Color‐Tunable Eu3+, Eu2+‐Activated CaSiO3 Nano Phosphor Extract from Agricultural‐Recycling‐Food‐Waste Materials for Display Applications

A series of Eu3+ and Eu2+ doped wollastonite is produced with a modified sol–gel technique using agricultural‐food waste materials. Rice husk ash (RHA) and eggshell (ES) are used as an usher for silica and calcium oxide, appropriately, at different concentrations (0.0, 0.2, 0.4, 0.6, 0.8, and 1 mol%). Most investigations have documented photoluminescence (PL) from Eu3+ ions caused by electronic transitions between 4f levels (5D0 → 7FJ) but there is limited information on emissions from Eu2+ ions. Fourier transform infrared (FTIR) spectra and X‐ray diffraction (XRD) studies reveal that varying amounts of europium ions do not have any effect on the structure of the host. PL spectra display that europium ions exist in both trivalent and divalent forms. A valence change from Eu3+ to Eu2+ ion is investigated using luminescence measurements. As‐prepared Eu3+ activated β‐wollastonite emitted as red and reduction atmosphere using argon gas produced Eu2+ activated phosphor gives off blue light. X‐Ray photoelectron spectroscopy reveals that throughout the processing of the samples, Eu3+ cations are partially reduced to Eu2+ cations in argon gas atmosphere. These findings prove the feasibility of fabricating white light‐emitting diodes (white‐LEDs) for three‐band type (RGB) phosphors utilizing only one host crystal.

DOI: 10.1002/adpr.202200266 A series of Eu 3þ and Eu 2þ doped wollastonite is produced with a modified sol-gel technique using agricultural-food waste materials. Rice husk ash (RHA) and eggshell (ES) are used as an usher for silica and calcium oxide, appropriately, at different concentrations (0.0, 0.2, 0.4, 0.6, 0.8, and 1 mol%). Most investigations have documented photoluminescence (PL) from Eu 3þ ions caused by electronic transitions between 4f levels ( 5 D 0 ! 7 F J ) but there is limited information on emissions from Eu 2þ ions. Fourier transform infrared (FTIR) spectra and X-ray diffraction (XRD) studies reveal that varying amounts of europium ions do not have any effect on the structure of the host. PL spectra display that europium ions exist in both trivalent and divalent forms. A valence change from Eu 3þ to Eu 2þ ion is investigated using luminescence measurements. As-prepared Eu 3þ activated β-wollastonite emitted as red and reduction atmosphere using argon gas produced Eu 2þ activated phosphor gives off blue light. X-Ray photoelectron spectroscopy reveals that throughout the processing of the samples, Eu 3þ cations are partially reduced to Eu 2þ cations in argon gas atmosphere. These findings prove the feasibility of fabricating white light-emitting diodes (white-LEDs) for three-band type (RGB) phosphors utilizing only one host crystal. light transparency, low thermal expansion, and the best matrix for luminous materials. Recent reports on Eu 3þ /Eu 2þ reduction process phosphor discuss ZnWO 4 :Eu 3þ /Eu 2þ phosphor synthesized by hydrothermal method using highly expensive chemicals in the fabrication process. [15] NaCaPO 4 :Eu phosphor was fabricated with chemicals using a solid-state reaction (SSR) method at high temperatures. [16] Eu 3þ /Eu 2þ -doped calcium boroaluminate glasses were made with a high-temperature melt quench method. [17] Sr 2 MgSi 2 O 7 :Eu 3þ phosphor was fabricated by co-precipitation method using (NO 3 ) 2 ,Mg(NO 3 ) 2 ·6H 2 O, TEOS, and Eu 2 O 3 as precursors at high temperatures. [18] β-calcium orthosilicate activated with europium was prepared with the help of SSR method at high temperatures (1250°C). [19] According to the literature, luminescent materials are made with high-priced chemicals as precursors using very high-temperature synthesis techniques, and limited production. Despite the fact that many investigators use agricultural waste as starting material, they prepare phosphor using very high-temperature processes. The SSR approach is appealing from an economic standpoint, but it necessitates extremely high temperatures. This fabrication route causes exaggerated grain development, resulting in nonhomogeneous final output stage products. Wet chemical synthesis approaches like sol-gel, hydrothermal, precipitation, and microemulsion fabrication processes utilize low processing temperature, uniformity, and reduced particle size. [20,21] The sol-gel approach is a manageable, favorable chemical pathway that allows for better control of particle size and phase uniformity, as well as faster manufacture of extremely pure products at low sintering temperatures. [22,23] Based on our understanding, we are the first to report an easy, economical, modified sol-gel procedure to synthesis and an environmentally sociable phosphor generated through agro-food-waste materials.
The present research work is primarily concerned with the transformation of agro-food-waste materials into affordable nano phosphor materials and also minimizing environmental pollution. In this article, we describe the preparation of blue and red emission phosphors by the reduction of Eu 3þ ion to Eu 2þ in a single host material. The results of this study indicate that it might be a potential choice for an environmentally friendly white-LED phosphor generated from agricultural food waste.

