Bright green emission from f-MWCNT embedded co-doped Bi³⁺ + Tb³⁺:polyvinyl alcohol polymer nanocomposites for photonic applications

Abstract

A bright, dazzling green emission has been obtained from functionalized multi walled carbon nanotube (f-MWCNTs)-embedded Bi³⁺ + Tb³⁺:PVA polymer nanocomposites under UV excitation. We successfully synthesized Tb³⁺:PVA, Bi³⁺ + Tb³⁺:PVA and Bi³⁺ + Tb³⁺ + f-MWCNTs:PVA polymer films by a solution casting method. For these polymer films, their XRD and FTIR spectral profiles were analyzed for structural details and ion–polymer interaction mechanism. Tb³⁺ doped at different concentrations in PVA polymer films displayed a green emission at 546 nm (⁵D₄ → ⁷F₅) under 370 nm (⁷F₆ → ⁵L₁₀) excitation. Upon co-doping with Bi³⁺ in the Tb³⁺:PVA polymer film, the film exhibits enriched green emission compared to single Tb³⁺:PVA under 320 nm excitation due to the energy transfer from Bi³⁺ to Tb³⁺. Surprisingly, a common excitation band was found at 320 nm for Tb³⁺, Bi³⁺ and f-MWCNTs. After dispersion of the f-MWCNTs in the co-doped Bi³⁺+Tb³⁺:PVA polymer films, the photoluminescence properties were remarkably enhanced, and a prominent green emission was observed at 546 nm. The green emission of Tb³⁺ is significantly enhanced though an efficient energy transfer process from Bi³⁺ to Tb³⁺ and f-MWCNTs to Tb³⁺. The possible energy transfer mechanism was clearly demonstrated via several fluorescent methods. The energy transfer mechanism was substantiated with fluorescence lifetime decay dynamics. From these results, these f-MWCNTs-embedded Bi³⁺+Tb³⁺:PVA polymer films could be suggested as promising candidates for green luminescent materials for photonic devices.

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1. Introduction

In recent years, trivalent rare earth ion-doped organic luminescent materials have gained abundant interest in their processing for their use in the field of photonics, such as display devices, optical sensors and light emitting diodes (LEDs). Much attention has been focused on these promising materials that consist of lanthanide–organic frameworks because of the intrinsic physical properties of the trivalent lanthanide ions.¹ Rare earth doped complexes are widely used in several photonic devices because of their superior advantages such as narrow emission bands for high color purity, long lifetime, large stokes shifts, high quantum efficiency and good processability. These versatile materials could also be considered as light converting molecular devices (LCMD).² Nevertheless, rare earth–organic complexes are found to be potential candidates for luminescence and electroluminescence devices, detector optical displays, telecommunications, optical signal displays, energy-harvesting devices and for use in a variety of areas such as fluoroimmunoassay.³ Polymer-rare earth complexes have more advantages such as light weight, ease of fabrication, design flexibility and excellent mechanical properties and transparency over other inorganic complexes.⁴ Polyvinyl alcohol (PVA) exhibits semicrystallinity and has several advantages such as solubility in water, chemical resistance, high mechanical strength, good processability, good charge storage capacity and high dielectric strengths over other polymers. PVA is widely used in the fields of contact lenses, synthetic fibers, papers, textile, coatings and binding industry.⁵ Because of the electronic transitions between the 4f orbitals, the rare earth ions exhibit prominent luminescence characteristics. The emission transitions yield sharp spectral lines in the optical spectra because the 4fⁿ shells are shielded by the 5S² and 5P⁶ orbitals. Specifically, some lanthanide ions, such as Sm³⁺, Dy³⁺, Eu³⁺ and Tb³⁺, can emit a characteristic fluorescence with a high photoluminescence efficiency and long fluorescence lifetime due to the f–f transition. Terbium (Tb³⁺) has unique properties over the other rare earth ions. It shows an intense green emission at 546 nm due to the ⁵D₄ → ⁷F₅ electronic transition. Tb³⁺ consists of broad excitation bands in the short and long wavelength regions due to 4f⁸ → 4f⁷ 5d¹ and 4f⁸ → 4f⁸ electronic transitions, respectively. Hence, if any sensitizer ion possessing strong absorption or excitation in the UV region is situated near Tb³⁺ ions in a complex, they can easily enhance the Tb³⁺ emission through energy transfer process (sensitizer for Tb³⁺ ions).⁶ In this perspective, Bi³⁺ has been co-doped with Tb³⁺ to enhance the Tb³⁺ ion emission performances because Bi³⁺ exhibits strong excitation in the UV region due to the ¹S₀ → ³P₀ transition. It can activate Tb³⁺ ions efficiently as a sensitizer to obtain the intense green emission.⁷ In spite of the addition of Bi³⁺ ions to the Tb³⁺ ion doped complexes, the green emission of the Tb³⁺ ions has been reduced, probably due to a concentration quenching effect above the optimized concentration of Bi³⁺ ions. The luminescence quenching occurs due to ionic aggregation, and this aggregation acts as a quenching center.⁸ To overcome this situation, we can choose another strategy, which is the addition of a suitable nanofiller, to obtain further luminescence efficiency from the activator ion.⁹ Homogeneous dispersions of nanofiller in the polymer matrix could become a new field of research for several advanced applications. Much research effort has been made to obtain an improvement in the luminescence efficiency while retaining the mechanical properties and stability towards metallic anodes by the incorporation of nanofillers.¹⁰ Nevertheless, by encapsulating the MWCNTs in the rare earth complexes like phosphors and nanostructures, the photoluminescence properties of the rare earth ions are drastically decreased.¹¹ However, in some cases, the MWCNT assembled rare earth complexes show prominent photoluminescence features under long UV exposure. This might be due to the relatively rigid environment of the MWCNTs. Moreover, the MWCNTs can protect the complexes from decomposing under UV irradiation.¹² Based on this unique character of the MWCNTs, we made an attempt to incorporate the f-MWCNTs into a terbium-doped PVA polymer complex. In our present study, f-MWCNTs were dispersed in co-doped Bi³⁺+Tb³⁺:PVA polymer films at different concentrations to obtain enhanced emission intensities of Tb³⁺.

