1 Introduction

Nanostructure such as CdS, SnO2, TiO2, ZnS, ZnO, etc. with particular morphologies has drawn growing attention in the latest years due to their technological applications and unique optical characteristics [1,2,3,4,5]. Zinc oxide (ZnO) is a versatility of the II–VI direct band gap semiconductor with a large 3.37 eV band gap with a large 60 meV excitonic binding energy [6,7,8,9]. The band gap and optical characteristics of ZnO nanostructure can be customized by altering the shape and size of the particles and by doping ZnO structures with separate components. The wide-band gap energy of ZnO is an adaptable host lattice for doping various components [10]. Semiconductors with rare earth (RE) doped materials are essential because of the prospective applications of such materials in optoelectronic systems, optical communication, X-ray panel display, scintillators and biological labels [11, 12]. The elevated fluorescence effectiveness of the doped nanostructure of RE metals has been of great concern in the growth of next-generation photonic instruments and biolables. In comparison to 3d metal and non-metallic doping transitions, ZnO semiconductors with RE doped ions have been of great concern due to their f → f or f → d internal orbital transitions, which give highly intensive ultraviolet (UV) emissions, as well as visible and infrared areas [13, 14]. Moreover, the optical properties of RE ions depend mainly on the local environment or host material symmetry [15, 16]. Despite its precious technological interests, many studies on RE doped semiconductors have been conducted. The Eu3+ ion is a representative dopant of several distinct compounds among the RE3+ ions and the red release of Eu3+ is thoroughly explored and extensively used for light emission systems [17,18,19,20,21]. The luminescence characteristics of Eu3+ doped ZnO have become more interesting because of their enormous potential for plasma displays, fiber lasers, bio-images, solar cells, and displays in terms of emission in general [22, 23]. Trivalent europium ion is the prevailing doping agent for different compounds with a 4f emissions scheme with red emissions [24]. The issues remain for the nanotechnology community to manipulate the dopant content and to control the aspect ratio of the nanostructure. The implementation of RE3+ ion into ZnO was noted as not as easy as ionic radii, electric charge, and coordination number problems are involved [25, 26]. Nanorods, nanobelts, nanoneedles, nanobowls, and nanonails have been employed in many synthesis techniques. Examples of these methods are co-precipitation, sol–gel, combustion, solvothermal, microwave, etc. [21, 26,27,28,29]. An alternative is a gel-combustion reaction technique, as it does not involve intermediate composition or steps of calcinations, to make a synthesis of these Eu-doped ZnO compounds quick and cheap [30,31,32]. Moreover, the stoichiometric composition and crystallites size can easily be controlled which significantly impacts the luminescent properties of the Eu-doped ZnO compounds.

This article reports a systems research on the synthesis of macroporous honeycomb-like ZnO nanoparticles doped with various concentrations of Eu3+ ion by the gel-combustion method. Moreover, there was a comparative assessment from the X-ray diffraction (XRD) operation of the mean crystallite size of pure and doped ZnO. Williamson-Hall (W–H) plots and Size-Strain plots (SSP) estimate for the strain associated with undoped and different concentration of Eu3+ ion-doped ZnO nanoparticles as a result of lattice deformation. The study was extended in the form of investigating the structure and the photoluminescence (PL) characteristics of the resulting samples in detail.

2 Experimental details

2.1 Materials

Europium nitrate pentahydrate (Eu(NO3)3⋅5H2O, 99.9%), zinc nitrate hexahydrate (Zn(NO3)2⋅6H2O, 98%), and citric acid (C6H8O7, 99.5%) as a fuel used in this work were of analytical grade were provided by Sigma-Aldrich. Distilled water was used for the preparation of stock solutions by dissolving stoichiometric quantities of metal salts.

