Materials Today Communications 38 (2024) 107944 Available online 22 December 2023 2352-4928/© 2023 Elsevier Ltd. All rights reserved. Multi-colour emission from ZnO[SO4]:Eu2+ nanophosphors: Effects of SO4 2- and Eu2+ doping concentration M.L.A. Letswalo a,*, L. Reddy a, A. Balakrishna a,*, M.A. Mamo b, R.M. Erasmus c, O.M. Ntwaeaborwa c,* a Department of Physics, University of Johannesburg, Johannesburg ZA2006, South Africa b Department of Chemical Science, University of Johannesburg, Johannesburg ZA2006, South Africa c School of Physics, University of the Witwatersrand, Johannesburg, WITS2050, South Africa A R T I C L E I N F O Keywords: Solid-state reaction Phosphors Photoluminescence Energy transfer A B S T R A C T Blue-emitting ZnO-SO4:Eu2+ powder phosphors were synthesized via solid state reaction method. Based on the X- ray diffraction (XRD) data, hexagonal wurtzite structures of ZnO were formed. Further structural elucidation via Raman spectroscopy showed a peak at 439 cm− 1 associated with E2hgh optical mode that is a distinctive char- acteristic band of hexagonal wurtzite ZnO phase. The ZnO powders were composed of distorted spherical nanoparticles, which were agglomerated together as confirmed from scanning electron microscopy images. After incorporation of SO4 2- and Eu2+ into the ZnO lattices sites, the particle shapes became irregular. When excited at 351 nm with Xenon lamp, our samples emitted UV and blue-green visible light associated, respectively, with excitonic recombination and intrinsic structural lattice defects in ZnO. Both the photoluminescence (PL) peak intensity and positions of these emissions were influenced by SO4 2- and Eu2+ ions. The blue emission was dependent on the Eu2+ content with the largest intensity measured from the sample doped with 1.0 mol% of Eu2+ ions. Mechanisms of energy transfer and concentration quenching effect are discussed. 1. Introduction ZnO is a semiconducting material that has been studied widely for application in lighting [1], gas sensors, solar cells and a wide variety of optoelectronic devices [2,3]. It is characterized by UV and visible light emission resulting from electron-hole recombination in the bandgap and intrinsic lattice defects, respectively. It has a bulk bandgap of 3.37 eV and the exciton binding energy of 60 meV [4], which is useful in opto- electronic devices. Although ZnO has a large and direct bandgap, it is highly capable of absorbing energy from the incident beam (e.g. pho- tons) and generates photo excitations and emissions. In addition, ZnO is used as host for lanthanides and alkali earth ions to prepare phosphors that emits ultraviolet or visible or infrared light [5–7]. Doping of ZnO offers advantages such as: (1) tailoring of the bandgap by repositioning the conduction and valence bands, (2) enhancing absorbance, and (3) enhancing ZnO conductivity and charge carrier mobility. Many re- searchers have doped ZnO to increase charge traps, which reduce bulk recombination thereby allowing efficient separation of photogenerated electrons and holes [8–10]. Our earlier report showed significant improvement in green photoluminescence due to incorporation of sul- phate ions in the zinc lattices sites [11]. In this paper, we report the effect of SO4 2- on the blue photo- luminescence of Eu2+ ions incorporated in ZnO crystal. Since Eu2+ is sensitive to the local crystal field of the host lattice compared to other popular dopant ions, its broad emission bands can be shifted across the visible spectrum. The blue emission was enhanced by a transfer of en- ergy from ZnO[SO4] to Eu2+. Energy transfer from the host lattices en- hances photoemission intensity of rare-earths dopant ions that can in turn be used to improve performance and life span of light emitting diodes (LEDs), fluorescent lamps, fibre amplifiers