2. Experimental

2.1 Functionalization process of MWCNTs

The raw material of the MWCNTs was refluxed in an H₂SO₄–HNO₃ mixture in a 3 : 1 (v/v) ratio for 20 h. The obtained mixture was filtered properly after dilution. In order to remove the traces of solvents and excess acid from this solution, we washed the mixture several times under vacuum with DD water. The obtained material was kept at 110 °C for 12 h in an oven. The final product was crushed and properly ground with a cleaned mortar to get f-MWCNT powder.¹³

2.2 Preparation of MWCNT embedded Bi³⁺ + Tb³⁺:PVA polymer nanocomposite films

Bi³⁺ and Tb³⁺ ion doped polymer composite films were prepared by employing a solution casting method. Standard chemicals of polyvinyl alcohol (PVA) (MW = 186 × 10³), bismuth nitrate pentahydrate [Bi(NO₃)₃·5H₂O], terbium nitrate pentahydrate [Tb(NO₃)₃·5H₂O] and multiwalled carbon nanotubes (MWCNTs) were purchased from Sigma-Aldrich. Precursor chemicals were taken in appropriate weight percentage ratios using triple distilled water as a solvent. Initially, PVA and the dopant salts were properly dissolved in triple distilled water in one beaker. The functionalized MWCNTs were dispersed in water at various concentrations (0.01, 0.02, 0.03, 0.04, 0.05, 0.06 and 0.07 mg) in a separate beaker using an ultrasonicator. These two solutions were intermixed in another separate beaker and continuously stirred at RT (∼30 °C) for 10–12 h until the mixture was homogeneous. The obtained solution was cast onto polypropylene dishes, and those solutions were allowed to evaporate slowly at RT. The final product was dried upon warming to remove all traces of the solvent. The dried f-MWCNT embedded co-doped Bi³⁺ + Tb³⁺:PVA polymer nanocomposite films were collected for further characterization.

3. Characterization

X-ray diffraction profiles of the rare earth doped PVA polymer with and without f-MWCNTs were recorded on a SEIFERT 303 TT X-ray diffractometer (XRD) with CuKα (1.5405 Å). It was operated at 40 kV and 50 mA of voltage and anode currents, respectively, at an angle ranging from 10° to 90° and a scan speed of 10° min⁻¹. FT-IR spectra of the prepared samples were carried out on EO-SXB IR spectrometer in the range of 4000–500 cm⁻¹. Phillips TECHNAI FE 12 transmission electron microscopy (TEM) was used for particle size and shape confirmation and operated at 120 kV. The TG measurements for the pure PVA, Tb³⁺:PVA, Bi³⁺:PVA and Bi³⁺ + Tb³⁺ + f-MWCNTs:PVA polymer films were carried out using thermo-gravimetric analysis and differential scanning calorimetry, Model: SDTQ600TA Instrument (specimens were scanned in the nitrogen atmosphere from 30–60° at a heating rate of 10 °C min⁻¹) from RT to 700 °C. The photoluminescence (excitation and emission) spectra of Tb³⁺:PVA, Bi³⁺:PVA films and Bi³⁺ + Tb³⁺ + f-MWCNTs:PVA polymer nanocomposites were recorded on a Scinco FluoroMate FS-2 visible fluorescence spectrometer with a Xe arc lamp of 150 W as an excitation source for a steady state emission spectrum measurement.

4. Results and discussion

4.1 XRD analysis

X-ray diffractometer data was recorded before functionalization and after functionalization of the MWCNTs and are presented in the insert of Fig. 1(f). The intense diffraction peak at 2θ of 25.2° corresponds to the (002) plane, and the two low intensity peaks at 42.6° and 52.7° corresponds to (100) and (004) planes of the MWCNTs.¹⁴ X-ray diffraction investigations were performed to confirm the microstructural changes that take place as well as to characterize the molecular structure of the various polymer composites. The diffraction patterns of pure PVA, Tb³⁺:PVA, Bi³⁺:PVA, co-doped Bi³⁺ + Tb³⁺:PVA polymer composite films with and without f-MWCNTs are shown in Fig. 1(a)–(e). The un-doped PVA polymer film exhibits an intense diffraction peak at 19.2°, a weak shoulder at 22.3° and a low intensity peak at 40.2°, which correspond to the characteristic reflections of the (101), (200) and (111) planes of the monoclinic unit cell (JCPDS: 53-1587) of PVA, respectively.^15,16 Upon doping Bi³⁺ or Tb³⁺ ions in the PVA polymer matrix, the intensity of the characteristic diffraction peaks decreased. This variation in intensity suggests that the strong interactions between the polymer chains and the dopant ions lead to the disruption of the semicrystalline phase of the PVA bonding scheme.¹⁷ Due to the addition of a very small amount of f-MWCNTs in the co-doped PVA polymer system, the diffraction peaks pertaining to f-MWCNTs were not found.

Fig. 1 (a) XRD profiles of pure (a) PVA, (b) Bi³⁺:PVA, (c) Tb³⁺:PVA, (d) (Bi³⁺ + Tb³⁺):PVA, (e) (Bi³⁺ + Tb³⁺) + f-MWCNTs (0.05 mg):PVA polymer films and (f) f-MWCNTs.

4.2 TEM analysis

The TEM images of the MWCNTs, which are in an agglomerated form and are hydrophobic, are shown in Fig. 2(a). After functionalization, they were separated from each other and possessed a hydrophilic nature, as shown in Fig. 2(b). These functionalized MWCNTs were more convenient agents to disperse in the polymer composites for further investigation. The functionalized MWCNTs were well dispersed in the polymer composite films and are shown in Fig. 2(c). The f-MWCNTs and polymer are clearly represented in Fig. 2(c). In the figure, the black background could be the polymer, and the tube-like structures indicate the f-MWNTs.