Materials
Rice husk ash (RHA) was collected from a local rice mill and eggshell (ES) was obtained from the college canteen. Sodium hydroxide pellets (NaOH), hydrochloric acid (HCl), and europium (III) nitrate hexahydrate purchased from Sigma-Aldrich were the raw materials used in this experiment.

Methods
The synthesis process and extraction of SiO 2 through rice husk and CaO from ESs was explained in the previous work. [24] Briefly, a stoichiometric amount of CaO obtained from ES was dissolved in 2 mole hydrochloric acid and stirred for 1 h to obtain a clear calcium chloride solution (sol A). Sodium silicate solution was obtained by boiling the rice husk ash (RHA) with 2 mole NaOH solution (sol B). The obtained precursors are calcium chloride solution (sol A) and sodium silicate solution (sol B). An appropriate amount of europium nitrate hexahydrate was added to sol A. After 1 h of stirring, dropwise of sol B was added with continuous stirring. A white gelation is formed and was kept at ambient temperature for 3 days in a sealed container. After 3 days, the seal was removed and washed 3 times with DI water and filtered every time. The filtered samples were dried at 80°C for one day and then 130°C for another 7 h. The obtained samples were fine grinded in mortar and calcinated at 600°C for 2 h. Finally, as-prepared samples were sintered at 800°C for further analysis. The diagram of heat treatment of Eu 3þ /Eu 2þ -activated CaSiO 3 with a modified sol-gel technique is shown in Figure 1. For heat treatment for Eu 3þ , the sample was sintered at 800°C for 7 h and the holding time was 2 h. For Eu 2þ , the prepared sample was sintered at 850°C for 2.20 h in an inert gas atmosphere and the holding time was 3 h. In both processes, the temperature automatically came to room temperature.

Characterization Techniques
Powder X-ray diffraction patterns (XRD) were recorded in the 2θ range of 10°-60°with the help of PANALYTICAL XPERT POWDER, UK. Absorption spectra were measured by an Analytic Jana 210 plus model (UV-vis) spectrometer. FTIR (APHA-IIBRUKER) analysis was recorded in the span of 400-4000 cm À1 with the KBr pellet technique used to assess functional groups in the sample. Horiba Jobin Yvon Fluorolog-3 (FL3-21) was used to obtain all luminescence properties at room temperature (RT). A 450 W xenon lamp was connected to a double grating excitation monochromator and a single grating emission monochromator that operate in the 200-800 nm wavelength range. Zeiss Gemini FE-SEM from  were used to examine the morphology and existence of elements and particle size of the sample. X-ray photoelectron spectra (XPS) studies were carried out on Physical electronics, using PHI 5000 VersaProbe III model. Time-resolved photoluminescence (TRPL) or lifetime fluorescence spectroscopy equipped with HORIBA investigated the change in fluorescence over time of a sample when irradiated with UV, visible, or near-IR light. This decay in fluorescence can be measured over a wide time range: from picoseconds to milliseconds and beyond. Decay curves were measured with a 447 nm pulsed laser source. The spectrometer introduced ultraviolet or visible light using a photon source, like a laser, a xenon lamp, or LEDs. The light passed through a monochromator that selects a specific wavelength, often using a diffraction grating. A diffraction grating is a plate of glass or metal ruled with very close parallel lines, producing a spectrum by diffraction and interference of light. The light that exits comes out at a specific angle depending on its wavelength. The spectrometer focused the monochromatic wavelength toward the sample. The sample emitted a wavelength, which travels to the detector. The detector was usually set at a 90°angle to the light source to avoid any interference from the transmitted excitation light. Photons emitted hit a photodetector. Computer software connected to the detector generates a decay spectrum, a graphical representation that shows lifetime of the sample.