and solid-state lasers [12–14]. Enhanced photoemission from different rare-earths ions because of phono-mediated transfer of energy from substitutional components has been reported. Ntwaeaborwa et al. [14] observed transfer of energy from ZnO to Ce3+ embedded in an amorphous glassy SiO2 host. Dhlamini et al. [15] and Mhlongo et al. [16] have, respec- tively, demonstrated improved photoemission from Tb3+ and Pr3+ dopant ions because of transfer of energy from ZnO nanoparticles encapsulated in glassy SiO2 host lattices. Usually, transfer of energy * Corresponding authors. E-mail addresses: letswalom@uj.ac.za (M.L.A. Letswalo), bavula@uj.ac.za (A. Balakrishna), martin.ntwaeaborwa@wits.ac.za (O.M. Ntwaeaborwa). Contents lists available at ScienceDirect Materials Today Communications journal homepage: www.elsevier.com/locate/mtcomm https://doi.org/10.1016/j.mtcomm.2023.107944 Received 18 October 2023; Received in revised form 8 December 2023; Accepted 20 December 2023 mailto:letswalom@uj.ac.za mailto:bavula@uj.ac.za mailto:martin.ntwaeaborwa@wits.ac.za www.sciencedirect.com/science/journal/23524928 https://www.elsevier.com/locate/mtcomm https://doi.org/10.1016/j.mtcomm.2023.107944 https://doi.org/10.1016/j.mtcomm.2023.107944 https://doi.org/10.1016/j.mtcomm.2023.107944 http://crossmark.crossref.org/dialog/?doi=10.1016/j.mtcomm.2023.107944&domain=pdf Materials Today Communications 38 (2024) 107944 2 between ZnO and dopant ions takes place via primary excitation energy capture by intrinsic defect state of ZnO and subsequent transfer to the dopants and/or via free or bound excitonic state levels of ZnO. Balak- rishna et al. [17] and Letswalo et al. [11] reported transfer of energy from substituted anionic groups such as BO3 3-, PO4 3- and SO4 3- to Sm3+ and Eu3+ in molybdate hosts. The proposed energy transfer mechanisms between substituted groups and luminescent ions are speculative and therefore more research needs to be conducted to get a better under- standing of these energy transfer mechanisms. Unlike most rare-earths dopant ions whose narrow emission bands are fixed because their 4 f orbitals are shielded from the local crystal field, the 4 f orbitals of Eu2+ are not shielded from the crystal field of the host lattice [18] and therefore their energy levels can be easily split, and the radiative tran- sitions can be shifted. This makes Eu2+ an interesting dopant ion with exceptional luminescent properties and a wide range of applications. Eu2+ produces broad range of visible emissions in different host lattices Fig. 1. X-ray diffraction spectra of undoped and SO4 2- and (0.5 – 3 mol%) Eu2+ doped ZnO. Fig. 2. (a) the Rietveld fits of (a) undoped, (b) SO4 2- and (c) (1 mol%) Eu2+ doped ZnO crystal structures. Table 1 Crystallographic data, structure Rietveld refinement parameters and particle size for ZnO, ZnO[SO4] and ZnO[SO4]:Eu2+ phosphors. Sample Parameter ZnO ZnO[SO4] ZnO [SO4]:1.0Eu2+ Crystal phase Hexagonal Hexagonal Hexagonal Space group P63mc P63mc P63mc Lattice parameters (Å) a = b = 3.2488c = 5.2054 a = b = 3.2539c = 5.20598 a = b = 3.2524c = 5.2162 Rp 2.73 2.52 4.25 Rwp 3.71 3.46 3.42 Rexp 2.72 2.51 4.39 Cell volume (Å3) 47.58 47.77 47.60 Density (g/cm3) 5.68 5.66 5.67 Particle size (nm) (Scherrer’s method) 87 70 64 M.L.A. Letswalo et al. Materials Today Communications 38 (2024) 107944 3 [18–22]. Red photoluminescence from Eu-doped ZnO nanostructures fabricated by microemulsion have been reported by various researchers [23–27]. Photoemission from Eu2+ ion is a result of electronic transition from excited 4 f65d energy states to 4 f7 ground energy levels. Electron transition from the ground energy state (4 f7 configuration) to the excited energy state (4 f65d configuration) is caused by absorption of primary excitation energy and emission results from electronic