2.2 Synthesis of macroporous honeycomb-like ZnO and Eu3+ doped ZnO nanoparticles

The gel-combustion technique for the design of ZnO and various concentrations of Eu3+ (1%, 3%, and 5%) doped ZnO nanoparticles was implemented. In a beaker, the required molar proportions of zinc nitrate, europium nitrate and, citric acid were mixed to achieve a 100 ml aqueous solution. The solution was mixed at room temperature, maintaining unity between the citric acid and the metal cation ratio. The homogeneous solution was clear and converted into a gel which was dried at 130 °C on the hot plate until the solution became viscous. Further drying makes the foam harder. The dried gel was ignited with a burning matchstick. The gel was combusted immediately and produced a fluffy porous, fine white powdery mass. The resulted materials were heat-treated at 700 °C in the tube furnace and then milled to achieve the fine powders.

2.3 Characterization

A series of techniques were adopted for the characterized such as; X-ray diffraction (XRD) using a Shimadzu diffractometer (model 6000) with CuKα radiation (λ = 1.5418 Å) in a wide range of Bragg angles (2θ) varying from 20° to 80° with a scanning rate of 2°/min at room temperature. The presence of functional groups in the samples was determined by Fourier Transform Infrared (FTIR-8400S) spectrophotometer. Field emission scanning electron microscopy (FESEM) (ZEISS EVO-50) was performed to observe the morphology of the samples. The optical absorption spectra of the samples were recorded in the range of 350–600 nm on Shimadzu-Pharmaspec-1600 UV–Visible (UV–Vis) spectrophotometer. The room temperature photoluminescence (PL) properties were measured using 325 nm monochromatized xenon lamp as the excitation source at room temperature.

3 Results and discussion

3.1 XRD analysis

Figure 1 displays XRD patterns of pure ZnO and Eu3+ doped ZnO nanoparticles prepared at different doping percentages (1%, 3% and, 5%). Sharp characteristic peaks were observed for all samples in the patterns matching the perfect wurtzite structure of ZnO (JCPDS data card no: 89-0510). The synthesized samples were pure ZnO without any secondary phase or impurities. In the inset of Fig. 1 when the doping concentration of Eu3+ ions increased, it was visible that the peak corresponding to (1 0 1) XRD patterns related to ZnO shifted to lower angles. R. Singh et al. and V. Kumar et al. also noted similar changes in ZnO with RE doping [33, 34]. In addition, a small rise in the ZnO lattice parameters was noted at doping (Table 1 and Fig. 2), which was in excellent agreement with the literature information where it was stated that this sort of outcome appeared owing to the substitution of Zn2+ (radius 0.074 nm) with Eu3+ (radius 0.095 nm) in the ZnO crystal [34]. Furthermore, the peaks of diffraction of ZnO increased in width and decreased in intensity with a rise in the minimal content of Eu3+, suggesting that Eu3+ doping inhibited the development of crystallite size. Mohanty et al. attributed this shifting peak to lattice mismatch and distortion and the strain of crystals [35].

Fig. 1
figure 1

XRD patterns of ZnO and distinct concentrations (1%, 3%, and 5%) of Eu3+ doped ZnO nanoparticles. Inset shows the peak shifting character of the plane (1 0 1) towards lower angles for Eu3+ doped ZnO

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Table 1 Variation of the crystallite size, micro-strain, and the lattice constant of ZnO with different Eu3+ doping content
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Fig. 2
figure 2

Deviation with distinct levels of Eu3+ ions in the lattice parameters a and c

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The lattice constants for undoped and distinct concentrations of Eu3+ doped ZnO nanoparticles (1%, 3%, and 5%) were calculated using the following equation [36].

$$\frac{1}{{d}_{hkl}^{2}}= \frac{4}{3} \left(\frac{{h}^{2}+hk+{k}^{2}}{{a}^{2}}\right)+ \frac{{l}^{2}}{{c}^{2}}$$
(1)

where dhkl is the interplanar spacing, h, k, l are miller indices, a and c are lattice constants.