Results and Discussion
3.1. PXRD Studies Figure 2 depicts the XRD patterns obtained at each phase of the synthesis process of Eu 3þ -doped (0.0, 0.2, 0.4, 0.6, 0.8, and 1.0 mol%) β-wollastonite. The peaks specified with diamond character (♦) are β-wollastonite peaks and correspond to COD database code # 9 011 452. [25] The dopant Eu 3þ peaks are indicated with a dot character (•) and correspond to ICSD code #53 437. From the diffraction peaks, the monoclinic structure of Eu 3þ -doped β-wollastonite was confirmed and matched with data in the literature. [26,27] The temperature at which the β-wollastonite phase forms is lower than the SSR approach, which can be attributed to the synthesis process. The results further show that the dopants (Eu ions) had no significant effect on the crystalline structure of the samples, and no other impurity phase was found in the entire pattern. This is due to the 8-coordinated Eu 3þ (1.066 Å) and 7-coordinated Eu 3þ (1.01 Å) ionic radius nearly identical to the 8-coordinated Ca 2þ (1.12 Å) and 7-coordinated Ca 2þ (1.06 Å), respectively. [28] 3.2. FTIR Studies Figure 3 shows the FTIR spectrum of Eu 3þ -activated and undoped (0.0-1.0 mole%) CaSiO 3 . The Si-O-Si bending vibrational mode peaks were observed at 456 and 568 cm À1 . [29] The symmetric stretching vibration of Si-O-Si was linked to the peaks at 646 and 684 cm À1 . [30] At 800 cm À1 , a Si-O-Si symmetric stretching vibration band was detected. The nonbridging siliconoxygen bond of Si-O-NBO can be linked to the vibrational bands detected at 903 and 943 cm À1 . The IR absorption peaks were noticed at 1021 and 1068 cm À1 , which matched the asymmetric

Morphological Studies
Figure 4a-e shows the field emission scanning electron microscopy (FESEM) images and EDAX spectrum of CaSiO 3 :Eu 3þ / Eu 2þ nano phosphor. The surface morphology of undoped CaSiO 3 shows many agglomerates of nonuniform sizes and shapes. The dopant concentration and reduction atmosphere process affect the morphology of the sample. From Figure 4b-e, the prepared powder's morphology reveals that the particles are overlapping and aggregated. The sizes of the particles were irregular with rod-like shapes. The particles also had crystal and rod-like morphologies, as seen in Figure 4b-e. The rods were shaped and sized differently. There were both small and large rods are present. These results reveal that this morphology consists of nano-sized rods. EDS elemental analysis revealed the presence of constituent elements in phosphor in quantitative form. There were other peaks that corresponded to the grid. [15,33,34] Transmission electron microscopy (TEM) was used to examine the CaSiO 3 :1.0 Eu 3þ sample in detail, as shown in Figure 5. Agglomeration of irregular and rod-like nanoparticles were visible in the morphology. Agglomerates arise primarily as a result of the high surface energy of the nanoparticles. TEM images revealed that the CaSiO 3 :1.0Eu 3þ sample had an average particle size of 86.5 nm. The high crystalline nature was confirmed by selected area electron diffraction (SAED). Materials in the nano range identified during the TEM investigation have a wide range of applications in optoelectronic devices. [35,36] 3.4. Absorbance Spectrum of Undoped and Eu 3þ -Doped CaSiO 3 Nano Phosphor Figure 6 depicts the UV-vis absorbance spectra of undoped and Eu 3þ -doped CaSiO 3 nano phosphor while the inset shows direct bandgap measurement of all fabricated samples with their corresponding energy bandgaps. All the spectra were recorded at room temperature in the range of 200-1000 nm.    A concentration-dependent reduction in the bandgap was identified after Eu addition. The computed energy bandgap of the host material indicates there was sufficient bandgap to accommodate the luminescent center within the host material. [37,38] The bandgap energies reduced from 4.043 to 3.928 eV. Because of the slight change in the activator concentration and lower particle size, the optical bandgap shifted. Figure 7 shows the photoluminescence excitation (PLE) spectra of Eu 3þ -doped CaSiO 3 (1.0 mol%) nano phosphor recorded at 613 nm. This spectrum was taken in the range of 320-550 nm and comprised sharpened peaks due to 4f-4f transitions of Eu 3þ . From the near UV region to the visible region, the bands of excitation spectra were high, particularly the 7 F 0 ! 5 L 6 transitions at 392 nm. [19] Other characteristic bands recorded at 360, 380, 392, 402 and 463 nm were transitions due to 7 F 0 ! 5 L 7 , 7 F 0 ! 5 D 4 , 7 F 0 ! 5 L 6 , 7 F 0 ! 5 D 3, and 7 F 0 ! 5 D 2 respectively. The strong characteristics features of the absorption band at 320-550 nm show that this nano phosphor can be successfully activated by a UV LED chip, and has the potential to be used in white-LEDs. [39] To obtain PL spectra, the fabricated samples were excited at 392 nm and are shown in Figure 8. The PL spectra comprised a series of narrow bands recorded at 581-604, 604-640, 640-665, and 680-715 nm due to transitions of 5 D 0 ! 7 F J (J = 1,2,3 and 4) individually. [40,41] The present work shows that the most strong band at 613 nm ( 5 D 0 ! 7 F 2 ) matched with the literature reports. [42,43] Out of all emission peaks, the peak at 613 nm due to the electric dipole transition of 5 D 0 ! 7 F 2 was the strongest, and it is well known that this transition is a hypersensitive transition and is strongly influenced by the surrounding environment. The chromaticity color coordinates (CIE) diagram of Eu 3þdoped CaSiO 3 phosphors were calculated according to the Commission International de l 0 Eclairage (CIE 1931), and presented in Figure 9. Table 1 shows the coordinates determined for the current phosphors. The red CIE color coordinates were derived from high-intensity bands of the emission spectra under 392 nm excitation. In the present work, phosphor color coordinates closely match the standard NTSC red light value (x = 0.67, y = 0.33). While increasing the dopant concentration into Ca 2þ sites, color coordinates shift toward the red region.    The present phosphor emits red color under ultraviolet light excitation, which shows it is good for optical properties. The Mc Camy empirical formula Equation (1) was used to evaluate the color correlated temperature (CCT) values. The CCT values were calculated from the color coordinates (x, y) of the CIE diagram.

