are assigned to the electronic transitions (8S7/2 ! 6D7/2), (8S7/ 6 8 6 2 ! I11/2), and ( S7/2 ! P3/2), respectively [31]. Along with these prominent excitation bands, moderate bands at 229 and 241 nm attributed to host (PVA) lattice absorption [32] were also observed. Several emission bands associated with a broad band from 336 to 487 nm were observed in the emission spectrum of the Gd3+–PVA film, as shown in Fig. 6(b). However, the small and sharp lowintensity peak at 312 nm was due to the presence of Gd3+ ions, and it is assigned to the electronic transition (6P7/2 ! 8S7/2). As per the literature, the sharp peaks between 200 and 400 nm are assigned to characteristic forbidden intra-configuration 4f–4f transitions of Gd3+ ions [31]. Several researchers analyzed the emission characteristics of pure PVA at room temperature [32]. It was shown that under UV excitation, the PVA emission spectrum exhibits four emission peaks centered at 364, 399, 428, 450, and 467 nm; similar bands have also been reported in earlier studies [33]. These bands might be assigned to the recombination of free charge carriers at the defects in the PVA film. From the emission spectral features of the Gd3+: PVA film in our study, we strongly believe that the band representing the emission from Gd3+ ions emission overlapped the strong emission band of PVA. The fluorescence excitation spectra of the 0.1Eu3+–PVA film are shown in Fig. 7. They consist of relatively narrow excitation bands, which correspond to the ground state absorption of 7F0. We observed the excitation bands at 364, 378, 385, 396, 416, and 464 nm, and these bands are assigned to the electronic transitions 7 F0 ! 5D4, 7F0 ! 5L7, 7F0 ! 5G2, 7F0 ! 5L6, 7F0 ! 5D3, and 7F0 ! 5D2, respectively [34]. The band at 396 nm [7F0 ! 5L6] was most prominent. By using this 396 nm excitation band, we obtained the fluorescence emission spectra of the Eu3+-doped PVA matrix with different concentrations of Eu3+ ions, these spectra are shown in Fig. 8. We observed five emission bands under the excitation at 396 nm. All the samples exhibited strong red luminescence at 618 nm, which originated from the forced electric–dipole transition 5D0 ! 7F2. In addition to this prominent red emission, the spectra also exhibited characteristic 4f–4f emission transitions within the 4f6 configuration of Eu3+: 5D0 ! 7F0 at 580 nm, 5D0 ! 7F1 at 594 nm (magnetic dipole transitions), 5D0 ! 7F3 at 688 nm, and 5 D0 ! 7F4 at 695 nm [35].

Fig. 7. Excitation spectrum of 0.1Eu3+: PVA composite film.

7

Fig. 8. Emission spectra of Eu3+: PVA composite films with Eu3+ concentrations of 0.025, 0.05, 0.075, 0.1, 0.15, and 0.2 wt%.

The fluorescence intensity was significantly enhanced when the Eu3+ ions in the PVA matrix was increased, with the highest intensity corresponding to a Eu3+-ion concentration of 0.1 wt%. Based on the fluorescence features, the optimized concentration for Eu3+ ions was determined to be 0.1 wt%. The emission intensities of the spectral profile were drastically reduced when the Eu3+-ion concentration was increased to 0.1 wt%, possibly owing to the effect of concentration quenching. Ion–ion interactions were initiated by the decreasing distance between interacting activators as the dopant concentration was increased, leading to concentration quenching. In order to increase the emission performance of the 0.1Eu3+: PVA (wt%) polymer matrix, we introduced the secondary rare earth ions such as Gd3+ at different concentrations. When the co-dopant concentration of Gd3+ ions was increased from 0.05 to 0.2 wt% in the 0.1Eu3+: PVA (wt%) polymer matrix, the photoluminescence features of the Eu3+ ions were slightly enhanced under 270 nm excitation, as shown in Fig. 9. This could have been caused by the efficient energy transfer from the Gd3+ ions to Eu3+ ions in the PVA composite. It can be seen that the emission transition intensity of 5D0 ! 7F2 in the co-doped polymer system became stronger than in the Eu3+-doped PVA

Fig. 9. Emission spectra of co-doped PVA composites of 0.1 wt% of Eu3+ ions and 0.025, 0.05, 0.1, 0.15, and 0.2 wt% of Gd3+ ions.