The average crystallite sizes (D) of all the samples were calculated with the (1 0 1) peak using Scherrer formula [37].

$$D= \frac{0.9\lambda }{{\beta }_{hkl}cos\theta }$$
(2)

where λ is the wavelength of X-ray corresponding CuKα (1.5406 Å), βhkl is the full width at half maximum and θ indicates the Bragg peak position. In doped products, the calculated crystallite sizes were slightly less than pure ZnO and tabulated in Table 1. This reduction in crystallite size of doped ZnO was primarily due to the creation of Eu–O–Zn on the doped product surface, which inhibits the development of crystallite size [38]. This type of inhibitory effect has also been found with different RE ions for ZnO doped [38,39,40]. In addition, Scherer plots (SP) were built for the ZnO and Eu3+ doped ZnO nanoparticles by placing (1/cosθ) on the x-axis and βhkl (rad) along the y-axis as shown in Fig. 3. The crystallite size was extracted from the slope of the fit line by fitting data. βhklCosθ versus 4Sinθ was plotted for ZnO and Eu3+ doped ZnO nanoparticles as shown in Fig. 4, based on the Williamson-Hall (W–H) analyzes. A linear fit would achieve if plotted, βhklCosθ versus 4Sinθ for isotropic strain deformation [41]. In this work, no linear character of the W–H plot was observed. Moreover, for Eu3+ doped ZnO nanoparticles, the non-linear character (dispersed information spots) of the W–H model has been more influential relative to undoped ZnO. This verified that the line extension in XRD models was due to anisotropic strain in ZnO for Eu3+ doped ZnO nanoparticles. The linear fitting information with the W–H plot slope and intercept indicate strain and average crystallite size, respectively. Moreover, an upgraded evaluation of the size-strain variables might be extracted in isotropic instances of line extension because of an average size-strain plot (SSP) that has the benefit that less weight is given to data from high angle reflections in which the accuracy is often lower. In this estimate, it is not specified that a Lorentzian function describes the crystallite size profile and a Gaussian function describes the strain profile [42]. In Fig. 5, like to W–H methods, the term (dhklβhklCosθ)2 is plotted concerning (d2hklβhklCosθ) for the all- orientation peaks of undoped and Eu3+ doped ZnO nanoparticles with the wurtzite hexagonal phase from 2θ = 20° to 80°. The crystallite size was calculated by the slope of the linear data and the y-intercept root, which provides the strain. Table 1 sum up the average crystallite size and strain for undoped and Eu3+ doped ZnO nanoparticles. By studying the models, the findings that arose from the SSP technique were found to be more precise than the W–H because the information was placed in this design more precisely, with all elevated intensity levels affecting the linear fit. In addition, the strain of Eu3+ doped ZnO nanoparticles improved relative to the undoped ZnO strain. Comparable performance concerning micro-strain was also reported by K.G. Gopchandran et al. [16] upon Eu-doping with ZnO nanophosphor. This variation in the micro-strain indicates the substitution of Eu3+ ions into ZnO lattice.

Fig. 3
figure 3

Scherrer plots for estimating the size of crystallites

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Fig. 4
figure 4

XRD peak broadening of ZnO and various concentrations (1%, 3%, and 5%) of Eu3+ doped ZnO nanoparticles in the Williamson-Hall plots

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Fig. 5
figure 5

Size-Strain plots to calculate the crystallite size and strain

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The volume of the nanoparticles [36] was calculated using Eq. (3):

$$V= \left(\frac{4}{3}\right)\pi {\left(\frac{D}{2}\right)}^{3}$$
(3)

The volume of the hexagonal unit cell [36] was predicted using Eq. (4) to compile the XRD evaluation.

$$v={0.866a}^{2}c$$
(4)

The proportion (V/v) then indicates the number of unit cells in crystallite [36] calculated with the Eq. (5):

$$n=0.6043 \left(\frac{{D}^{3}}{{a}^{2}c}\right)$$
(5)

This variation can be explained by a shift in the parameters a and c of the lattice.