transi- tion from the lowest energy band of the 4 f6d configuration to the 8S7/2 of the 4 f7 configuration. Because of the higher spatial length of the 5d orbitals, the 4 f65d levels can be easily split by the crystal field compared to the 4 f7 levels, and they are usually regarded as the metastable state, when the Eu2+ ion is incorporated in host lattices. The octahedral symmetry of the crystal field splits the 5d orbitals into two sub levels of t2 g and eg components [28,29]. We used soli-state reaction method to synthesize ZnO[SO4]:Eu2+ phosphors with different contents of Eu2+ and evaluated how SO4 2- ions incorporated in ZnO host influenced defect emission from the host and the blue emission associated with the 5d→4f transitions of the Eu2+ ions. The mechanism of transfer of energy by phonon mediated process from ZnO and ZnO[SO4] to Eu2+ that enhanced the blue emission is proposed. 2. Experimental 2.1. Materials and synthesis Solid-state reaction method was used to prepare ZnO, ZnO[SO4], and Eu2+ doped ZnO[SO4]. Varying concentrations of Eu2+ from 0.5 to 3 mol % were added in constant increments of 0.5. The starting materials were high-purity grade ZnO (99.99%), (NH4)2HSO4 (99.99%), and Eu2O3 (99.99%) powders. These materials were weighed stoichiometrically, mixed in acetone and ground for 45 min before sintering in a furnace in a reducing atmosphere (5%H2/95%Ar) at 1000 ◦C for 6 h. After cooling the mixture to room temperature, the final products were ground gently using a pestle and mortar resulting in a fine powder. 2.2. Characterization techniques The Bruker D8 Advanced X-ray diffractometer to conduct the X-ray diffraction analysis of the crystalline structure of the samples. A monochromatic source of radiation CuKα (λ = 0.15405 nm) was utilized, and measurements were taken within the range of 20 – 80◦ 2θ scans. Additionally, Nicolet Fourier transformed infrared (FTIR) spectrometry was used to examine the stretching frequency modes. The FE-SEM (field emission scanning electron microscope) and an energy dispersive spec- trometer (EDS) were employed to study the particle morphology and elemental composition. The Raman scattering of ZnO was evaluated by analysing lattice vibrational modes using a Horiba Jobin-Yvon Raman Spectrometer. Furthermore, PL measurements were conducted using Horiba 800 spectrophotometer, at room temperature, equipped with a monochromatic Xenon lamp used as an excitation source. Finally, the UV–vis reflectance spectra were acquired using a Lambda 950 UV–Vis spectrometer. Table 2 Fractional atomic coordinates and occupancy factors for ZnO[SO4]:Eu2+ deter- mined from the Rietveld refined structures. Atom x y z Occupancy Site Zn1 0 0 0 1.000 2a Zn2 0.5 0.5 0.5 1.000 6c Zn3 0.99353 0.49885 0.24366 0.990 6c Eu1 0.00648 0.50115 0.75634 0.010 6c S1 0.87445 0.71945 0.51039 1.000 12d S2 0.12555 0.28055 0.48961 1.000 12d S3 0.77074 0.39625 0.01972 1.000 12d S4 0.22926 0.60375 0.98028 1.000 12d O1 0.00232 0.70692 0.62526 1.000 12d O2 0.99768 0.29307 0.37474 1.000 12d O3 0.95711 0.70571 0.3624 1.000 12d O4 0.04289 0.29429 0.6376 1.000 12d O5 0.74941 0.59459 0.54545 1.000 12d O6 0.25059 0.40541 0.45455 1.000 12d O7 0.8991 0.25293 0.04852 1.000 12d O8 0.1009 0.74707 0.95148 1.000 12d Fig. 3. (a-d) SEM images and (a-c) EDS spectra of ZnO, ZnO[SO4], and Eu2+-doped ZnO[SO4] phosphors sintered in argon/hydrogen atmosphere. The insets are weight percentages of the elements. M.L.A. Letswalo et al. Materials Today Communications 38 (2024) 107944 4 3. Results and discussion 3.1. XRD powder diffraction analysis The XRD spectra of the powder samples are displayed in Fig. 1. The diffraction peaks are the same of as those of a hexagonal ZnO phase referenced in JCPDS card number 01–080-0074 [30]. The crystal structure was not influenced by small amounts of SO4 2- ions, suggesting