Influence of Eu3+ on the structural, optical and electrical properties of PEO–PVA: dual bandgap materials for optoelectronic applications

Solid-polymer electrolytes (SPE) based on rare-earth doping is a growing approach for the development of various optoelectronic and ion-conducting devices. Eu3+/PEO–PVA SPE was prepared by solution casting. The impacts of Eu3+ content on the microstructure, chemical composition, and complexation with the functional groups of the blend as well as on the film morphology were evaluated by X-ray diffraction, FT-IR spectroscopy, and FE-SEM microscopy. It was revealed that the film's crystallinity and optical transmittance can be tailored by Eu3+ content. Tauc's method illustrated that the films exhibit dual band gaps on both the low energy side (2.0–2.8 eV) and the high energy side (4.0–4.38 eV). In addition, the refractive index and optical conductivity of SPE were greatly enhanced with increasing Eu3+ content. The current–voltage characteristic curves were recorded at an applied voltage range of 0–10 V, and temperature range of 30–100 °C. The materials exhibited non-Ohmic behavior. The DC conductivity (σdc)\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\sigma }_{\mathrm{dc}})$$\end{document} values of the pure and 6 wt% Eu3+-doped blend were in the range of 1.16 × 10–6–2.05 × 10–6 S/cm and 1.73 × 10–6–3.36 × 10–6 S/cm, respectively. The relations between the current density and the electric field revealed that the Schottky emission is the most suitable conduction mechanism. The results indicate that Eu3+/PEO–PVA SPE is suitable for some optoelectronic applications and ion-conducting devices.