The atomic packing fraction (APF) [43] was calculated using the Eq. (6)

$$APF= \frac{2\pi a}{3c\sqrt{3}}$$
(6)

The measured value of APF was approximately 75.18%, while for bulk ZnO value of APF is 74%. This minor change in the APF value in nanoparticles was due to the size impact. This small rise in the APF value for doped ZnO shows that dopant has been incorporated into the host lattice.

The dislocation density (δ) [36] was determined from the size of the crystallite by the Eq. (7)

$$\delta = \frac{1}{{D}^{2}}$$
(7)

These values were included in Table 2, indicating that δ values improved with Eu3+ content from 1.7528 × 1015 to 2.6316 × 1015. This is the sign that lattice imperfection improves with reductions in crystallite size.

Table 2 Additional ZnO parameters with various Eu3+ doping content resulting from XRD
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Similarly, micro-strain (ε) [36] was calculated using the formula.

$$\left(\varepsilon \right)= \frac{{\beta }_{hkl}cos\theta }{4}$$
(8)

In particular, this sort of ε (Table 1) variability was due to the morphology, crystallite size, and particle shape conversion.

Another method to authenticate the presence of dopant is to estimate the Zn–O bond length (L) [44]. The subsequent relationship was therefore used to assess L.

$$L= \sqrt{\left(\frac{{a}^{2}}{3}+{\left(\frac{1}{2}-u\right)}^{2}{c}^{2}\right)}$$
(9)

where u is the positional parameter for wurtzite structure and is given by;

$$u= \frac{{a}^{2}}{{3c}^{2}}+ 0.25$$
(10)

Bond length for Zn–O was found to 1.9767 Å, and it increased (1.9773 Å) with Eu3+ content up to 5%, which also confirmed the replacement of Eu3+ ion in ZnO matrix. The variation in the volume of nanoparticles, volume of the hexagonal unit cell, number of atom per unit cell, dislocation density, APF and bond length are tabulated in Table 2.

3.2 FTIR analysis

FTIR spectra as shown in Fig. 6, details the functional properties of the synthesized powder. The Zn–O stretching mode is attributed to characteristic sharp peaks found in the range 474–525 cm−1, which also confirms the development of the ZnO product [45]. The absorption at 3436 cm−1 was attributed to O–H stretching mode of the water molecule on the surface of the samples.[46]. The band at 2934 cm−1 was due to C–H stretching vibrations and the absorption peak at 2384 cm−1 was due to the presence of CO2 molecules in the atmosphere [46, 47]. The band at 1564, 1035, and 759 cm−1 was related to the precursor materials [48, 49]. Zn–O stretching absorption bands for different content of Eu3+ doped ZnO nanoparticles are 474 (ZnO), 487 (1%), 512 (3%) and 525 cm−1 (5%). However, with the rise in Eu3+ content, a minute change in the position of the absorption band was noted. Faraz et al. [32] has also reported such a type of peak shift upon RE doping. This type of band shift is the indication of successful doping of Eu3+ ion into the Zn–O crystal structure.

Fig. 6
figure 6

FTIR spectra of different levels of Eu3+ doped ZnO nanoparticles (1%, 3%, and 5%)

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3.3 FESEM

Figure 7a, b shows a low magnified and high magnified FESEM picture of the nanoparticles of 1% Eu3+ doped ZnO. FESEM images are clear evidence of the existence of a macroporous honeycomb-like nature of gel-combustion synthesized material. Spherical Eu3+ doped ZnO nanoparticles are layer by layer self-accumulating and the holes between the particles are bound. The macropores were visible in Fig. 7b and the diameter of the pore varied from 0.3 to 1.3 μm. The feasible development method of the macroporous honeycomb-like structure was observed in our case as seen in Fig. 7b. Chikkahanumantharayappa et al. [50] reported such type of morphology occurs due to nucleation, growth, and self-accumulation. Were Zn2+ and OH− ions ionize first in solution after the hydrolysis process. Then the nucleation process begins between Zn2+ and OH− ions [51]. The increase in reaction time outcomes in greater development and aggregation of the nucleation crystallite. Ma et al. [52] and Nagabhushana et al. [53] have also reported the formation of macropores which were generated after the calcination phase due to the evasion of gasses, which in turn leave small holes on the sample surface, which was also observed in our case. This type of macroporous structural design was acquired by the self-accumulation of nanoparticles.