that these ions were well dispersed in the host lattice. However, at larger concentrations (> 1.0 mol%) of Eu2+ a minor diffraction peak (marked with an asterisk (*)) was detected at 28.8◦ and was indexed to a sec- ondary phase of Eu2O3 [31,32]. This peak can be attributed to incom- plete decomposition of the precursor. The crystal structures from the XRD data were refined using the Match and Full-Proof Rietveld refinement programme method and the results are presented in Fig. 2. The observed (black:colour online), calculated (red:colour online), and the difference (blue:colour online) XRD profiles of undoped and SO4 2- and (1.0 mol%) Eu2+ doped ZnO are presented in Fig. 2(a)-(c) respectively. The structural parameters and the 5% reliability factors (Rwp and Rp) or simply R-factors were determined. The refined ZnO and SO4 2- doped ZnO structures contain no impurities or secondary phases. Furthermore, no additional phases were detected in the 1.0 mol% Eu2+ doped ZnO[SO4] structure which is consistent with the Rietveld refinement results. The crystallographic data and structural parameters obtained from the refinement are listed in Table 1. The results indicate that the basic hexagonal structure of ZnO was not affected by the substitutional ions SO4 2- and Eu2+ ions and have the same space group P63mc. Additionally, the refinement R-values (Rp, Rwp and Rexp) were found to be in agree- ment with the hexagonal structure with the P63mc space group. The influence of adding SO4 2- ions and Eu2+ ions was identified through the lattice parameters and cell volume of the unit cell structure. The results revealed that the lattice parameters and cell volume were slightly increased after adding SO4 2- ions and Eu2+ ions. This ZnO lattice enlargement was related to the different ionic sizes of the Zn2+ (radius = 0.083 nm) and Eu2+ (radius = 0.117 nm) ions. Using the XRD data, Scherrer’s method was employed to determine the crystalline sizes of the prepared nanophosphors. The calculated particle sizes are presented in Table 1. The results indicated that the variation of the particle sizes in the nanometre scale followed an increasing trend of ZnO>ZnO[SO4] Fig. 4. EDS mapping of ZnO[SO4]:1.0Eu2+ phosphor powder. Fig. 5. FTIR spectra of undoped, SO4 2- and (1 mol%) Eu2+ doped ZnO phosphors. Fig. 6. Raman spectra of undoped, and SO4 2- ( 2.0 mol%) and Eu2+-doped ZnO phosphors. M.L.A. Letswalo et al. Materials Today Communications 38 (2024) 107944 5 > ZnO[SO4]:1.0Eu2+ profile. The reduction in the crystalline size of the ZnO after substituting with SO4 2- and Eu2+ ions is mainly attributed to the distortion of the ZnO lattice [33,34]. Table 2 displays the fractional atomic positions (x, y and z) and occupancy factors for ZnO[SO4]:Eu2+ determined from the Rietveld refined structures. 3.2. Scanning electron microscopy analysis The SEM images and EDS spectra of the powder samples are pre- sented in Fig. 3(a)-(d). As observed from Fig. 3(a), ZnO is made up of distorted spheres. The degree of distortion increased upon incorporation of SO4 2- (see Fig. 4(b). After incorporation of 1.0% and 2.0% of Eu2+, there was a further increase in particle distortion giving rise to particles with irregular shapes (Fig. 3(c) and (d)). The EDS data confirmed all the elements present in the materials, together with their atomic weight percentages shown in the insets of Fig. 3(a), (b), and (c). Carbon (C) and Iridium (Ir) probably came from the carbon tape mountings and coating layers (used to prevent charging) respectively. The elemental mappings of ZnO[SO4]:1.0Eu2+ powder are shown in Fig. 4. The mappings confirm an even distribution of Zn, S, O, Eu, and C elements. 