Introduction
Blending of two or more polymers is considered one of the most promising and advantageous green chemistry processes for creating new compositions with a wide variety of distinct properties [1,2]. Solid polymer electrolytes (SPE) have advantages over their liquid counterparts including ease of preparation, leakage-free, a wide range of operating temperatures, good shelf life, higher energy density, lower flammability, stability during the charge/discharge processes, thermal stability, and good mechanical behavior. Therefore, developing SPE based on polymeric blends with improved physicochemical properties, enhanced ionic conductivities, low cost and lightweight can be used for advanced flexible, stretchable, and all-solid-state organo-microelectronic energy storage, supercapacitors, harvesting devices, and for solar cells and sensors [3][4][5].
On the other hand, rare-earth (RE) doping was found to create considerable effects on the polymer's physical and chemical properties owing to their distinctive nature of 4f-4f electronic transitions shielded by the filled 5s and 5p shells. Therefore these RE/ polymer complexes have been used in a broad range of uses, such as LEDs, laser materials, fiber optics, signal amplification, broadband telecommunications, and some NIR photonic applications [6][7][8][9][10][11]. Zhang et al. [12] reported improvement in the thermal and fluorescence emission properties of the poly(styreneco-butyl acrylate) by incorporating Eu 3? /Yb 3? into the polymer matrix.
Poly(ethylene oxide), PEO is a normal chain polymer that attracts particular attention from researchers. Because of its hydrophilicity, nontoxicity, and biocompatibility, PEO has been widely used for biomedical and biomimetics applications [1]. In addition, it is still the prime choice for manufacturing ion-conducting flexible-type SPE films [4]. Due to its good conductivity, polar character, strong chemical strength, stability and reactivity, low glass-transition temperature, and chemical stability, PEO was recently used in electrical and industrial equipment [13]. Moreover, PEO has a high degree of crystallinity (X C ) * 73.6% [14], and the fast chain segmental motion facilitates the hopping of ion transportation. However, the poor thermo-mechanical properties of PEO represents a major drawbacks for work at higher temperatures. For SPE of improved efficiency, with enhanced thermal and physical properties, it is essential to blend PEO with another polymer. Dhatarwal and Sengwa studied the electrochemical parameters of PEO (0.75)/poly(vinylidene fluoride) PVDF (0.25) loaded with LiClO 4 and TiO 2 [4]. Loading Tb 3? into PEO/poly(vinyl pyrrolidone), PVP blend also gave a green emission at 546 nm with an excitation at 370 nm which means the blend act as a visible color luminescent material [15]. Chigome et al. [16] reported that embedding the Eu 3? -doped yttrium orthovanadate (YVO 4 ) in PEO nanofibers yield a red light emission upon excitation at 254 nm. This suggested the using of this phosphor material for color display panels.
Poly(vinyl alcohol), PVA is a water-soluble, chemical resistant, high mechanical resistant, high transparent polymer, and has a hydroxyl -OH grouped carbon backbone, that acts as a hydrogen bonding source [10]. Dash et al. [7] synthesized PVAcoated Eu 3? -doped (BiOCl, BiOBr, and BiOI) nanoflakes with a strong photoluminescence and photocatalytic properties towards RhB degradation under sunlight irradiation. Doping PVA with Ce 3? , Tb 3? , and GO at certain concentrations yielded dazzling green emissions that can be employed in photonic devices [8]. Kumar et al. [17] used PVA as a host material to design sensitive biosensor (Ag and Eu:Y 2 O 3 ) fluorescent film for the detection of hydrogen peroxide and glucose. Ragab [18] studied the role of (0-15 wt%) CsCl on the physical properties of PEO (60%)/PVA (40%). Mahmoud et al. [19] reported the influence of EuCl 3 , up to 5 wt%, on the thermal properties, optical parameters, the color changes of the PVA, and the photocatalytic degradation of pnitrophenol. Elsaeedy et al. [20] reported the possible use of Er 3? /PVA for varistor device fabrication and UV shielding. Salma and Rudramadevi [21] reported that introducing Sm 3? ions up to 0.4 wt% inside PVA/PVP made the matrix able to improve the reddish-orange photonic emission for luminescent applications. Moreover, Ding et al. [22] reported that Eu 0.3 Tb 0.7 ions/PVA organic complex can effectively shield the UV light and convert it to red light at 612 nm.
The need to blend PVA with PEO is arising due to the low strength of the etheric linkages (C-O-C) of PEO, which act as a functional group, compared to C=C=C in PVA. Moreover, this will enables the blend chains to form coordinative structures with the metal salts [15,18,23]. Recently, Eu 3? has attracted increased research interest because of its sharp red and green emissions in the visible region, where a small amount of external light can excite its 4f electrons. Eu 3? /PVDF and Eu 3? /PEO nanofibers could be used in the textile industries and for designing photoluminescent fabric [24]. The above-mentioned literature survey illustrates that the fabrication of PEO/PVA blends and the effect of RE ions on their physical properties have received little attention. There is no complete report available in the literature, to the authors' knowledge, on the effect of Eu 3? on the physical properties of the PEO-PVA blend. Therefore, this work focuses on the preparation of PEO/PVA blend and Eu 3? /blend by solution casting method which is a facile and less-expensive route. The structural, optical, and electrical properties, as well as the possible conduction mechanism in these SPE were investigated by various characterization techniques and the obtained results were discussed in the light of the published data.
2 Experimental section

Materials and preparation
Europium chloride (EuCl 3 Á6H 2 O) of molecular weight M W = 366.41 g/mol, purity 99.9, an average particle size of 40-60 lm from Nanoshel LLC, was used as the source of Eu 3? rare earth ions. To confirm this size, an FE-SEM image for EuCl 3 was taken and shown in Fig. S1 (see the Supplementary Materials file). PEO of M W = 3 9 10 5 g/mol, from Alfa Aesar, Germany, and PVA powder, 87-90% hydrolyzed, and M W = (3-7) 9 10 4 g/mol, from Sigma-Aldrich, Germany, were used for the blend formation, where the double distilled water (DDW) was used as a common solvent. Solution-cast preparation method was applied for making the PEO/PVA/rare earth films. 0.8 g of PEO was dissolved in 20 ml DDW at room temperature (RT) using magnetic stirring for about 20 min. 0.2 g of PVA was dissolved in 30 ml DDW using magnetic stirring at 80-85°C, for 1 h, cooled to RT, and then mixed with PEO solution under stirring for 30 min. For SPE films the ratio 9 (wt%) of EuCl 3 (x = 0, 2, 4, and 6 wt%) was dissolved in 15 ml DDW and mixed with the blend solution. Finally, the solutions were cast into glass Petri dishes and dried at 50°in a furnace till complete removal of the solvent. The films were peeled off the dishes easily and their thickness was evaluated using a digital micrometer and was in the range of 0.15-0.19 mm. A decrease in the film's transparency was seen with the increase in Eu 3? concentration. Moreover, the films were of good flexibility, and their surfaces appeared to be smooth and non-sticky.