Fig. 7
figure 7

SEM images of 1% Eu3+ doped ZnO nanoparticles with different magnification view: a 1 μm, b 200 nm

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3.4 UV–Vis spectroscopy

Figure 8 depicts the UV–Vis absorption spectra of prepared undoped and different content of Eu3+ doped (1%, 3%, and 5%) ZnO nanoparticles. A monotonous red shift in the absorption spectrum parallel to that of undoped ZnO nanoparticles was noted with an increase in Eu3+ ions concentration, which is the indication of the band gap narrowing of Eu3+ doped ZnO nanoparticles. This type of change in the band gap was also reported by Swart et al. [10] due to charge transfer between ZnO valance or conduction band concerning the 4f levels of Eu3+ ions. The energy gap energies were calculated by Eq. (11):

$${E}_{g}\left(eV\right)= \frac{hc}{\lambda } (nm)$$
(11)

where Eg is the band gap energy (eV), h is Planck’s constant (4.135667 × 10−15 eVs), c is the velocity of light (3 × 108 m/s), and λ is the wavelength (nm) of absorption onset. We then continued our research to calculate the energy gap of undoped and Eu3+ doped ZnO nanoparticles with the well-known Tauc's equation.

$$\alpha h\upsilon = A{\left(h\vartheta -{E}_{g}\right)}^{n}$$
(12)

where Eg is optical band gap, h is Planck’s constant and υ is the frequency of incident photons, A is a constant called the band tailing parameter, and n is the index with distinct values (2, 3, 1/2, and 1/3) that are indirect allowed, indirect forbidden, direct allowed and direct forbidden transitions, respectively [54, 55]. ZnO is a semiconductor with a direct band gap, n = 1/2. The optical band gap was determined from the extrapolation of the linear portion of (αhν)2 versus hν plot to the energy axis. The connection between (αhν)2 and hν for ZnO and Eu3+ doped ZnO nanoparticles was depicted in Fig. 9. The band gap energies of the doped products were less than the pure ZnO, as indicated in Table 3. Such type of decrease in the band gap related to RE-doped ZnO compared to pure ZnO was also reported by Ntwaeaborwa et al. [24]. The findings above showed that Eu3+ doped ZnO is capable of absorbing UV and visible regions of solar light. Therefore, the absorption property deduced that in sunlight photocatalysis the Eu3+ doped ZnO could be useful.

Fig. 8
figure 8

UV–visible absorption spectra of pure ZnO and various Eu3+ doping content of (1%, 3%, and 5%) ZnO nanoparticles

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Fig. 9
figure 9

The Plot of (αhυ)2 versus hυ of ZnO and various content of (1%, 3%, and 5%) Eu3+ doped ZnO nanoparticles

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Table 3 Calculated band gap based on the energy wave equation and Tauc plot
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3.5 PL studies