Please cite this article in press as: K.N. Kumar, et al., Dazzling red emission from TiO2 nanoparticles impregnated co-doped Gd3++Eu3+: PVA polymer nanocomposites for photonic applications, J. Ind. Eng. Chem. (2016), http://dx.doi.org/10.1016/j.jiec.2016.10.002

378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398

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matrix, with the spectrum obtained from the co-doped system with 0.1 wt% Gd3+ exhibiting predominant emission features. Hence, the optimized co-doping concentration of Gd3+ ions was determined to be 0.1 wt%. The emission intensities of the co-doped polymer matrix slowly decreased as the Gd3+ ion concentration was increased above 0.1 wt%, probably because of concentration quenching resulting strong interactions between the Gd3+ ions [36]. All these results revealed that the co-doping of Gd3+ ions with Eu3+ ions plays a pivotal role in improving the luminescence of Eu3+ in the co-doped PVA matrix. In this phenomenon, the Gd3+ ions played the role of sensitizers and Eu3+ ions were the activators. Successful emission photons from the Gd3+ ions under 270 nm excitation were collectively captured by the Eu3+ ions within the PVA matrix. Several approaches were made to further improve the photoluminescence properties of the co-doped (Gd3+ + Eu3+): PVA films. In the second part of the energy transfer, TiO2 nanoparticles were incorporated into the (Gd3+ + Eu3+): PVA matrix. The fluorescence excitation and emission spectral features of pure TiO2 nanoparticles are shown in Fig. 10(a) and (b). Several excitation bands were observed at 242, 250, 260, 274, 299, and 308 nm in the excitation spectrum of pure TiO2 nanoparticles, as shown in Fig. 10(a) with the excitation band at 299 nm being most prominent. The emission spectrum of pure TiO2 nanoparticles under excitation at 299 nm is shown in Fig. 10(b). The broad emission band at 370–480 nm comprised three emission bands at 399, 441, and 468 nm, which originated from the indirect band gap and surface recombination process [37]. The band at 468 nm was most prominent. A high number oxygen vacancies were possible in TiO2 nanoparticles because of the high surface to volume ratio. The

Fig. 10. (a) Excitation and (b) emission spectra of TiO2 NPs.

obtained emission bands might have been caused by emission from the radiative recombination of self-trapped excitons. The most common defects in TiO2 nanoparticles are obviously oxygen vacancies, which usually act as radiative centers. The emission peak at 399 nm is attributed to the emission of near-band-gap transition of anatase. The emission bands at 441 and 468 nm arose from the excitonic photoluminescence, probably due to the presence of surface oxygen vacancies and defects [38,39].

429

Mechanism of energy transfer from Gd3+ ions to Eu3+ions and from TiO2 nanoparticles to Eu3+ ions in co-doped PVA matrix So far, there have been no reports on the energy transfer from TiO2 nanoparticles embedded in co-doped polymer matrixes for photonic applications. In our study, we made a first attempt to enhance the photoluminescence efficiency of Eu3+ ions in the PVA matrix with TiO2 nanoparticles. We noticed the energy migration from TiO2 to Eu ions as a result of local interactions. Fig. 11 shows the photoluminescence spectra of the co-doped (Gd3+ + Eu3+): PVA films with and without TiO2 nanoparticles under 274 nm excitation. A remarkable enhancement of the Eu3+ emission intensities in the co-doped system was observed after the films were impregnated with TiO2 nanoparticles. There were two possible mechanisms for this enhancement: energy migration from TiO2 to Eu3+ ions, which enhanced the population of the excited ions; an increase in the lifetime of the emitting energy level of 5D0 of the Eu ions owing to the interaction with TiO2 nanoparticles. By using the excitation wavelength of 274 nm, which is related to the plasmon band of TiO2 nanoparticles, we obtained the emission spectra shown in Fig. 11. The absorption cross-section of the TiO2 nanoparticles was very large and the TiO2 nanoparticle domains were also excited to higher energy states. The excitations at the 5D0 level naturally enhanced the intensity of all the 5D0 ! 7FJ (J = 0–6) transitions as the concentration of Eu3+ ions was increased. However, the highest intensity was observed for the transition 5 D0 ! 7F2, which is an allowed hypersensitive electric–dipole transition. The remarkable enhancement of the emission intensity was due to hypersensitivity of the transition in the presence of TiO2 nanoparticles. The energy-migration probability is proportional to the superposition integral of two spectral shapes, that of the excitation of the donor (TiO2 nanoparticles in our study) and the acceptor (Eu3+ ions in citation). As shown in Fig. 12, there was considerable overlap between the emission spectrum of the TiO2 nanoparticles and the excitation spectrum of the Eu3+ ions. Based on the above discussion, we conclude that energy migration possibly took place

437

Fig. 11. Emission spectra of co-doped (0.1Eu3+ + 0.1Gd3): PVA (wt%) polymer nanocomposite films containing 0.005, 0.01, 0.015, 0.02, and 0.025 wt% of TiO2 NPs.