Fig. 2 TEM images of (a) non-functionalized MWCNTs, (b) functionalized MWCNTs, (c) f-MWCNTs (0.05 mg) embedded co-doped Bi³⁺ (0.15 wt%) + Tb³⁺ (0.15 wt%):PVA polymer nanocomposites.

4.3 FTIR analysis

Complex formation and intermolecular interaction studies in the solid dispersions were extensively analyzed from the Fourier transform infrared (FTIR) spectroscopy. FTIR spectra of the pure PVA, PVA:Bi³⁺, PVA:Tb³⁺, PVA:Bi³⁺ + Tb³⁺ and f-MWCNT embedded PVA:Bi³⁺ + Tb³⁺ polymer matrix were recorded in the range of 4000–530 cm⁻¹, as shown in Fig. 3. We observed a broad peak from 3670–3000 cm⁻¹, which centered at 3285 cm⁻¹. This was attributed to the symmetric stretching vibrations of the O–H from the intramolecular and intermolecular hydrogen bonds in PVA. The stretching vibrational peak –OH is sensitive to hydrogen bonds in PVA. The bands pertaining to symmetric and anti-symmetric stretching vibrational modes of the C–H alkyl groups were observed at 2848 cm⁻¹ and 2919 cm⁻¹.^18,19 The band at 1657 cm⁻¹ was observed, and it was attributed to the acetyl C=C group in the PVA polymer matrix. We observed two bands at 1581 cm⁻¹ and 1436 cm⁻¹, which were ascribed to aromatic C=C stretching and CH₂ bending vibrations, respectively. The wagging vibrational mode of the CH₂ group was observed at 1367 cm⁻¹. The combination of the C–H wagging vibrational and C–O stretching of the acetyl groups was observed at 1256 cm⁻¹. The band at 1092 cm⁻¹ was observed, and it was attributed to the hydroxyl C–O stretching.¹⁹ The most significant bands were observed at 952 cm⁻¹ and 836 cm⁻¹, and these bands were assigned to the C–C stretching and CH₂ stretching vibrations. The band assignments of the FTIR profiles of pure PVA, PVA:Bi³⁺, PVA:Tb³⁺, PVA:Bi³⁺ + Tb³⁺ and f-MWCNTs embedded PVA:Bi³⁺ + Tb³⁺ polymer matrix are listed in Table 1. Compared with pure PVA, the –OH stretching peak position slightly shifted to a higher wavenumber in the doped PVA polymer system, and a slight intensity change in the C=O stretching peak was observed in the doped PVA polymer matrix, suggesting the presence of hydrogen bonding interactions between the hydroxyl groups on the PVA molecular chains.

Fig. 3 FTIR profiles of PVA, Bi³⁺:PVA, Tb³⁺:PVA, (Bi³⁺ + Tb³⁺):PVA, and (Bi³⁺ + Tb³⁺) + f-MWCNTs (0.05 mg):PVA polymer films.

Table 1 FTIR bands assignment of the (a) PVA (b) PVA:Bi³⁺ (0.15), (c) PVA:Tb³⁺ (0.15), (d) PVA:Bi³⁺ (0.15) + Tb³⁺ (0.15) and (e) PVA:Bi³⁺ (0.15) + Tb³⁺ (0.15) + f-MWCNTs (0.05 mg)

Assignment of the bands	Wavenumber (cm^−1)					Ref.
	a	b	c	d	e
OH stretching vibrations	3285	3286	3287	3389	3394	18 and 19
CH symmetric stretching	2848	2849	2852	2857	2857	18 and 19
CH anti symmetric stretching	2919	2921	2921	2922	2928	18 and 19
Acetyl C=C group	1657	1664	1670	1678	1681	18
Aromatic C=C stretching group	1581	1584	1589	1592	1598	18
CH2 bending vibrations	1436	1438	1438	1438	1440	18
CH2 wagging vibration	1367	1367	1368	1372	1374	18
C–H wagging mode, C–O stretching of acetyl groups	1256	1257	1257	1259	1259	18
Hydroxyl C–O stretching	1092	1093	1093	1094	1094	19
C–C stretching vibrational mode	952	954	955	957	959	18
CH2 stretching vibration	836	838	839	842	848	18

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Upon doping with Bi³⁺ and Tb³⁺ ions with and without f-MWCNTs, the FTIR spectral profile of the PVA polymer matrix was found to be slightly modified. We observed the variation in the intensity in the FTIR spectra peaks upon doping, and this explores the information regarding the formation of the polymer complex between the pure polymer and dopant ions. The stretching vibrational mode of the carbonyl groups (C=O) was observed at 1734 cm⁻¹.

This band could be ascribed to the absorption of hydrogen ions from the OH groups of the main chain of the PVA polymer matrix. Interestingly, the intensity of the C=C band and other bands was found to decrease upon the addition of the rare earth ions, which indicates a decrease in the number of C=C groups along the PVA main chain. This could be attributed to the interaction between the C=C groups and the hydrogen, which was abstracted from the O–H groups of the PVA structure. Hence, the C=C π groups became C–C σ groups. The C–H symmetric vibrational band at 2848 cm⁻¹ shifted to a higher wavenumber (nearly 2855 cm⁻¹) upon the addition of the Bi³⁺ and Tb³⁺ ions and f-MWCNTs. The absorption of the hydrogen ions from the free OH groups plays a pivotal role in the formation of the complexes. Moreover, it was clearly observed that the OH stretching vibrational band at 3670–3000 cm⁻¹ exhibited a decreased pattern with the incorporation of the dopant ions and f-MWCNTs, and also it shifted to a higher wavenumber. This encouraging result suggested the formation of non-covalent bonds between Bi³⁺, Tb³⁺ and the OH groups of the PVA skeleton.²⁰ From the FTIR spectra of the f-MWCNTs dispersed co-doped PVA:Bi³⁺ + Tb³⁺ polymer nanocomposite, the polymer complex formed probably via weak physical forces rather than strong chemical bonding forces with the nanoparticle environment. We observed the decreased transmittance in the FTIR spectra with the addition of the dopant ions and f-MWCNTs; this might be due to the complex formation and the presence of the f-MWCNTs within the polymer matrix.²¹ Nevertheless, the significant bands of the f-MWCNTs, such as 1072 cm⁻¹, 1167 cm⁻¹, 1398 cm⁻¹ and 1733 cm⁻¹, are attributed to the stretching vibrations of C–O in the ether group, stretching vibrations of C–O in alcohol, O–H bending mode in the COOH group and the C=O in the COOH group, respectively. These pertinent bands regarding the f-MWCNTs are overlapping in the PVA complexed signals.²² The decrease in the FTIR peak intensity in the FTIR spectral profile supports the complex formation between the dopant ions and the PVA polymer matrix, which is in good agreement with the XRD analysis.