Figure 10 displays the room temperature emission spectra of ZnO nanoparticles. The spectra were recorded at 325 nm wavelength excitation source using a monochromatized xenon lamp. Under the excitation of 325 nm, two bands displayed ZnO nanoparticles: (1) Band of UV emissions below 400 nm and (2) The broad visible emission band from 400 to 600 nm. The sharp UV emission band at 383 nm was associated with the near band-edge (NBE) emission of ZnO [56]. This band origin is due to the direct recombination of ZnO free excitons [57, 58]. The wide noticeable range in 400 nm to 600 nm wavelength region is due to deep-level defects in the crystal such as vacancies and oxygen and zinc interstitials [59, 60]. We discovered that our samples showed a UV emission peak at 383 nm along with three defect peaks related to violet emission close to 418 nm, blue emissions close to 470 nm and a peak connected with green emission close to 541 nm in the PL spectra. The emission peak in the violet region (418 nm) may be due to the transition from the conduction band to zinc vacancy centers [61]. Certain groups have also determined that this form of emission is linked to the transition from zinc interstitial energy levels to the valence band with non-stoichiometric ZnO [62, 63]. The observed blue emission band at 470 nm was associated with the intrinsic defects emission within the ZnO host [64]. Some groups also indicate that this blue emission arises as a result of the recombination of zinc interstitial energy level to zinc vacancy energy level [57, 65]. Oxygen vacancies and intrinsic defects can be attributed to the origin of the green band (541) [66]. The observed green emission is also due to the level of impurity corresponding to the ZnO single ionized oxygen vacancy [67]. The green emission was described by Vanheusden et al. by the transition between photoexcited holes to ionized oxygen vacancies [68]. Such type of green emission in ZnO was also reported by Zeng et al. [62] due zinc vacancies, i.e. the transition between the electrons near the conduction band and deeply trapped holes at doubly ionized oxygen vacancies.

Fig. 10
figure 10

Room temperature photoluminescence emission spectra of ZnO nanoparticles excited at 325 nm

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The excitation (615 nm) and emission (465 nm) spectra of ZnO:Eu3+ (Eu3+  = 1%, 3%, and 5%) nanoparticles recorded at room temperature using monochromatized xenon lamp as the excitation source as shown in Figs. 11 and 12. The excitation spectra were evaluated by tracking the wavelength of emission at 615 nm. The excitation spectra, as shown in Fig. 11, comprises of five distinctive peaks at 402 (7F0 → 5L6), 416 (7F0 → 5D3), 465 (7F0 → 5D2), 501 (7F0 → 5D1), and 535 nm (7F1 → 5D1). These transitions are attributed in the ZnO lattice to intra-configurationally 4f → 4f transitions of Eu3+ ions [69,70,71]. Among these excitation peaks 7F0 → 5D2 (465 nm) provides the most intense Eu3+ emission spectral profile. Concerning the above result, 465 nm as a pump light was adopted to produce red emissions from all Eu3+ doped ZnO nanoparticles.

Fig. 11
figure 11

Room temperature photoluminescence excitation spectra of various concentrations (1%, 3%, and 5%) of Eu3+ doped ZnO nanoparticles

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Fig. 12
figure 12

Room temperature photoluminescence emission spectra of different concentrations (1%, 3%, and 5%) of Eu3+ doped ZnO nanoparticles

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Figure 12 shows, emission spectra (465 nm), which comprises five emission bands at 579, 591, 617, 652 and 706 nm, credited to Eu3+ transitions: 5D0 → 7FJ (J = 0, 1, 2, 3, and 4). Nouri et al. [5] and Shi et al. [72] have also reported such type of emission bands related to Eu3+ ion. The first emission peak at 579 nm (5D0 → 7F1) which is related to magnetic dipole transition and its intensity hardly differs with the Eu3+ ion bonding environment, while the peak at 591 nm (5D0 → 7F2) is an electrically permitted transition hypersensitive to the Eu3+ ion coordinating environment. As reported by Cao et al. [73], the information related variation in the location of Eu3+ ions symmetry depends on the 5D0 → 7F2 / 5D0 → 7F1 intensity ratio. Based on literature related to Ramakrishna et al. [74] and Van Tran et al. [75] it can be suggested that Eu3+ ions occupy without inversion symmetry in our case, which was observed in the form of higher intensity related to 5D0 → 7F2. These results conclude that Eu3+ ions are located on the host lattice without inversion symmetry. Similar results were also reported by Zamiria et al. [14], which matches in our case.