Please cite this article in press as: K.N. Kumar, et al., Dazzling red emission from TiO2 nanoparticles impregnated co-doped Gd3++Eu3+: PVA polymer nanocomposites for photonic applications, J. Ind. Eng. Chem. (2016), http://dx.doi.org/10.1016/j.jiec.2016.10.002

430 431 432 433 434 435 436

438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472

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9

+

Fig. 12. Overlapping emission spectrum of synthesized TiO2 NPs and excitation spectrum of Eu3+: PVA composite film. 473 474 475 476 477 478 479 480 481 482 483 484 485 486

from Gd3+ ions to Eu3+ ions and it unambiguously occurred between TiO2 nanoparticles and Eu3+ ions. However, the emission bands at 594 and 618 nm are attributed to 5D0 ! 7F1 and 5D0 ! 7F1 transitions, respectively, of Eu3+. Nevertheless, the 5D0 ! 7F2 transition was electrically allowed, and this transition was very sensitive to the environment of the Eu3+ ions. Meanwhile, the 5 D0 ! 7F1 transition was magnetically allowed and it was not influenced by the environment. The R value is a measure of the breaking of the centro-symmetry and the degree of disorder around the Eu3+ ions in any complex. This R value was determined from the relative intensity ratio between the 5D0 ! 7F2 and 5 D0 ! 7F1 transitions [40]. The R values calculated from the spectra shown in Figs. 8, 9 and 11 were 3.0, 2.4, and 2.8 for the 0.1Eu3+: PVA (wt%), (0.1Eu3+ + 0.1Gd3+): PVA (wt%), and (0.1Eu3+ + 0.1Gd3

+ 0.015TiO2NP): PVA (wt%) composites, respectively, indicating that there was a wide distribution of R values. This favorable result affirmatively indicates that the Eu3+ ions were surrounded by different local environments (Gd3+ ions and TiO2 nanoparticles) in the PVA matrix in different samples [41].

487

Decay analysis

492

In order to understand the energy-migration process, the lifetime decay dynamics of the monodoped (Gd3+ or Eu3+) and codoped (Gd3+ and Eu3+) polymer systems was systematically measured and extensively studied; the results are shown in Fig. 13. The lifetime of the excited state of the Gd3+ ions in the Gd3 + -doped PVA film was calculated to be 0.015 ms. The decay curve was well fitted with a single exponenetial function, as shown in Fig. 13(a). The luminescence (red emission at 618 nm) lifetime of the 5D0 excited states of the Eu3+ ions in the PVA composite under 396 nm excitation was calculated to be 0.17 ms. It can be seen in Fig. 13(d) that the decay curve of the monodoped Eu3+–PVA film was well fitted with a single exponential function. The lifetime is Q5 calculated by fitting the curve to the following equation

493

It = I0exp( t/t ),

(1)

506

where It at time t, I0 is the intensity at time 0, and t is the luminescence lifetime. For the co-doped system, the decay curves exhibited non-exponential behavior owing to the different sites of the Eu3+ ions. However, lifetime decay curve fitted the expression given by

508 507

I(t) = A1exp( t/t 1) + A2exp( t/t 2),

(2)

513

where I(t) is the emission intensity, A1 and A2 are constants, t1 and t 2 are the short and long lifetimes, respectively. The average lifetimes (t avg) of the Gd3+ and Eu3+ ions were determined by the equation

515 514

t avg ¼

A1 t 1 2 þ A2 t 2 2 : A1 t 1 þ A2 t 2

ð3Þ

Fig. 13. Decay curves of (a) red emission (618 nm) (Eu3+) and (b) blue emission (413 nm) (Gd3+) from co-doped (Gd3+ + Eu3+): PVA with and without TiO2 NPs. The emission lifetimes (t m) are also shown.