4.4 Thermal analysis

The variation in the thermal properties of the prepared polymer matrices were systematically analyzed by TG/DTA profiles. Thermograms were recorded for (a) pure PVA, (b) PVA:Bi³⁺, (c) PVA:Tb³⁺, (d) PVA:(Bi³⁺ + Tb³⁺), (e) PVA:(Bi³⁺ + Tb³⁺) + f-MWCNTs (0.05 mg) polymer films in the temperature range from 25 °C to 700 °C, as shown in Fig. 4. The weight losses observed for each step along with the corresponding temperature for all the samples is given in Table 2. The decomposition of pure PVA occurs in three steps. The initial weight loss (around 7%) at 95 °C may be due to degradation of the large polymer chains into smaller fragments through primary decomposition and the removal of absorbed water could be because PVA is a hydrophilic polymer.²³ The second decomposition occurs at 289 °C with a weight loss of 68%, which is due to decomposition of the acetate and side-chains of PVA. The third decomposition occurs at 420 °C with a weight loss of 17%. This third degradation of the polymer is due to oxidation combustion of the PVA polymer main chain. The complete decomposition of pure PVA polymer is at 465 °C.²⁴ In the case of Bi³⁺ and Tb³⁺ doped and co-doped PVA polymer films with and without f-MWCNT, the first weight loss was at 94 °C, 89 °C, 84 °C and 91 °C with weight losses of 6%, 7%, 8% and 4% for (b) PVA:Bi³⁺, (c) PVA:Tb³⁺, (d) PVA:(Bi³⁺ + Tb³⁺), (e) PVA:(Bi³⁺ + Tb³⁺) + f-MWCNTs (0.05 mg):polymer films, respectively. This could be due to volatilization of small molecules or evaporation of the absorbed water from the polymer composites. This could result from micro-Brownian motion of the main chain back bone of the PVA polymer matrix. A small percentage of the weight loss occurred in doped samples at 168 °C, 178 °C, 182 °C and 178 °C with a weight loss of 5%, 4%, 4% and 6%, respectively. This lower value of weight loss suggests a phase transition that indicates the existence of a physical transition upon the addition of dopant ions. The third degradation temperature point was noticed in all the doped samples at 245 °C, 229 °C, 232 °C and 233 °C with weight losses of 47%, 45%, 44% and 42% for (b) PVA:Bi³⁺, (c) PVA:Tb³⁺, (d) PVA:(Bi³⁺ + Tb³⁺), (e) PVA:(Bi³⁺ + Tb³⁺) + f-MWCNTs (0.05 mg) polymer films, respectively. This thermal degradation occurred due to heating rearrangement of the polyene structure to the polyaromatic form. The decomposition of polyenes results in the formation of macroradicals, which further decompose to form cis and trans derivatives. Later on, they can form polyconjugated aromatic structures as a result of intramolecular cyclization and condensation reactions according to the Diels–Alder mechanism. The final step of the degradation was observed in all the doped samples above 440 °C, and it was due to thermo-oxidation of the carbonized residue.²⁵ All the thermal degradation was substantiated by the endothermic and exothermic peaks of the DTA profiles of all the polymer samples. The glass transition and melting temperatures were determined from the DTA profiles of the polymer composites, and these values are reported in Table 2. The T_g values of the rare earth doped PVA polymer samples have a slight variation and a noteworthy variation in the f-MWCNTs embedded dual rare earth ion doped PVA polymer nanocomposite. This variation in the thermal properties is mainly due to the changes in the polymer interaction with respect to the hydrogen bonding and covalent bonding with the dopant ions, nanofiller dispersion, load transfer from the polymer molecules to the fillers, the crystallinity of the polymer matrix as well as the molecular packing of the polymer matrix. Among these, the polymer interactions between the polymer molecules and dopant salts or nanofillers were the most important because numerous other parameters such as crystallinity, molecular packing and degree of dispersion of nanofillers also depend on the interactions.²⁶ The thermal stability and melting temperatures of the rare earth ion doped PVA polymer composites and polymer nanocomposites decreased compared to pure PVA, and this might be due to a reduction in the number of intra and inter hydrogen bonds of PVA after the doping or dispersion of nanofiller. This is presumably due to the decrease in the degree of crystallinity as shown by XRD in earlier.²⁷

Fig. 4 TG/DTA profiles of (a) PVA, (b) Bi³⁺:PVA, (c) Tb³⁺:PVA, (d) (Bi³⁺ + Tb³⁺):PVA, (e) (Bi³⁺ + Tb³⁺) + f-MWCNTs (0.05 mg):PVA nanocomposite films.