3.3. Fourier transform infrared and Raman spectroscopy measurements The Fourier transform infrared (FTIR) spectra of undoped and SO4 2- and (1.0 mol%) Eu2+ doped ZnO are shown in Fig. 5. The well-known absorption band around 442 cm− 1 in the spectra is attributed to stretching vibrations of Zn-O bonds [35,36]. The incorporation of sul- phate compounds into the ZnO phosphors was confirmed by the peak located at 1206 cm− 1, which represented the SO4 2- structural groups [37, 38]. The transmittance bands at 1648 cm− 1 are assigned the C––O bonds, and those at 902 and 1375 cm− 1 are due to the C-O bonds [37–39]. The C-H antisymmetric stretching bonds belong to a low signal peak at 2929 cm− 1 [40–42]. The bands around 3609–3800 cm− 1 are attributed to the O-H groups [43]. Raman spectra of ZnO, ZnO[SO4], and 2.0 mol% Eu2+-doped ZnO [SO4] phosphors are shown in Fig. 6. The bands at 99, 337, 380, 408 and 439 cm− 1 are assigned to wurtzite ZnO. The 439 cm− 1 band is associ- ated with the E2 H mode of the non-polar optical phonons of ZnO. The bands at 327 and 380 cm− 1 are associated with the A1 symmetry (A1T) of ZnO nonpolar optical phonons in the 2E2 L and TO modes [44]. The peak at 205 cm− 1 is due to lattice oxygen vacancies [45]. The enlarged portion of the spectra in the 1000–1025 cm− 1 range, with major peaks at 1016 and 1005 cm− 1, is shown in the inset. These peaks possibly orig- inated from the stretching vibrations of the sulphate anions [46]. The small band located at 584 cm− 1 is attributed to E1 symmetry for LO modes, which is due to lattice O-vacancies, Zn-interstitials, and free carriers [44,45]. Upon incorporation of Eu ions, the E2 H Raman peak intensity decreases and there was a shift in the peak position from 437 cm− 1 for ZnO to 439 cm− 1 for the 2.0 mol% Eu2+-doped ZnO[SO4] sample [45]. This confirms the successful substitution of Zn2+ ions by Eu2+ ions. 3.4. Diffuse reflectance spectroscopy Fig. 7(a) and (b) show, respectively, the diffuse reflectance spectra (DRS) and Tauc plot of undoped, SO4 2- and Eu2+ doped ZnO phosphors. All the DRS spectra exhibit prominent ZnO characteristic absorption edge in the range of 375 to 450 nm [47–52]. ZnO[SO4] has a relatively higher reflectance of ̴85% compared to all other samples, while the least reflection of ̴ 45% was recorded from ZnO[SO4]:0.5 mol%Eu2+. The weak absorption bands located at 466, and 525 nm are assigned to the 7F0 →5D2 and 7F0→5D1 transitions of Eu3+ [53,54] respectively, which further confirms the presence of Eu2O3 as confirmed from the XRD data. The band gap energies were estimated from the Tauc plots in Fig. 7(b) [54]. The largest bandgap of 3.204 eV was determined from ZnO[SO4] and the narrowest bandgap of 3.137 eV was determined from 0.5% Eu-doped ZnO[SO4]. The bandgap energies did not follow any particular trend but were rather fluctuating as the concentration of Eu2+ was varied from 0.5 to 3 mol%. This could be due to varying particles sizes, defect states, anion or cation vacancies, and interstitials (Oi/Vo) that don’t occupy fixed positions in the bandgap of ZnO [55]. Again, it is most likely that the presence of Eu2O3 secondary phase (as confirmed from the XRD data) influenced the bandgap energy values. 3.5. Photoluminescence (PL) spectroscopy Fig. 8(a) – (d) show the PL spectra measured when the powders where excited using a monochromatic Xenon lamp. Fig. 8(a) shows the spectra of undoped, and SO4 2- and (1.0 mol%) Eu2+ doped ZnO. The major blue emission at 409 nm is associated with the 4 f65d1→4f7 transition of Eu2+ [51], while the broad emission bands peaking at 507 and 521 nm (Fig. 8(b)) are due to structural defects in ZnO. These bands also display small humps at 413 and 436 nm, which are associated with oxygen/zinc interstitials and oxygen/zinc vacancies. A marginal in- crease in the PL intensity and a slight red-shift of the spectrum were observed after incorporating SO4 2- ions into the ZnO host. The observed changes are most