PL Spectra of Eu 3þ -Doped CaSiO 3 Nano Phosphor
CCT ¼ À 449n 3 þ 352n 2 À 6823n þ 5520. 33 (1) Here n = (xÀx e )/(yÀy e ), x e = 0.332, y e = 0.186. Table 1 shows the CCT values of Eu 3þ -doped CaSiO3 phosphors for varied Eu 3þ ion concentrations. Lamps with CCT values of <3200 K are mostly used as warm light sources, and those with CCT values of >4000 K are typically known as cold light sources. [44] The CCT values in the present study were between 1716 and 1902 K for various Eu 3þ ion concentrations and were <3200 K. So, warm light sources for solid-state lighting (SSL) approaches can benefit from these phosphor properties.
TRPL, a potent and nondestructive technology, was used to validate the optical characteristics of prepared samples. Under 392 nm excitation, the luminescence decays for 5 D 0 ! 7 F 2 (613 nm) emission of CaSiO 3 :Eu 3þ phosphors are appeared in Figure 10, as a basis of Eu 3þ ion concentration. The exponential decay curve Equation (2) was used to analyze the optical behavior of the prepared samples.
Here, I(t) is the phosphorescence intensity at time t, I is the initial intensity, A 1 are constants, and τ is the decay time for transition. [12] TRPL of all the samples is tabulated in Table 1. From the table, it was noticed that the decay period of all the activated samples is more than 1 ms, implying that Eu 3þ -activated CaSiO 3 can be employed for bioluminescence probes. [45] For all concentrations of CaSiO 3 :Eu 3þ phosphors, the lifetimes are shown to vary marginally. The slight differences in decay time for different Eu ion concentrations can be attributable to the manufactured phosphors having varied particle sizes. This strongly shows that Eu 3þ ions' local environments may differ at different points in the lattice. This means that radiation-less energy transfer between Eu 3þ ions is extremely unlikely and unaffected by Eu 3þ ion concentration. [46,47] 3.6. PL Spectra of Eu 2þ -Doped CaSiO 3 Nano Phosphor Figure 11 and 12 show the PLE and PL spectra of Eu 2þ -CaSiO 3 prepared in a reduction atmosphere at different excitation wavelengths. The precursors for europium were trivalent forms. It is possible to convert divalent form using the reduction atmosphere process and this can be obeyed by the PL spectra of samples. When the sample was heated in Ar-gas reduction temperature at 850°C for 3 h, Eu 3þ ions were reduced to Eu 2þ ions. The excitation spectra consist of broadband in the span of 330-380 nm with more intensity peaks at 345 and 365 nm as well as have some shoulder peaks. From the spectra, it was concluded that the spectral band features related to Eu 2þ transitions, not Eu 3þ transitions. In contrast, excitation spectra of Eu 3þ ions show keen peaks from 300 to 550 nm, which correspond to f-f transitions. [48] The PL spectra of Eu 2þ consist of broadband in the span of 400-500 nm, with a maximum intensity peak at 447 nm due to 4f 6 5d 1 -4f 7 allowed transition. The emission spectra, in contrast, revealed a very faint emission of Eu 3þ . As an outcome, the majority of Eu 3þ ions have been converted to Eu 2þ ions throughout the reduction procedure, while very little quantity of Eu 3þ remains.