Please cite this article in press as: K.N. Kumar, et al., Dazzling red emission from TiO2 nanoparticles impregnated co-doped Gd3++Eu3+: PVA polymer nanocomposites for photonic applications, J. Ind. Eng. Chem. (2016), http://dx.doi.org/10.1016/j.jiec.2016.10.002

488 489 490 491

494 495 496 497 498

499 500 501 502 503 504 505

509 510 511 512

516 517

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Table 3 Calculated emission lifetimes. Polymer composite (concentration in wt%) PVA: PVA: PVA: PVA: PVA:

518 519 520 521 522 523 524 525 526

527 528 529 530 531 532 533 534 535 536 537 538

540 539 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555

0.1 Gd3+ 0.1 Eu3+ 0.1Gd3+ + 0.1Eu3+ 0.1Gd3+ + 0.1Eu3+ 0.1Gd3+ + 0.1Eu3+ + 0.015TiO2 NPs

558 559 560 561 562 563 564 565 566 567 568 569 570

Emission wavelength (nm)

Lifetime (ms)

270 396 270 270 274

313 618 313 (Gd3+) 618 (Eu3+) 618 (Eu3+)

0.015 0.17 0.01 0.18 0.19

However, the decay curves of the monodoped (Gd3+ or Eu3+) PVA films slightly deviated from the single exponenetial nature when compared with decay curves of the co-doped PVA composite, possibly due to modification of the fluorescent dynamics by the co-doped ions. The lifetime of the excited state of Gd3+ ions in the co-doped (Gd3+ + Eu3+)–PVA film was calculated to be 0.01 ms. This reduced lifetime clearly indicates the occurrence of an energy transfer from the Gd3+ ions to the Eu3 + ions in the co-doped system [42], as suggested by the nonexponential nature of the decay curve, which is indicative of an interaction between the acceptor and donor ions. Since the availability of the donor (Gd3+) ions for each acceptor (Eu3+) was higher in the co-doped polymer composite, the average distance between the acceptor and donor ions was decreased, leading to electronic multipole–multipole interactions. This condition was preferable for energy transfer. Consequently, the above lifetime decay dynamics strongly suggests that an efficient energy transfer process from Gd3+ to Eu3+ took place in the PVA film [43]. According to Dexter’s theory, the probability of the energy transfer through multipolar interaction can be expressed by following equation: Z Q f D ðEÞF A ðEÞ PðRÞa b A dE; ð4Þ Ec R td where P represents the probability of energy transfer, t D is the donor emission decay time, QA is the absorption cross section of the acceptor, R is the distance between the donor and the acceptor ions, and b and c are the energy-transfer parameters [44]. The calculated lifetimes of Gd3+ and Eu3+ in monodoped and codoped polymer matrixes are presented in Table 3. It can be seen that the lifetime of the gadolinium emission in the co-doped polymer matrix was lower than that in the monodoped system. The energy-transfer probability is inversely proportional to the decay time t D according to Eq. (1). Therefore, the lower lifetime decay of the Gd3+ emission strongly affirms the energy-transfer pathway from Gd3+ to Eu3+ ions [44]. In order to clearly understand the energy-migration process from Gd3+ to Eu3+ ions, the energy-transfer efficiency (hET) from the sensitizer Gd3+ ions to activator Eu3+ ions was estimated by following equation:

hET = 1  t d/t d0, 557 556

Excitation wavelength (nm)

(5)

where t d0 is the intrinsic lifetime of the sensitizer (Gd3+) and t d is the lifetime of the sensitizer while in the presence of the activator (Eu3+). The ET efficiency was evaluated and found to be 33.33% [45]. By co-doping PVA with Gd3+ along with Eu3+, the Eu3+ emission peak (618 nm) intensity was remarkably enhanced under the indirect excitation at 270 nm. Consequently, its lifetime was also considerably increased. The emission lifetime of the Eu3+ ions in the co-doped polymer matrix was found to be 0.18 ms. The lifetime of the 5D0 levels of Eu3+ was higher in the co-doped system than in the monodoped polymer system. The lifetime decay curve of Eu3+ ions in the co-doped polymer under indirect 270-nm excitation was well fitted by a non-exponential function, as shown in Fig. 13(e), confirming the efficient energy migration from the Gd3+ to Eu3+ in the PVA composite [46].

Fig. 14. Partial energy-level diagram of energy transfer from Gd3+ to Eu3+ and from TiO2 NPs to Eu3+ in PVA composite films.