Characterization techniques
The film's micro-structural properties and crystallinity were investigated using X-ray diffraction (XRD) analysis utilizing a Rigaku mini flex diffractometer with Cu Ka radiation (k = 1.5406 Å ) in the 2h range of 5°-75°. The Fourier transform infrared (FTIR) transmittance spectra were recorded in the wavenumber range of 400-4000 cm -1 by a Bruker vertex 70 Spectrophotometer coupled with a diamond attenuated total reflection unit. The surface morphology of the PEO/PVA SPE and the chemical composition analysis were studied by a high-resolution field-emission scanning electron microscope (FE-SEM), Quanta EFI 250-Philips Company, coupled with an EDS unit. Optical transmittance and absorption spectra of the samples were recorded at RT utilizing a UV-3600 UV-Vis-NIR spectrophotometer (Shimadzu, accuracy of ± 0.2 nm) in the range of 200-1350 nm. The current-voltage (IV) curves of PEO-PVA and 6 wt% doped blend were recorded using a computerized Keithley 2635A in the temperature range of 30-100°C and DC voltage 0-10 V. To make good Ohmic contact, the samples were coated with conductive silver paste. Figure 1 displays the XRD patterns of PEO/PVA blend and Eu 3? /blend SPE films. The un-doped PEO/PVA blend exhibits two main intense/sharp peaks at 2h = 19.24°and 23.28°which could be attributed to the (120) and (112) reflection planes of PEO [15,16]. It is known that PVA alone exhibits a broad peak around 19.7°associated with the (101) plane [17]. This PVA reflection plane may be merged into the (120) plane of PEO. These results confirm the semicrystalline nature of the host blend matrix as also found in the literature [18]. Insertion of Eu 3? created a significant alteration in the blend structure, where the peak intensity is significantly reduced with increasing Eu 3? content to 4 and 6 wt%, showing a considerable decrease in the blend crystallinity. Doping Y 3? at 8.88 wt% in PVA shifted the main peak from 19.7°to 19.98° [10].