probably due to transfer of energy, non-radiatively, between SO4 2- and ZnO as illustrated in the deconvoluted PL emission spectra in Fig. 9(a) and (b). The photoemission spectra of ZnO-SO4 with varying concentrations of Eu2+ are presented in Fig. 8(c). The highest Fig. 7. (a) Diffuse reflectance spectra and the (b) Tauc plots from which the band gap energies were estimated. M.L.A. Letswalo et al. Materials Today Communications 38 (2024) 107944 6 Fig. 8. PL spectra of (a) undoped, and SO4 2- and 1 mol% Eu2+ doped ZnO, (b) ZnO[SO4] and ZnO (enlarged), (c) ZnO[SO4]: Eu2+ with varying content of Eu2+, and (d) ZnO:Eu2+ (1 mol%) and ZnO[SO4]:Eu2+ (1 mol%). M.L.A. Letswalo et al. Materials Today Communications 38 (2024) 107944 7 Fig. 9. (a)-(h): De-convoluted PL spectra of SO4 2- and Eu2+ doped ZnO phosphors. M.L.A. Letswalo et al. Materials Today Communications 38 (2024) 107944 8 photoemission was observed from the 0.5 and 1 mol% of Eu2+ doping. The peak position for these two spectra is located at 490 nm, confirming that the emissions are attributable to the 4 f65d1→4f7 transitions of the Eu2+dopant ions. The PL peak intensity decreased with increasing Eu2+ dopant concentrations. The decrease of PL intensity with increasing Eu2+ dopant concentration is attributed to effects of concentration quenching. In addition, there is a relative increase in the well-known UV emission at ~ 389 nm, which is attributed to band-to-band transition in ZnO. The ~ 389 nm emission is maximum for 2.5 mol% of Eu2+ doping, and the main emission peak is blue-shifted from 490 nm, which is indicative of the changes in radiative transitions. This blue-shifted emission is associated with intrinsic defects in ZnO[SO4] [52,53]. The quenching of photoluminescence at 490 nm with increasing concentration of Eu2+ is known to occur via cross-relaxation processes between two identical centres. The decrease in the PL intensity was noticeable when the concentration of Eu2+ exceeded 1.0 mol%. To evaluate the concentration quenching effect of the ZnO[SO4]:xEu2+ phosphors, the critical distance Rc between identical Eu2+ ions was calculated using the following formula proposed by Blasse [54]: Rc = 2( 3V 4πC0N ) 1 3 (1) where N is the number of ions per unit cell, V is the volume of the unit cell, C0 is the ideal activator (Eu2+) concentration. Using N = 1.0, C0 = 0.01, and V = 47.77 Å, the Rc value for Eu2+ ions were found to be 20.89 Å [55]. This value is greater than 5 Å, pointing to lack of transfer of energy between Eu2+ ions in the ZnO-SO4 host. It shows that non-radiative transfer of energy was not possible at higher concentra- tions of Eu2+ ions in this electric multipolar interaction and this contributed to the quenching of luminescence from Eu2+. Fig. 8(d) compares the photo-emission spectra of ZnO:Eu2+ (1 mol%) to that of ZnO[SO4]:Eu2+ (1 mol%). For the same molar concentration of Eu2+, the PL emission from ZnO:Eu2+ is more intense than that of ZnO[SO4]: Eu2+, suggesting that the transfer rate of primary excitation energy from ZnO to Eu2+ was faster and more efficient compared to the transfer rate from ZnO[SO4] to Eu2+. Deconvoluted PL spectra of the powder phosphors are shown in Fig. 9(a)-(h) The emission bands at ~ 392 – 383 nm [~3.17 - 3.23 eV] in Fig. 9(a) are attributed to band-to-band recombination in ZnO. The blue emission band at ~ 414 – 436 nm [~2.99 – 2.84 eV] in Fig. 9(b) are assigned to zinc vacancies (VZn), while those at ~ 513 – 584 nm [~ 2.77 - 2.12 eV] are due to electronic transitions from the conduction band/ zinc interstitial states to zinc vacancy. The major blue emission bands in Fig. 9(c)-(d) at ~ 2.56 eV are attributed to 4 f 65d1- 4 f 7Eu2+ transitions. The emission bands in Fig. 9(e)-(h) are associated with intrinsic defects, i.e. transition from the conduction band to oxygen vacancy (Vo +)/zinc vacancy/electronic transition from zinc interstitial (Zni) to oxygen interstitial (Oi) [56,57]. The blue emission bands in Fig. 9(c)-(d) are shifting with varying concentration of Eu2+. Studies conducted by H. Sugimoto et al. [51] reported similar emission bands [52,53,58]. From Fig. 9(a)-(h), the emission peaks associated with various defect levels such as the green emission located at 512 nm (2.42 eV) in 0.5 and 1.0 mol% Eu2+ doped ZnO[SO4] phosphor are less intense than those observed in the undoped ZnO (Fig. 9(a)). This energy loss in ZnO emission centres is probably due to incorporation of Eu2+ ions to which energy was transferred from the Zn-O anti-bonding orbitals [54,59]. A further analysis of the decrease in the blue emission with increasing doping concentration of Eu2+ was performed using the Dexter formula [60–63]: I/x = k[1 + β(x) θ 3 ] − 1 (2) where I is the emission intensity, x is the activator concentration, k and β are the host constants under the same excitation conditions and θ is the electric multipolar character. As shown in Fig. 10 (a), a double- logarithmic graph of log (I/x) as a function of log (x) was constructed to determine the value of θ, and the relationship between these is dis- played in Fig. 10 (b). For θ = 6, 8, and 10, the multipolar interactions for transfer of energy are dipole-dipole, dipole-quadrupole and quadrupole- quadrupole, respectively [61]. The slope (θ 3) of log (I/x) as a function of log (x) curve was calculated to be –1.681, from which the value of θ was found to be 5.04, which is very close to 6. The interaction that is assigned to the value of θ = ~6 is dipole-electric dipole. This further confirms that the concentration quenching in Eu2+ doped ZnO[SO4] nanophosphors was a result of electric dipole-electric dipole interactions. The proposed simplified energy level diagrams of energy transfer mechanisms in our samples are depicted in Fig. 11 (a-c). Fig. 11 (a) shows the simplified energy level diagram of ZnO phosphor, which de- scribes the possible electronic transitions extracted from deconvoluted PL spectra of ZnO. The defect emission band located at 391 nm (3.17 eV) is related to the electron-hole recombination in the bandgap of ZnO. The other emission bands centred at 414 nm (2.99 eV) and (439 nm) 2.82 eV are attributed to zinc vacancies VZn, and zinc interstitials (Zni) respec- tively. The deconvoluted broad band centred at 530 nm (~2.44 eV) is related to singly ionized Vo + oxygen vacancy. Similar transitions were observed in ZnO[SO4] (Fig. 11 (b)), except for the emission peak at 584 nm (~2.12 eV), which is assigned to oxygen interstitials (Oi). Fig. 11 (c) shows a simplified energy level diagram of ZnO demon- strating energy transfer mechanism from ZnO host to Eu2+. The primary excitation energy was captured by ZnO followed by excited electrons Fig. 10. (a) Intensity against Eu2+ concentration of ZnO[SO4]:xEu2+ (0–3.0 mol%) and (b)Linear fitting of log (x (Eu2+)) against log (I/ x (Eu2+)) with different Eu2+ concentration for ZnO[SO4]:Eu2+ phosphors. M.L.A. Letswalo et al. Materials Today Communications 38 (2024) 107944 9 Fig. 11. Simplified energy level diagrams of light emission mechanisms based on deconvoluted PL emission spectrum of (a) ZnO, (b) ZnO[SO4], and (c) ZnO[SO4]:1.0Eu2+. M.L.A. Letswalo et al. Materials Today Communications 38 (2024) 107944 10 moving from Zn-O anti-bonding orbitals to similar orbitals in Eu2+ with a subsequent radiative relaxation to the ground state. Energy transfer probably took place by phonon mediated processes. Furthermore, we recorded the luminescence decay curves of ZnO: xEu2+ (x = 1.0 mol%) and ZnO[SO4]:xEu2+ (x = 1.0 mol%) phosphors excited at 351 nm while monitoring the 490 nm emission at room temperature (see Fig. 12). These lifetime decay curves were fitted with double-exponential function [64]. The average lifetime values of ZnO:1.0Eu2+ and ZnO[SO4]:1.0Eu2+ were found to be 30.16 and 18.91 ns, respectively. This result confirms that the transfer rate of captured primary excitation energy to Eu2+ from ZnO was faster and more efficient than that from ZnO[SO4] to Eu2+. 