Upon the addition of TiO2 NPs (0.015 wt%) to the co-doped (Gd3+ + Eu3+)–PVA system, the lifetime of 5D0 levels of the Eu3+ ions was increased, possibly due to the coupling effect of the plasmonic field of nanoparticles and rare earth ions. This might have affected the lifetime of the rare earth ions’ radiative level and result in an energy migration. After TiO2 NPs were impregnated along with both Gd3+ and Eu3+ in the co-doped polymer composite, the lifetime of the red emission from Eu3+ ions was found to be 0.19 ms. The lifetime improvement by the addition of Gd3+ and TiO2 NPs to the Eu3+-doped PVA complex confirms the occurrence of energy transfer from the Gd3+ ions and TiO2 NPs to the Eu3+ ions. The lifetime decay curve of the Eu3+ ions in the TiO2-impregnated codoped PVA matrix appeared non-exponential, which supports the earlier conclusion of an efficient interaction taking place between the acceptor and donor ions [47]. The energy transfer mechanism is illustrated by a schematic energy level diagram shown in Fig. 14.

571

Conclusion

587

We successfully synthesized PVA composite films doped with Gd3+ ions, Eu3+ seperately and both Gd3+ and Eu3+ ions, and with and without TiO2-nanoparticle impregnation by solution casting. The XRD profiles revealed that the composite films of PVA doped with rare earth ions were semi-crystalline. FTIR spectral analysis affirmed the complex formation and ion–polymer interactions while both types of rare earth ions were incorporated into the polymer composite along with TiO2 nanoparticles. The thermal stability and decompositon dynamics of films of pure PVA, Gd3+ doped, and Eu3+ doped PVA with and without TiO2 nanoparticles were evaluated by TG/DTA profiles. The optical properties were systematically analyzed by optical absorption and luminescence

588

Please cite this article in press as: K.N. Kumar, et al., Dazzling red emission from TiO2 nanoparticles impregnated co-doped Gd3++Eu3+: PVA polymer nanocomposites for photonic applications, J. Ind. Eng. Chem. (2016), http://dx.doi.org/10.1016/j.jiec.2016.10.002

572 573 574 575 576 577 578 579 580 581 582 583 584 585 586

589 590 591 592 593 594 595 596 597 598 599

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623

spectral studies. Upon with UV excitation, prominent red emission at 618 nm (5D0 ! 7F2) of Eu3+ ions was observed and other transitions of Eu3+ were clearly accounted for. The optimized concentration of the Eu3+ ions was found to be 0.1 wt% in the singly doped Eu3+: PVA matrix. The PL efficiency of Eu3+ ions was increased through an energy-transfer process by co-doping with Gd3+. The optimized sensitizer concentration of the Gd3+ ions in the co-doped (Gd3+ + Eu3+): PVA system was found to be 0.1 wt% based on the fluorescence spectral analysis of the co-doped polymer system. Surprisingly, the emission of the Eu3+ ions in the co-doped polymer complex was remarkably enhanced by the addition of TiO2 nanoparticles to the co-doped (Gd3+ + Eu3+): PVA system, probably because of the efficient energy transfer from the TiO2 nanoparticles to Eu3+ ions. In the (Gd3+ + Eu3+): PVA system with embedded TiO2 nanoparticles, an energy migration from the TiO2 nanoparticles to Eu3+ ions also took place. The sensitizing effect of the TiO2 nanoparticles and Gd3+ ions significantly improved the emission of the Eu3+ ions. This dual energy-transfer phenomena was substantiated by overlapping spectral profiles and the results of photoluminescence studies. The energy-migration pathway was clearly elucidated through partial energy-level diagrams as well as lifetime decay dynamics. We believe these red luminescent polymer nanocomposite materials are promising candidates for red luminescent photonic applications.

624 Q6

Uncited reference

601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622

625

[11].

626

Acknowledgment

627

630

This research work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of the Science ICT & Future Planning (NRF2015R1A1A3A04001268).

631

References

628 629 Q7

632 633 634 635

636

637

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Please cite this article in press as: K.N. Kumar, et al., Dazzling red emission from TiO2 nanoparticles impregnated co-doped Gd3++Eu3+: PVA polymer nanocomposites for photonic applications, J. Ind. Eng. Chem. (2016), http://dx.doi.org/10.1016/j.jiec.2016.10.002