XRD and FTIR spectral analysis
In addition, no peaks related to Eu 3? or Eu 2 O 3 are seen in the XRD patterns of Fig. 1, indicating that the inserted ions were introduced into the amorphous regions of the blend. To account for this point quantitatively, the percent crystallinity X C ð%Þ of the PEO/PVA blend in these SPE films was calculated using the relation based on the integrated area of blend diffraction peaks and the hump area in the range of 2h = 16°-28°in XRD patterns, according to [10]: The obtained X C ð%Þ values are listed in Table 1. As noted, the X C of the blend is 46.23% which is smaller than the value reported for PEO [14]. This value initially increased to 48.75% at 2 wt% Eu 3? content, then decreased to 27.89 with increasing Eu 3? content up to 6 wt%. This means that low Eu 3? content makes some ordering character inside the blend matrix, while the electrostatic interaction between Eu 3? and the polymer chains is dominant at high Eu 3? contents that deactivate the admission of the polymeric chains [5,18]. The decrease of SPE film' crystallinity and reduction of the peaks' intensity were also noticed in the XRD patterns of Er 3? /Yb 3? codoped PEO/PVP blend systems [9].
FTIR spectral analysis is a technique that gives complete information about the functional groups and their interactions and/or complexation with the additives. Figure 2 displays the FTIR spectra of PEO/ PVA SPE films. The spectrum of the un-doped blend has a broad band around 3350 cm -1 . The intensity of this band is small but greatly improved after EuCl 3-6H 2 O addition. Similarly, the band at 1735 cm -1 is assigned to the C=O stretching vibrations. The first band (of O-H) confirms the existence of PVA and the second one confirms the existence of PEO in the film. C-H symmetric vibration is seen at 2880 cm -1 and a tiny peak at 2945 cm -1 is also observed and arising due to C-H asymmetric vibration. Moreover, the bending vibrations of C-H of the CH 2 group is seen at 1445 cm -1 and the asymmetric bending of CH 2 is at 1347 cm -1 . No obvious peak shifts could be seen, but the intensity of most of these peaks depends on the salt content. Farea et al. [25] reported a shift from 1345 to * 1337 cm -1 for CH 2 bending after loading the PVA/PEO with CsBr salt. The two neighboring bands at 1242 and 1278 cm -1 are owing to C-O, and C-O-C bending modes [26]. The symmetric/asymmetric stretching of C-O-C appears at 1100 cm -1 [27]. The two sharp peaks at 957 and 847 cm -1 are assigned to CH 2 rocking motion and C-O stretching in PEO, respectively [28]. The Assignments of the FTIR characteristic bands of the samples are listed in Table S1 (see the Supplementary Materials file). No changes in the position of these bands mean that the added salt did not change the chemical structure of PEO/PVA.

Surface morphology and compositional analysis
At very low magnification, the film appears through SEM as shown in Fig. S2 (see the Supplementary Materials file). Figure 3 shows FE-SEM images for PEO/PVA blend and the blend loaded with 2 and 6 wt% Eu 3? . The blend films, Fig. 3a, exhibits rough, cracks-free, and homogenous surface confirming the complete miscibility of the mixed polymers. Loading 2 and 6 wt% Eu 3? made the surface smooth and ruffles like the fingerprint, Fig. 3b, is found on some areas of the film. Under higher magnification (9.77 kx), no important variation is seen in the surfaces of the blend and 2 wt% Eu 3? -doped films, the insets of Fig. 3a, b. But the 6 wt% Eu 3? -doped PEO/PVA SPE film appears porous with a texture of net-like structure (Fig. 3c, c 0 ). This change in the film surface morphology is certainly due to the added salt.     Figure 5a, b shows the transmittance spectra (T%) and the absorption index k [k ¼ ak 4p , where a the absorption coefficient ð Þ ¼ 2:303 Absorbance film thickness ]. PEO (0.8)/PVA (0.2) shows 70-75% transmittance. The blend composed of PEO (0.1)/PVA (0.9) showed transmittance of * 82% in the visible and IR regions [3]. 2 wt% Eu 3? loading slightly increased T to * 76.5% in the visible and IR regions, due to the slight improvement of film's crystallinity as reported in the XRD results. However, increasing Eu 3? content reduced T to about 60% at k = 700 nm. The transmittance in the range of 60-75% of these SPE films makes them suitable to be involved in many optoelectronic applications. Moreover, Fig. 5b confirms that the films are transparent throughout the visible region and the absorption is in the UV region. Adding Eu 3? ions to the blend matrix increases its ability to absorb UV rays at 275 nm. This band could be assigned to p-p* electronic transitions due to the presence of C=O and C=C unsaturated bonds in the blend. PVA sole polymer was found to exhibit a characteristic peak at 278 nm [17]. The absorption band at 216 nm (see the inset of Fig. 5b) is assigned to the n-p* transition and it shifted to 210 nm with increasing Eu 3? ions content. This change is an indication of the chelate formation of Eu 3? coordinated with the hydroxyl group of PVA [29]. Similar observations were noticed for PEO/PVA/CsCl [18] and PVA/Eu 3? SPE systems [19].

