Optical and morphological investigation of Y₂O₃:Tb³⁺ (1 mol%) and Bi³⁺ co-doped Y₂O₃:Tb³⁺ (1 mol%) nanoparticles

Overview, in recent years, rare-earth doped nanophosphors have attracted a lot of attention because of their exceptional optical qualities and possible uses in sensors, solid-state lighting, and display technologies. Because of its great thermal stability, wide bandgap, low phonon energy, and exceptional chemical durability, yttrium oxide (Y₂O₃) is regarded as an outstanding host material among several host lattices [1]. In addition, yttrium oxide, a type of rare earth oxide, is crucial for a range of photonic applications. As a host material, yttrium oxide is widely used in fluorescent lamps, lasers, plasma display panels, sensing devices, cathode ray tubes, various detection devices, and field emission displays. Y³⁺ shares similar ionic radii and chemical characteristics with other rare earth ions, making yttrium oxide an effective host material for these ions. When doped with rare earth ions, yttrium oxides exhibit impressive luminescent properties, including prolonged luminescent decay times and narrow emission lines. These attributes make yttrium oxide particularly appealing for use in optical communication, display technologies, and solar cells. We improved the rare earth emissions in doped yttrium oxide through co-doping with some sensitizers along with rare earth ions, which is of significant interest due to the potential for non-radiative energy transfer [2-4].The addition of Bi³⁺ enhances the photoluminescence intensity of Tb³⁺, even in lower amounts.

Furthermore, Bi³⁺ doping can slightly alter the structural parameters of Y₂O₃ nano-particles as Bi³⁺ has a slightly larger ionic radius (Bi³⁺: 1.03 Å compared to Y³⁺: 0.90 Å), its substitution leads to local lattice distortions, which may create conditions that promote luminescence and affect defect-related non-radiative centers [5, 6]. This capability is useful for creating tailored nanophosphors suitable for a range of optoelectronic and bioimaging applications [7].  Additionally, Y₂O₃:Tb³⁺ phosphors are characterized by their excellent thermal and chemical stability, extended lifespans, and high quantum efficiency qualities that are critical for use in advanced optoelectronic applications like field emission displays and white LEDs [8]. Recent progress in nanotechnology has facilitated the creation of Y₂O₃:Tb³⁺ nanophosphors, which enhance performance in biomedical imaging owing to their small size and compatibility with biological systems [9].

Doping Y₂O₃ nanoparticles with Bi³⁺ can affect their morphology, grain size, and surface defect states, typically resulting in improved crystallinity and decreased non-radiative losses [10]. Terbium (Tb³⁺) is the main activator ion in Y₃O₃:Tb³⁺, Bi³⁺ nanophosphors, a type of luminous materials that generate bright green light because of the ⁵D₄ → ⁷F_J transitions, especially at ~545 nm. By absorbing excitation energy (often in the near-UV range) and effectively transferring it to Tb³⁺ ions, the use of bismuth (Bi³⁺) as a sensitiser improves the photoluminescence efficiency and increases the intensity of green emission[11,12].  By increasing surface area, reducing light scattering, and producing quantum confinement effects, doping both Tb³⁺ and Bi³⁺ into the Y₂O₃ matrix at the nanoscale can increase luminous characteristics, making these nanophosphors appropriate for high-resolution displays and cutting-edge optical devices [13].  Furthermore, the crystal field environment can be influenced by Bi³⁺ co-doping, which may adjust emission properties [14].

This research involved the synthesis of a Y₂O₃:Tb³⁺ and   Bi³⁺   co-doped Y₂O₃:Tb³⁺   nano-phosphors using combustion methods. The phosphor was analyzed through techniques including X-ray diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), Scanning electron microscopy (SEM) with Energy Dispersive X-ray spectroscopy (EDX), and Photo-luminescence (PL).

Synthesizes of Y₂O₃: Tb³⁺ nano-phosphors

Y₂O₃ nano phosphors doped with 1 mol % Tb³⁺ were produced using a solution combustion method, which is a wet-chemical process. This approach requires both a fuel and an oxidizer. In the combustion process, metal nitrates serve as the oxidizing agent, while urea acts as the reducing agent. The proportions of yttrium nitrate, terbium nitrate, and urea are determined by the overall oxidizing and reducing valencies of the oxidizer and fuel [15-16]. Yttrium oxide nanophosphors doped with terbium were created by combining yttrium nitrate [Y(NO₃)₃·XH2O], terbium nitrate [Tb(NO₃)₃·6H₂O], and urea [CO(NH₂)₂] in double-distilled water. The chemicals were blended according to their stoichiometric ratios to produce a uniform solution. This solution was then placed in a silica crucible and heated in a preheated muffle furnace at a consistent temperature of 600°C for 15 minutes. Following the combustion process, the resulting sample was foamy, which was then crushed into a fine powder and further calcined at 1000°C. This powder was subsequently collected for analysis and characterization.