Table 2 TG/DTA results for (a) PVA, (b) PVA:Bi³⁺ (0.15), (c) PVA:Tb³⁺ (0.15), (d) PVA:[Bi³⁺ (0.15) + Tb³⁺ (0.15)], (e) PVA:[Bi³⁺ (0.15) + Tb³⁺ (0.15) + f-MWCNT (0.05 mg)] polymer films

Sample	TG (degradation temperature with weight loss) (°C) (%) ± 2				DTA
	I^st	II^nd	III^rd	IV^th	Tg (°C)	Tm (°C)
(a)	95 (7%)	—	289 (68%)	420 (17%)	113	339
(b)	94 (6%)	168 (5%)	245 (47%)	430 (14%)	129	332
(c)	89 (7%)	178 (4%)	229 (45%)	432 (10%)	111	264
(d)	84 (8%)	182 (4%)	232 (44%)	431 (10%)	119	253
(e)	91 (4%)	178 (6%)	233 (42%)	433 (9%)	121	255

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4.5 Photoluminescence analysis

The photoluminescence excitation and emission spectral profiles of PVA:Bi³⁺ polymer film are shown in Fig. 5. The outer nS² configuration in Bi³⁺ is of great importance in the field of luminescence. The Bi³⁺ ion has been widely used as a luminescence sensitizer in various rare earth doped complexes such as phosphors, glasses and polymer complexes.²⁸ Bi³⁺ possesses a 6S² electronic configuration, and its photoluminescence properties are dependent on the host composition. The ground state of Bi³⁺ is ¹S₀, and it consists of four excited states of ³P₀, ³P₁, ³P₂ and ¹P₁ in increasing energy according to the electronic configuration of Bi³⁺. However, the transitions of ¹S₀ → ³P₀ and ¹S₀ → ³P₂ are completely spin forbidden since the two levels of ³P₁ and ¹P₁ are mixed by spin–orbit coupling. The spin selection rule in the Bi³⁺ ion is considerably relaxed by the strong spin–orbit coupling. Hence, the transitions of ¹S₀ → ³P₁ and ¹S₀ → ¹P₁ are expected to have reasonable absorption strengths. In the case of Bi³⁺ doped samples, the absorption and emissions bands are attributed to the transitions of ³P₁ → ¹S₀ and ¹S₀ → ³P₁, respectively.²⁹ In our present case, the major excitation band is noticed at 300 nm apart from the other two low intensity bands at 385 nm and 370 nm in the PVA:Bi³⁺ polymer complex, as shown in Fig. 5(a), and this excitation band is assigned to the corresponding electronic transition of ¹S₀ → ³P₁. Using this excitation, we can measure the emission spectrum, and the major emission is observed at 400 nm, as shown in Fig. 5(b). This emission band is assigned to the corresponding electronic transition of ³P₁ → ¹S₀.³⁰

Fig. 5 (a) Excitation and (b) emission spectra of Bi³⁺ (0.15 wt%):PVA polymer film.

Fig. 6 shows the excitation spectrum of the PVA:Tb³⁺ polymer composite film. Several excitation bands are observed in the excitation spectrum at 327 nm, 340 nm, 353 nm, 370 nm, 379 nm and 489 nm, and these bands are assigned to the corresponding electronic transitions as ⁷F₆ → ⁵D₁, ⁷F₆ → ⁵L₆, ⁷F₆ → ⁵D₂, ⁷F₆ → ⁵L₁₀, ⁷F₆ → ⁵G₆ and ⁷F₆ → ⁵D₄, respectively.³¹ We observed the prominent excitation band at 370 nm (⁷F₆ → ⁵L₁₀) compared to the other excitation bands. The emission spectra of the PVA:Tb³⁺ polymer composite films at different concentrations of Tb³⁺ ions were studied using 370 nm (⁷F₆ → ⁵L₁₀) as the excitation wavelength, which is shown in Fig. 7. There were four emission bands observed in the emission spectra centered at 491 nm, 546 nm, 586 nm and 621 nm. These emission bands were assigned to the corresponding electronic transitions as (⁵D₄ → ⁷F₆), (⁵D₄ → ⁷F₅), (⁵D₄ → ⁷F₄) and (⁵D₄ → ⁷F₃), respectively.³² The prominent green emission was observed from the PVA:Tb³⁺ polymer composite films at 546 nm (⁵D₄ → ⁷F₅). Among the various concentrations of Tb³⁺ ion doped PVA polymer composites, the 0.15 wt% concentration of the PVA:Tb³⁺ polymer displayed a predominant emission intensity. Hence, the 0.15 wt% concentration of the Tb³⁺ ions was the optimized concentration for better green emission. In order to enhance the fluorescence performance of the Tb³⁺ (0.15 wt%):PVA polymer composite, we introduced a second suitable sensitizer ion, such as Bi³⁺ ion, in different concentrations. Different concentrations (0.01, 0.02, 0.03, 0.04, 0.05, 0.075, 0.1, 0.15, 0.175, 0.2, 0.25 and 0.275 wt%) of Bi³⁺ ions were added to the Tb³⁺ (0.15 wt%):PVA polymer matrix for the preparation of co-doped samples for further investigation.

Fig. 6 Excitation spectrum of Tb³⁺ (0.15 wt%):PVA polymer film.

Fig. 7 Emission spectra of Tb³⁺ (0.025, 0.05, 0.075, 0.1, 0.15 and 0.2 wt%):PVA polymer films.