UV-Vis study
It is possible to determine the electron transition nature of Eu 3? /PEO/PVA SPE films using the a values, where the optical band gap (E g ) of the films is connected to a by Tauc's formula: (aht) 1/m-= B(ht -E g ), where ht is the incident photon energy, ht (eV) = 1242/k (nm)), B is a constant, and the m value determines the nature of the possible optical transition. For example, m = 1/2 or 2 for the direct or indirect allowed transition, and m = 3/2 or 3 for the direct or indirect forbidden transitions. The values of E g can be obtained by extrapolating the linear part of the curves to a point of zero absorption. Figure 6a, b shows the plots of (aht) 2 vs. ht and (aht) 1/2 vs. ht for the direct (E gd ) and indirect (E gi ) optical band gap, respectively. The E gd and E gi values were obtained by extending the linear parts of curves of (aht) 2 vs. ht and (aht) 1/2 vs. ht, respectively, to zero absorption. It is very interesting to note that the films have a band edge at the lower energies. The values of E gd and E gi in the lower energy side are also listed in Table 1. For the high energy region both E gd and E gi were decreased from 5.3 and 4.38 eV to 4.4 and 4.0 eV, respectively, with increasing Eu 3? ions content from 0.0 to 6 wt%. In the low energy region, the values also decreased from 3.9 (the inset of Fig. 6a) and 2.8 eV to 3.6 and 2.0 eV. This dual-band gap was also reported for NiO-doped PVA films, where E gi were increased from 5.4 to 5.8 eV, in the high energy region and . Two E gi were also reported for Er 3? /PVA [20] and the organic alpha-sexithiophene/n-Si at 1.9 eV (onset) and the fundamental one at 2.09 eV [31]. Similar to the E g decrement after Eu 3? incorporation was also reported for La 3? /PVA/PVP SPE, and Sm 3? /PVA/PVP where the E g was decreased from 5.16 to 4.96 eV after doping with 15 wt% La 3? [11], and from 5.0 to 4.6 eV after loading 0.5% Sm 3? . Elsaeedy et al. [20] reported the shrinking of E gd and E gi of PVA from 5.6 and 4.98 eV to 5.08 and 4.47 eV by adding 37 wt% Er 3? . This also indicates that Eu 3? is more effective than La 3? , Sm 3? , and Er 3? for narrowing the E g of the SPE films.
The refractive index (n) is one of the most important parameter for optical device fabrication, coatings, and ant-reflection applications. The n value is related to the material's optical band gap through the following equation [11]: The obtained values of n are listed in Table 2. As seen, n increased from 1.956 to 2.096 with increasing Eu 3? content from 0.0 to 6 wt%. Another relation that relates n with E g can be written as [21]: n ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi 12:417 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi E g À 0:365 p s ð2Þ and the obtained values of n are also given in Table 2. As noticed, the derived n values from Eq. 2 are higher by a constant value (* 0.4) compared with those derived from Eq. 1. However, in both cases, the n values are inversely proportional with E g . Increasing n values indicates the improvement of the films' reflectivity by the formation of scattering centers inside the blend matrix.
Moreover, the optical conductivity of the SPE films is related to n and a by the following equation [3]: where c is the light velocity (3 9 10 8 m/s). Figure 7 shows the dependence of r on the incident photon energy. The r vs. ht curves can be divided into four regions, see Fig. 7, where in the region (i) the low energy region (ht \ 2.5 eV) and (iii) the plateau region (2.5 \ ht \ 4.4 eV), r values appear constant. In the regions (ii) 2.5\ht\4.4 eV and (iv) ht [ 5.0 eV, The values of r increase at region (ii) and sharply increase in region (iv). This sharp increase indicates that the incident photon (UV region) has enough energy to excite the electrons/charge carriers to higher states and thus increase the conductivity of the material [32]. At higher energies, ht [ 5.8 eV the transitions reach the saturation state. Moreover, the values of r are significantly enhanced with increasing Eu 3? ions content. This finding is consistent with E g decrement in doping.  Figure S4 shows the I-V curves of PEO/PVA and 6 wt% Eu 3? /blend in the applied voltage range of 0.0-10 V and temperatures in the range of 30-100°C. At low temperatures, I marginally increases with V. As the applied temperature increase, I increases significantly. This means that the temperature has a decisive effect on I. The linearity or non-linearity of these curves could be determined using the equation: I ¼ zV y [33], where z is a constant, and y is the non-linear coefficient parameter. The value of y could be used as an indication of the conduction mechanism in the polymers [34]. The Ohmic behavior is preeminent if y = 1, and at y = 2 the trap-space-charge-limited is the leading mechanism. But if x [ 2, the conduction mechanism is governed by the space-charge-limited [35]. Figure 8a, b shows the plots of ln I vs. ln V, where two regions (i and ii) with different slopes can be distinguished. The obtained values of slopes (y) are listed in Table 2 The conductivity (r dc Þ of the two films was calculated by using the relation; r dc ¼ I:d V:A , where A and d are the cross-sectional area and sample thickness, respectively. As seen from Fig. 9a, b, at RT (300 K) the r dc value is very low but increases with temperature. The Eu 3? /blend exhibits higher conductivity compared with the pure blend. For example, at 373 K, the r dc of the pure blend is in the range of 1.16-2.05 9 10 -6 S/cm and for the doped SPE film r dc = 1.73-3.36 9 10 -6 S/cm. This result is consistent with the UV/Vis data, where E g of the blend was more shrinkable after loading Eu 3? compared to the pure blend. The obtained conductivity values are consistent with the published data, where Sundaramahalingam et al. [27,36] reported that r ac of PEO/PVP raised from 1.34 9 10 -8 to * 1.59 9 10 -6 S/cm with increasing LiBr to 4.0 wt% but then decreased to 2.45 9 10 -7 S/cm at 5.0 wt% loading ratio. Also, they found a maximum ionic conductivity of 6.157 9 10 -7 S/cm at 30°C for PEO (0.67) /PVP (0.27) doped with 6 wt% NaNO 3 .