Synthesizes of Bi³⁺ co-doped Y₂O₃:Tb³⁺   nano-phosphors

By using the solution combustion approach, Tb³⁺-doped (1 mol %) and Bi³⁺codoped (1 mol %) Y₂O₃ nanophosphor were created.  By combining yttrium nitrate [Y(NO₃)₃.XH₂O], terbium nitrate [Tb(NO₃)₃.6H₂O], bismuth nitrate pentahydrate, and urea [CO(NH₂)₂] in double-distilled water, terbium doped, Bi co-doped Y₂O₃ nanophosphor was created. A homogenous solution was created by mixing all the components in a stoichiometric ratio. The homogeneous solution in the silica crucible was held at 600°C for 15° minutes in a muffle furnace that had been warmed.  The sample was foamy when the combustion process was finished, the resulting frothy sample was easily pulverised into a fine precursor powder using an agate mortar. In order to attain better crystallinity, it was also annealed for two hours at 1000 °C.  The sample was also safely kept in a desiccator for different characterisations.

Materials Characterization

The Multipurpose Versatile XRD System (Smart Lab 3kW, Rigaku) was utilized for powder XRD and thin film XRD analysis of Bi³⁺ ions co-doped with Y₂O₃:Eu³⁺ phosphor samples, covering a 2θ range from 10⁰ to 70⁰, to assess the structure and phase purity. The surface morphology of the Y₂O₃:Tb³⁺ and Bi³⁺ activated phosphor samples was examined with a High-Resolution Field Emission Scanning Electron Microscope with EDS (FE-SEM) (JSM-7610F Plus, JEOL). Luminescence decay profiles for the   Y₂O₃:Tb³⁺, Bi³⁺ samples were recorded using a Quanta Master 8450-22 Spectrofluorometer (Horiba), capable of measuring fluorescence across the UV-VIS-NIR range (~200 nm to 3000 nm). X-ray photoelectron spectroscopy (XPS, AXIS Supra) was employed to analyze the chemical bonding configurations.

Structural Analysis 

XRD

Figure 1 presents the X-ray diffraction patterns of Y₂O₃:Tb³⁺ (1mol %), and Y₂O₃:Tb³⁺ (1mol %), Bi³⁺ (1 mol %) nanophosphors, which were calcined at 1000 °C. The results show that, when compared to the standard JCPDS data, there are no observable impurity peaks in the XRD patterns. This indicates that the synthesized samples are single-phase and that the trivalent terbium and bismuth ions have successfully incorporated into the Y₂O₃ host without changing the crystal structure.

Figure 1: Displays the powder X-ray diffractograms for the pure host and the Y₃O₃:Tb³⁺, Bi³⁺   samples, as well as the standard JCPDS data for the Y₂O₃ host.

Assessment of structural parameters

XRD data was used to determine the structural characteristics, including   crystallite size, dislocation density, and   microstrain. As stated below, the Debye-Scherrer’s formula (1) [17] and the Hall-Williamsons equation (2) [18] were used to calculate the crystallite size (D).

 (Debye – Scherrer Formula)         (1)

  (Hall – Williamsons equation)       (2)

Here, the microstrain in the samples is denoted by ε, and the full-width at half-maximum (FWHM) is represented by β. The following relations (3), (4) [19] were used to compute the dislocation density (δ), and microstrain (ε).

          (3)

          (4)

 The structural characteristics of the Y₂O₃:Tb³⁺ (1mol %) and Y₂O₃:Tb³⁺ (1mol %), Bi³⁺ (1mol %) nanophosphors are compiled in Table 1.

Scanning electron microscopy (SEM) and Energy Dispersive X-ray spectroscopy (EDS)

Morphological studies

The surface morphology and crystallite sizes of the synthesised phosphor powder were further examined using a scanning electron microscope. Figure 2a–b displays SEM micrographs of Y₂O₃:Tb³⁺, Bi³⁺   phosphor materials along with   morphological pictures.  It is quite evident from the SEM micro-graph that the sizes of the nanoparticles ranges from 80 to 100 mm and are spherical in shape. According to SEM pictures, all of the created compositions have almost identical crystallization which confirms the homogeneity and formation of single phase.