Both the excitation spectra of the Tb³⁺ singly doped and co-doped PVA:Tb³⁺ + Bi³⁺ polymer composite films from monitoring the emission wavelength at 546 nm are shown in Fig. 8. The prominent excitation band is observed at 320 nm in the co-doped PVA:Tb³⁺ + Bi³⁺ polymer composite film. This indicates the presence of Bi³⁺ ions along with Tb³⁺ ions in the PVA polymer matrix. A slight difference was noticed between the excitation spectra with respect to the intensity and broadening of the spectral peaks. Apart from the existence of the same excitation bands of the Tb³⁺ ion in the co-doped sample, the excitation peak intensity at 320 nm was enhanced, and a slight broadening of the spectral peaks was observed in the co-doped PVA polymer system compared to the singly doped system. Other excitation band intensities pertaining to the Tb³⁺ ions decreased. Therefore, it could be understood that their absorption and emission intensity were enhanced near the UV source. This favourable situation also strongly suggested the presence of Bi³⁺ ions along with Tb³⁺ ions and possible energy transfers were taking place from the Bi³⁺ ions to the Tb³⁺ ions in the co-doped PVA:Tb³⁺ + Bi³⁺ polymer matrix.³³ Using this 320 nm excitation, we obtained the photoluminescence spectra of PVA:Tb³⁺ (0.15 wt%) + Bi³⁺ polymer composite films by varying the Bi³⁺ ion concentration, as shown in Fig. 9. Interestingly, we observed the significant emission properties in the co-doped PVA polymer matrix compared to the Tb³⁺ singly doped PVA polymer system. The emission intensities were appreciably enhanced with the addition of Bi³⁺ ions in different concentrations, as shown in Fig. 9. It is clearly shown that the green emission intensity of the ⁵D₄ → ⁷F₅ transition becomes very strong in the co-doped system than the single Tb³⁺ doped PVA polymer matrix. This could be due to possible energy transfer from the Bi³⁺ ions to the Tb³⁺ ions in the co-doped PVA polymer system.³⁴ Herein, Bi³⁺ ions act as a sensitizer, and Tb³⁺ ions act as an activator. Among all the concentrations of Bi³⁺ ions, 0.15 wt% concentration of the Bi³⁺ ion doped Tb³⁺ (0.15 wt%):PVA polymer system exhibits predominant emission features over the others. Hence, we found that the optimized concentration of the sensitizer of Bi³⁺ was 0.15 wt% based on the photoluminescence spectral features of the co-doped PVA polymer matrix. The emission intensities of the co-doped polymer matrix slowly decreased with the increasing Bi³⁺ ion concentrations above 0.15 wt%. This might be due to the concentration quenching effect, which probably occurred as a result of an energy transfer due to more Bi³⁺–Bi³⁺ interactions than Bi³⁺–Tb³⁺.³⁵ All the above results demonstrate that Bi³⁺ co-doping plays a pivotal role in improving the luminescence properties of Tb³⁺ in the co-doped PVA polymer matrix. In this phenomenon, successful emission photons of Bi³⁺ ions are collectively absorbed by the Tb³⁺ ions within the PVA polymer matrix through energy transfer.

Fig. 8 Excitation spectra of Tb³⁺ (0.15 wt%):PVA and Bi³⁺ (0.15 wt%) + Tb³⁺ (0.15 wt%):PVA polymer films.

Fig. 9 Emission spectra of co-doped Tb³⁺ (0.15 wt%) + Bi³⁺ (0.01, 0.02, 0.03, 0.04, 0.05, 0.075, 0.1, 0.15, 0.175, 0.2, 0.25 and 0.275 wt%):PVA polymer films.

Another strategy to confirm the energy transfer phenomenon is the study of overlapped spectra of excitation and emission spectral profiles of the activator and sensitizer ions, respectively. This energy transfer takes place mainly due to radiative energy transfer through the emission of sensitizer ions and re-absorption by activator ions or the non-radiative condition between both of them. Herein, the principle condition for the energy transfer phenomenon is that the emission spectrum of the sensitizer should overlap with the excitation spectrum of the activator. The non-radiative energy transfer is accompanied by the electric multipole interactions and exchange interactions. The possible energy transfer probability is proportional to the energy overlap between the emission of the sensitizer and excitation of the activator. The energy overlap plays a pivotal role in the energy transfer phenomena.³⁶ The overlapped spectral profile of the emission of Bi³⁺ and excitation spectrum of the Tb³⁺ ion is shown in Fig. 10. We observed the well overlapped spectral region between the emission spectrum of Bi³⁺ and the excitation spectrum of Tb³⁺. Hence, this overlapped spectral profile satisfies the condition for an energy transfer between the Bi³⁺ and Tb³⁺ ions. This encouraging result suggested that the possible energy transfer was taking place between the Bi³⁺ and Tb³⁺ ions, and this is the only reason for the significant enhancement of the Tb³⁺ ion emission intensities in the co-doped system. The schematic energy level for the energy transfer mechanism from Bi³⁺ to Tb³⁺ in the PVA polymer matrix is shown in Fig. 11.

Fig. 10 Overlapped emission spectrum of Bi³⁺ and excitation spectrum of Tb³⁺:PVA polymer films.

Fig. 11 Partial energy level scheme diagram of energy transfer from Bi³⁺ to Tb³⁺ and f-MWCNTs to Tb³⁺in PVA polymer composite films.

In order to obtain further improvement in the photoluminescence properties of the co-doped PVA:(Bi³⁺ + Tb³⁺) polymer films, several approaches for obtaining the prominent photoluminescence properties have been made. In another part of the energy transfer, f-MWCNTs were used to incorporate into the PVA:(Bi³⁺ + Tb³⁺) PVA polymer matrix. Fig. 12(a) and (b) shows the fluorescence excitation and emission spectra of pure f-MWCNTs. From earlier reports, we realized that no emission was observed from the nonfunctionalized MWCNTs due to their metallic nature.³⁷ There are some reports on the photoluminescence properties of MWCNTs after functionalization.³⁸ Herein, we observed the major excitation band at 300 nm. The prominent emission was observed at 472 nm under the excitation of 300 nm in the pure f-MWCNTs. This emission could be attributed to the presence of defects at the nanotube surface after functionalization.³⁹

Fig. 12 (a) Excitation and (b) emission spectra of f-MWCNTs.