I-V characteristics, and conduction mechanism
Regarding the conduction mechanism, the current density J, where J ¼ I A , and the field strength E, where E ¼ V d are related by the following equations [37][38][39]: where T is the temperature (K), k B is Boltzmann constant, J o and r o are constants, and bðsÞ and bðPFÞ are related to Schottky emission and   Table S2. As noticed in Fig. 10a, b and Table 2, the values of b are in the range of (1.3-1.53) 9 10 -5 that are very close to the theoretical value of bðsÞ. Thus, the Schottky emission is the most suitable conduction mechanism in PEO/PVA SPE composites.

Author contributions
Conceptualization, SE-S and SS and AMES; methodology, SE-S and SS; validation, SE-S and SS and AMES; formal analysis, SS, SE-S and AMES; investigation, SS and AMES; resources, SE-S and SS and AMES; data curation, SE-S and SS and AMES; writing-original draft preparation, SE-S and AMES; writing-review and editing, SE-S and SS and AMES; visualization, SE-S and SS and AMES. All authors have read and agreed to the published version of the manuscript.

Funding
Open access funding provided by The Science, Technology & Innovation Funding Authority (STDF) in cooperation with The Egyptian Knowledge Bank (EKB).

Data availability
The authors declare that the data supporting the findings of this study are available within the article and in the attached Supplementary Material of this article.

Declarations
Conflict of interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. There are no interests to declare towards any financial interests/personal relationships which may be considered as potential competing interests.
Statement of novelty This research paper presents a simple and low-cost method for the fabrication of Eu 3? /PEO-PVA SPE. Various characterization techniques; XRD, FT-IR, FE-SEM, UV-Vis-NIR spectroscopy. Besides, the IV characteristics. Doping at 2.0-6.0 wt% of Eu 3? significantly improved the optical, semiconducting and electrical properties of the blend. The results indicated that the produced SPE are best suitable for optoelectronic applications and ion-conducting devices.
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