Possible Methods of the Reduction Mechanism
Eu 2þ ions may replace all cationic sites Ca 2þ and Si 4þ when included in the crystal structures of CaSiO 3 . It is strenuous for Eu 2þ ions to be restored by Si 4þ ions because of the different ionic radii and the allowable oxygen-coordination number. [28] As a result, Eu 2þ ions exclusively replace Ca 2þ ions in CaSiO 3 .
The easing of Eu 3þ to Eu 2þ in CaSiO 3 :Eu in reduction atmosphere can be explicated using the charge compensation   mechanism hypothesis. [49,50] When trivalent Eu 3þ ions are doped into CaSiO 3 , Ca 2þ ions are replaced. Two Eu 3þ ions must be substituted for three Ca 2þ ions to preserve charge balance. Each replacement of every two Eu 3þ ions in the compounds will result in one vacancy defect V // Ca with two negative charges and two positive defects of Eu Ca • . The vacancy of V // Ca thus works as an electron donor, while the two Eu Ca • defects act as electron acceptors. As a result of the thermal provoke, the negative charges in vacancy defects of V // Ca are shifted to Eu 3þ sites, reducing Eu 3þ to Eu 2þ . The following equations could be used to represent the entire process.
X-ray photoelectron spectroscopy was manipulated to perceive the elements and assess the valence of Eu to further establish the co-existence of Eu 2þ and Eu 3þ in CaSiO 3 :Eu. The full survey spectrum presented in Figure 13 confirms the presence of Ca, Si, and O, which is compatible with EDS characterization findings. The typical binding peaks of 3d 3/2 and 3d 5/2 chemical states be into Eu in different states of 2þ (1125, 1156.7 eV) and 3þ (1135.5, 1165.2 eV), respectively, in the high-resolution X-Ray photoelectron spectrum of Eu are shown in the inset of Figure 13. From the aforementioned information, the present sample series CaSiO 3 : Eu consists of more than one emission site of Eu 2þ ions and unreduced Eu 3þ ions in the sample. [51,52] Figure 14  The luminescence decay curves of CaSiO 3 :Eu 2þ are indicated in Figure 15. The lifetime with CIE coordinates is tabulated in Table 2. The experimental lifetime for all the samples in a microsecond agrees with data from other literature and confirms these characteristics for the Eu 2þ ions. [53,54]

Conclusions
In this work, a series of Eu 3þ /Eu 2þ -doped CaSiO 3 was presented and the influence of reduction atmosphere for white light color  www.advancedsciencenews.com www.adpr-journal.com tuned application was properly studied with structural and optical analysis. From the diffraction peaks, the monoclinic structure of Eu-doped β-wollastonite was confirmed. PL spectra displayed that europium ions exist in both trivalent and divalent forms. Valence changes from Eu 3þ to Eu 2þ ion were investigated using luminescence measurements. The CIE chromaticity diagram illustrates that an optimum white light can be obtained by combining the emission of Eu 2þ in blue and Eu 3þ in red-orange.
The findings prove the feasibility of fabricating white-LEDs for three-band type (RGB) phosphors utilizing only one host crystal.   www.advancedsciencenews.com www.adpr-journal.com