4.5.1. Energy transfer from Bi³⁺ and f-MWCNTs to Tb³⁺ ions simultaneously in co-doped PVA polymer composites. There are no reports on the energy transfer based on the photoluminescence properties of f-MWCNT impregnated PVA:Tb³⁺ polymer matrices as of now. We attempted to use f-MWCNTs to enhance the luminescence efficiency of the Tb³⁺ ions in the PVA polymer matrix. The remarkable green emission was observed from the co-doped PVA:(Bi³⁺ + Tb³⁺) composites under the excitation of 320 nm, as shown in Fig. 13. Surprisingly, the remarkable green emission enhancement was observed in the f-MWCNTs impregnated co-doped PVA:(Bi³⁺ + Tb³⁺) polymer nanocomposites rather than the f-MWCNTs-free samples under 320 nm, as shown in Fig. 13. This encouraging result could be responsible for the energy transfer from the f-MWCNTs to the Tb³⁺ ions, which increased the population of the excited Tb³⁺ ions, or there might be an increase in the lifetime of the emitting energy level ⁵D₄ corresponding to Tb³⁺ due to the interaction with f-MWCNTs. Upon increasing the f-MWCNTs concentration in the co-doped system, the emission intensities were enhanced, and a prominent emission was observed with 0.05 mg of f-MWCNTs impregnated in the co-doped PVA polymer nanocomposite. Hence, the optimized concentration of the f-MWCNTs was 0.05 mg. The reason for the enhancement in the Tb³⁺ emission intensity with the addition of f-MWCNTs was that the possible energy transfer occurred from f-MWCNTs to Tb³⁺. Defects like oxygen vacancies generally act as radiative centers in complexes. Essentially, the absorption cross-section band for the f-MWCNTs is very large, and numerous f-MWCNTs domains are excited from lower to higher energy states under UV excitation. Moreover, the tubular structures of the f-MWCNTs might possess good surface to volume ratios. Due to the presence of high surface to volume ratios, the defects will also be ultimately enhanced. The surface defects and tubular morphology altogether could produce more luminescence centers, which acted as sensitizer portions to enhance the Tb³⁺ emission intensity.⁴⁰ The result of the ⁵D₄ level would naturally enhance the intensity of all the ⁵D₄ → ⁷F_J (J = 0–6) transitions with increasing Tb³⁺ ions. The relative increase in the intensities of the different bands will depend on the intrinsic probabilities for the individual transitions. However, the largest increase in intensity is found for the transition ⁵D₄ → ⁷F₅, which is a magnetic dipole allowed hypersensitive transition. This hypersensitive nature is most responsible for the large increase in the intensity seen in the presence of f-MWCNTs. From the above discussion, we concluded that the possible energy transfer takes place from Bi³⁺ to Tb³⁺ and f-MWCNTs to Tb³⁺ unambiguously. The superposition integral of the spectral shapes between the donor (f-MWCNTs) emission and acceptor (Tb³⁺) excitation is proportional to the energy transfer probability.⁴¹ We observed the overlapped spectral region between the emission spectrum of the f-MWCNTs and excitation spectrum of Tb³⁺, as shown in Fig. 14(a). Moreover, there are no overlapped spectral regions between Bi³⁺ excitation and f-MWCNTs emission, which suggests that the energy transfer might not take place from the f-MWCNTs to Bi³⁺, as shown in Fig. 14(b). From this discussion, we suggest that the energy transfer took place from Bi³⁺ to Tb³⁺ and f-MWCNTs to Tb³⁺ under 320 nm of excitation.

Fig. 13 Emission spectra of co-doped f-MWCNTs (0.01, 0.02, 0.03, 0.04, 0.05, 0.06 and 0.07 mg) embedded Bi³⁺ (0.15 wt%) + Tb³⁺ (0.15 wt%):PVA polymer nanocomposite films.

Fig. 14 Overlapped spectral profiles of (a) emission spectrum of synthesized f-MWCNTs and excitation spectrum of Tb³⁺:PVA polymer film and (b) emission spectrum of synthesized f-MWCNTs and excitation spectrum of Bi³⁺:PVA polymer film.

CIE chromaticity diagram pertaining to PVA:Tb³⁺ (0.15), PVA:Bi³⁺ (0.15) + Tb³⁺ (0.15) and PVA:Bi³⁺ (0.15) + Tb³⁺ (0.15) + f-MWCNTs (0.05 mg) polymer composites is shown in Fig. 15. We observed low violet-green and light green emissions in the singly doped and co-doped PVA:Tb³⁺ (0.15) and PVA:Bi³⁺ (0.15) + Tb³⁺ (0.15) polymer composites, respectively. A certain increment was noticed in the color index in the co-doped PVA:Bi³⁺ (0.15) + Tb³⁺ (0.15) sample compared to the singly doped PVA:Tb³⁺ (0.15) polymer composite. CIE chromaticity coordinates (x and y) for (i) PVA:Tb³⁺ (0.15), (ii) co-doped PVA:Bi³⁺ (0.15) + Tb³⁺ (0.15) and (iii) f-MWCNTs (0.05 mg) impregnated co-doped PVA:Bi³⁺ (0.15) + Tb³⁺ (0.15) polymer nanocomposites are presented in Table 3. By dispersion of the f-MWCNTs at a 0.05 mg concentration, the chromaticity coordinates could be turned to the green position under 320 nm. This could be external evidence for the energy transfer process from the f-MWCNTs to the Tb³⁺ ions in the PVA polymer matrix.

Fig. 15 CIE chromaticity diagram for Tb³⁺ (0.15 wt%):PVA, co-doped Bi³⁺ (0.15 wt%) + Tb³⁺ (0.15 wt%):PVA, f-MWCNTs (0.05 mg) embedded co-doped Bi³⁺ (0.15 wt%) + Tb³⁺ (0.15 wt%):PVA polymer nanocomposites.

Table 3 CIE chromaticity coordinates of PVA:Tb³⁺ (0.15), PVA:Bi³⁺ (0.15) + Tb³⁺ (0.15) and PVA:Bi³⁺ (0.15) + Tb³⁺ (0.15) + f-MWCNTs (0.05 mg) polymer composites under the common excitation of 320 nm

Sl. no.	Sample composition (wt%)	CIE coordinates (x, y)
1	PVA:Tb^3+ (0.15)	(0.1924, 0.2842)
2	PVA:Bi^3+ (0.15) + Tb^3+ (0.15)	(0.1992, 0.3664)
3	PVA:Bi^3+ (0.15) + Tb^3+ (0.15) + f-MWCNT (0.015)	(0.1998, 0.3942)

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4.6 Decay analysis

Another strategy to confirm the energy transfer phenomena is lifetime decay analysis. The decay curve profiles pertaining to singly doped Tb³⁺ ions and co-doped PVA polymer composites have been extensively studied and are shown in Fig. 16. In the case of singly doped Tb³⁺:PVA polymer system, the luminescence (green emission at 546 nm) lifetime for the ⁵D₄ excited states of the Tb³⁺ ion with an excitation of 370 nm was calculated, and it was found to be 0.451 ms. However, we observed the decay curve of the singly doped Tb³⁺:PVA polymer system that fitted well in the single exponential function, as shown in Fig. 16. It can be seen that the lifetime decay curve is well fitted to follow a single exponential decay. The lifetime is calculated by fitting the curve to the following equation:

I_t = I₀ exp(−t/τ) (1)

where I_t and I₀ are the intensities at time t and 0, respectively, and τ refers to the luminescent lifetime. In the co-doped system, the decay curves exhibit non-exponential behavior due to the different sites of the Tb³⁺ ions. However, the lifetime decay curve is fitted by the expression given as

I(t) = A₁ exp(−t/τ₁) + A₂ exp(−t/τ₂) (2)

where I(t) is the emission intensity, A₁ and A₂ are constants, τ₁ and τ₂ are the short and long lifetimes, respectively. The average lifetimes (τ_avg) of the Tb³⁺ ions were determined by the formula:

Fig. 16 Decay curves of Tb³⁺ emissions in the single and co-doped PVA polymer matrix with and without f-MWCNTs.