3.6. Commission International de I′Eclairage (CIE) and correlated colour temperature (CCT) The CIE (x, y) colour coordinates of ZnO, ZnO[SO4] and a series of ZnO[SO4]:xEu2+ with different concentrations of Eu2+ ion were evalu- ated, and are presented in Table 3. The schematic CIE diagram is dis- played in Fig. 13. It shows that the emission colours were tuneable from blue to white to bluish-green in the visible region. ZnO and ZnO[SO4] produced white light with colour coordinates (0.33,0.36) and (0.32,0.36) respectively. In general, Eu2+ doping produced tuneable blue to bluish- green emission associated with the 5d→4 f transition of Eu2+ions. This tunability was dependent on the Eu2+ concentration. In addition, the correlated colour temperature (CCT) of the synthesized materials was influenced by SO4 and Eu2+ doping, and day-white-light to blue-skylight were attained as the Eu concentration was varied. These results indicate that our materials can be useful in colour display applications. 4. Conclusions We successfully prepared ZnO[SO4]:Eu2+ nanophosphors via high temperature solid state reaction method. We investigated the optical and photoluminescence properties of ZnO[SO4]:Eu2+ nanophosphors. Upon excitation at 351 nm, ZnO and ZnO[SO4] nanophosphors exhibited emission bands in the UV and visible regions. These broad emission bands were associated with excitonic recombination and native defects in ZnO. However the ZnO[SO4]:Eu2+ nanophosphors exhibited one more additional blue emission peak at 495 nm, which was ascribed to the 5d-4f transitions of Eu2+ ions. The luminescent decay curves sug- gested that energy transfer took place from the host lattices to Eu2+ dopants. This was also confirmed through dipole–dipole interaction theoretical calculations. The calculated colour coordinates indicated that the Eu2+ emission lay in the blue colour region of the CIE diagram. From all the results, we concluded that the prepared ZnO[SO4]:Eu2+ phosphors was showing colour tuneable behaviour which can play a crucial role in white LED and NUV LED applications. Ethical Procedure The research meets all applicable standards with regard to the ethics of experimentation and research integrity, and the following is being certified/declared true. As an expert scientist and along with co-authors of concerned field, the paper has been submitted with full responsibility, following due ethical procedure, and there is no duplicate publication, fraud, plagiarism, or concerns about animal or human experimentation. Conflict of interest None of the authors of this paper has a financial or personal rela- tionship with other people or organizations that could inappropriately influence or bias the content of the paper. It is to specifically state that “No Competing interests are at stake and there is No Conflict of Interest” Fig. 12. The decay curves for ZnO[SO4]:0.5Eu2+ and ZnO [SO4]:1.0Eu2+ phosphors. Table 3 Undoped ZnO, ZnO[SO4] and ZnO[SO4]:xEu2+ and CCT values and the corre- lating colours are coordinated by CIE1931 chromaticity. Sample CIE 1931 Coordinates x y CCT (K) Color ZnO 0.33 0.36 5596 White ZnO[SO4] 0.32 0.36 6007 White ZnO:1.0:Eu2+ 0.26 0.35 9235 Bluish-Green ZnO[SO4]:0.5Eu2+ 0.24 0.34 10985 Bluish-Green ZnO[SO4]:1.0Eu2+ 0.24 0.35 10568 Bluish-Green ZnO[SO4]:1.5Eu2+ 0.24 0.30 13610 Greenish-blue ZnO[SO4]:2.0Eu2+ 0.23 0.30 14829 Greenish-blue ZnO[SO4]:2.5Eu2+ 0.21 0.21 18761 Dark-Blue ZnO[SO4]:3.0Eu2+ 0.23 0.27 19890 Blue Fig. 13. 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