In our case, however, an increase in peak intensity was observed with the addition of doping concentration, providing electronic transitions of Eu3+ ions for peak intensities. Because of their charge inequality in Zn2+ sites, the reasons for Eu3+ ion incorporated deficiencies are either related to their lattice mismatch or creating an oxygen vacancy. Such types of findings were detailed by Layek et al. [41] concerning the rise in emission intensity peaks following RE ion doping. No quenching impact was seen in our situation when the concentration of dopant was increased up to 5% (Eu3+). Such an impact confirms the high solubility of RE metal ions and also suggests that Eu3+ doping may have increased the efficiency of photoluminescence to achieve red emission nanoparticles.

As shown in Fig. 12, when the concentration of Eu3+ doping increased from 1 to 5%, Eu3+ doping had a significant impact on position 5D0 → 7F2, and the transition position 5D0 → 7F2 changed from 617 to 619 nm. Theoretically, the 7F2 level has three crystal-field sublevels (A1, E1, E2). The A1 and E1 sublevels are nearer to the energy level reported by Lopez-Romero et al. [76] and Linare et al. [77]. As shown in Fig. 12, for Eu3+ doping levels of 1%, 3% and 5%, the positions of 5D0 → 7F2 transition are 617 nm, 617.6 nm, and 619 nm. We think in our situation that the sublevels (A1, E1, E2) are behind the occurrence of shifting PL peak positions. We conclude that the shift in the peak position of 5D0 → 7F2 was due to the enhancement of the crystal field around Eu3+ ions and then the emission occurred from the main emitting level 5D0 to the 7F2 level A1, E1 and E2 when the concentration of Eu3+ doping varied (1%, 3%, and 5%), respectively.

A PL assessment is used to assess the feasible mechanism for the energy transfer technique from the ZnO host to Eu3+ ions. The Eu3+ ion peaks are the result of transitions in the PL spectra between their intra-4f levels of electron energy. When the Eu3+ doped ZnO nanoparticles, were excited at 465 nm, the ZnO host absorbs the excitation energy and encourages the electrons from the valence band into the ZnO host conduction band. Some of the excited electrons in the conduction band edge transit on the radioactively to surface defects of nanoparticles trapped by replaced Eu3+, oxygen vacancy, zinc interstitial, then transferred to the main emission level 5D0 of the Eu3+ ions resulting in the emission of 5D0 → 7F0, 7F1, 7F2, 7F3, and 7F4 [27].

4 Conclusions

The ZnO:Eu3+ nanoparticles with distinct levels of Eu3+ ions were synthesized effectively using the gel-combustion technique. The assimilation of Eu3+ has created a major effect on both the shape and luminescence efficiency of ZnO:Eu3+ nanoparticles. It was found that the crystallite size calculated by Scherrer's technique, W–H plots, and SSP was 19.49–23.88 nm. The increase in the lattice parameters of the hexagonal unit cell was observed with increased Eu3+ content. This is due to the difference in Zn2+ and Eu3 + ionic radii. Analysis of the FTIR verified the existence of Eu3+ dopant peak change from 474 cm−1 to 525 cm−1. FESEM studies revealed that the nanoparticle obtained was a macroporous honeycomb-like structure. Tauc relation confirmed the decreased energy gap with the increase of Eu3+ content. Due to distinct types of faults, broad emission spectra were noted in the observable region for undoped ZnO. The photoluminescence emission spectra of Eu3+ doped ZnO nanoparticles exhibited five intense emission bands at 579, 591, 616, 652, and 706 nm, which were attributed to 5D0 → 7F0, 5D0 → 7F1, 5D0 → 7F2, 5D0 → 7F3, and 5D0 → 7F4 transitions of Eu3+ ions when excited with the 465 nm wavelength, originated from intra-4f transition of Eu3+ ions, respectively. The emission intensity increased with Eu3+ doping and the highest intensity peak was observed at 617 nm (red emission) attributable to the electric dipolar transition of 5D0 → 7F2. Besides, the increase in the concentration of Eu3+ resulted in a PL peak position shift corresponding to transitions from the main 5D0 level to the 7F2 level of A1, E1, and E2. The present study shows that RE doped ZnO nanoparticles can find avenues for potential applications in optoelectronics, gas, and biosensors, photocatalysis and multicolor displays.