However, the Tb³⁺ co-doped PVA polymer film decay curves slightly deviated from the single exponential nature compared with the decay curve of the singly doped PVA polymer composite. This could be due to the modification of the fluorescent dynamics by the co-doped ions. The non-exponential nature of the decay curve regarding the co-doped system was observed. This favorable existence of the non-exponential nature of the decay profile indicated the interaction between the acceptor and donor ions. The availability of the donor (Bi³⁺) ions per acceptor (Tb³⁺) ion will be more in the co-doped polymer composite; therefore, the average distance between the acceptor–donor ions decreased, which led to electronic multipole–multipole interactions. This condition is preferable for energy transfer. Consequently, the abovementioned lifetime decay dynamics strongly suggested that the efficient energy transfer process takes place from Bi³⁺ to Tb³⁺ in the PVA polymer film.⁴²

By co-doping with Bi³⁺ and Tb³⁺, the Tb³⁺ emission peak (546 nm) intensity was remarkably enhanced under the excitation at 320 nm, which is related to Bi³⁺. Consequently, its lifetime was also considerably increased. The emission lifetime of the Tb³⁺ in the co-doped polymer matrix was 0.803 ms. It can be observed that the lifetime of the ⁵D₄ levels of the Tb³⁺ in the co-doped system increased more than the singly doped polymer system. The co-doped polymer lifetime decay curve of Tb³⁺ ions with the excitation of 320 nm fit well to a non-exponential function, as shown in Fig. 16. This is confirmed that efficient energy migration has been taking place from the Bi³⁺ to Tb³⁺ in the PVA polymer composite.⁴³

Upon adding the f-MWCNTs (0.05 mg) to the co-doped (Bi³⁺ + Tb³⁺):PVA polymer system, the lifetime of the ⁵D₄ levels of the Tb³⁺ ions increased. This could be due to the coupling effect of the plasmonic field of the nanoparticles and rare earth ions. This might influence the rare earth ion radiative level lifetime and energy migration. The f-MWCNTs were impregnated along with both Bi³⁺ and Tb³⁺ in the co-doped polymer composite, and the lifetime of the green emission regarding the Tb³⁺ ions was 0.857 ms. The lifetime improvement by the addition of Bi³⁺ and f-MWCNTs to the Tb³⁺ doped PVA complex confirmed the existence of energy transfer from the Bi³⁺ ions and f-MWCNTs to Tb³⁺. The lifetime decay curve pertaining to the Tb³⁺ ions in the f-MWCNTs impregnated co-doped PVA polymer matrix had a non-exponential nature, which supports the efficient interaction between the acceptor and donor ions.⁴⁴ The energy transfer mechanism was evidently interpreted by the schematic energy level diagram and is shown in Fig. 11.

5. Conclusion

We successfully prepared Bi³⁺, Tb³⁺ doped and co-doped PVA polymer composite films with and without functionalized multiwalled carbon nanotubes (f-MWCNTs) using a solution casting method. The XRD profiles revealed that the prepared PVA polymer composite films exhibit a semi-crystalline nature. FTIR spectral analysis confirmed the complex formation and ion–polymer interactions with the incorporation of both rare earth ions into the polymer composite along with the f-MWCNTs. The thermal stability and decomposition dynamics of the pure PVA, Bi³⁺, Tb³⁺ doped PVA polymer composite films with and without f-MWCNTs were evaluated by TG/DTA profiles. The optical properties were systematically analysed by optical absorption and luminescence spectral studies. Terbium doped PVA polymer composite films exhibited a green emission with a UV excitation source. The prominent green emission at 546 nm (⁵D₄ → ⁷F₅) of the Tb³⁺ ions was observed, and other transitions of Tb³⁺ were clearly explained. The optimized concentration of the Tb³⁺ ions was 0.15 wt% in the singly doped Tb³⁺:PVA polymer matrix. PL efficiency of Tb³⁺ ions was increased by co-doping with Bi³⁺ through the energy transfer process. The optimized sensitizing concentration of the Bi³⁺ ions in the co-doped Bi³⁺ + Tb³⁺:PVA polymer system was 0.15 wt% based on the fluorescence spectral analysis of the co-doped polymer system. Surprisingly, the emission performances of the Tb³⁺ ions in the co-doped polymer complex were remarkably enhanced by the addition of the f-MWCNTs to the co-doped Bi³⁺ + Tb³⁺:PVA polymer system. This could be due to efficient energy transfer taking place from the f-MWCNTs to the Tb³⁺ ions. In the f-MWCNTs embedded Bi³⁺ + Tb³⁺:PVA polymer system, an energy migration between the f-MWCNTs and Tb³⁺ ions took place in the PVA polymer. The sensitizing effect of the f-MWCNTs and Bi³⁺ ions significantly improved the emission performance of the Tb³⁺ ions. This dual way energy transfer phenomena were substantiated by several fluorescence methods. The energy migration pathway was clearly elucidated through partial energy level scheme diagrams and lifetime decay dynamics. These bright green luminescent polymer nanocomposite materials could be suggested as promising candidates for green luminescent photonic applications.

Acknowledgements

This study was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of the Science ICT & Future Planning (NRF-2015R1A1A3A04001268).

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