Figure 2 (a–b).  Shown SEM morphology of   Y₂O₃:Tb³⁺, Bi³⁺ phosphor at various magnifications.

Elemental analysis

Moreover, High-Resolution Field Emission Scanning Electron Microscope with EDS (FE-SEM) (JSM-7610F Plus, JEOL)   was used to evaluate the elements and its composition of the bismuth doped prepared phosphors. Figure 3 displays FE-SEM-EDS images of sample with   1.0 weight% of Tb and 5 wt % Bi co-doped Y₂O₃. The spectra distinctly show that the samples contained only terbium (Tb), bismuth (Bi), oxygen (O), and yttrium (Y), with no other impurities detected. These peaks show that the synthesized phosphors contain composite components.  The inset of Figure 3 lists the spectra of the region of the characterized phosphor that is being considered as well as the atomic percentage of the original reactant components. 

The homogeneous distribution of elements across the whole particle range is further demonstrated by the elemental mapping displayed in Fig. 4(a–e), with yttrium and oxygen elements exhibiting comparatively higher concentration content.

Figure 3. Energy dispersive spectroscopy and field emission scanning electron microscopy images of Tb and Bi doped   Y₂O₃ nanophosphors.

Figure 4.  Y₂O₃:Tb³⁺, Bi³⁺   nanophosphor calcined at 1000 ^◦C   (a) Element overlay, (b) Oxygen, (c) Terbium, (d) Yttrium, and (e) Bismuth elements.

Photoluminescnce (PL) Analysis:

The PL excitation and emission spectra were recorded for both Y₂O₃ doped with Tb³⁺ (1 mol %) shown in Fig. 5(a-b) and with co-doped Bi³⁺ in Fig. 6(a-b). Firstly, in Fig 5 (a), the excitation spectrum recorded at 545 nm emission, which shows a broad band ranging from 300 to 338 nm that corresponds to the intra-configurational transition from the ground state ⁷F₆ of the 4f⁸ configuration to  the 4f⁷5d¹ configuration of Tb³⁺ and the highest peak is observed at 307 nm associated with O^2- - Tb³⁺ charge transfer centers.

Whereas, in Fig 5 (b) the excitation  maxima occurring at 307 nm which is attributed to the ¹S₀ → ³P₁ transition of Bi³⁺ ion and a shoulder around 290 nm is correspondent to the 4f⁸ →4f⁷5d¹ transition of Tb³⁺ ion. The excitation efficiency is enhanced throughout the spectral range from 300 to 400 nm as compared with the excitation spectrum of Y₂O₃:Tb³⁺. In Fig 5(b), the emission spectra of Y₂O₃:Tb³⁺ observed at 307 nm and according to that the transitions takes place from ⁵D_J levels to ground state of Tb³⁺ ions, giving rise to sharp line emissions. The emission below 480 nm originates from ⁵D₃ → ⁷F_J transition and above 480 nm originates from ⁵D₄ → ⁷F_J transition of Tb³⁺ ions. It exhibit broad emission lines located at 484 nm, 490 nm, 544 nm, 551 nm. All these emissions are originates from ⁵D₄ → ⁷F_j transitions and the emission corresponds to ⁵D₃ → ⁷F_J was not observed in this case. The peaks at 484-490 nm are attributed to  ⁵D₄ → ⁷F₆  transition which is forced electric dipole allowed transition whereas the emission peaks at 544-551 nm are attributed to ⁵D₄ → ⁷F₅  transition of Tb³⁺ ions which is magnetic dipole allowed [20-22]. The dominant and most intense emission occurs at 544 nm responsible for the green light. In Fig 6 (b), Under the 307 nm,  it exhibit broad emission lines located between 484-551 nm. But here also some small peaks appeared at 583 and 596 corresponding to ⁵D₄ → ⁷F₄ and at 613 and 628 nm corresponding to ⁵D₄ → ⁷F₃. Overall, the the new peak appeared in Fig. 6 (a) indicates that the energy is transferred from ³P₁ level of Bi³⁺ ion to the ⁵D₄  level of Tb³⁺ ion.  In this case, Bi³⁺ ions absorb radiation and, subsequently energy is transferred from Bi³⁺ ions to Tb³⁺ ions and emits light via ⁵D₄ → ⁷F_j transitions [23]. The energy transfer mechanism, where Bi³⁺ ion acts as a ‘sensitizer’ for Tb³⁺ emission is presented in Fig 7. (Schematic diagram).

Figure 5 (a) The excitation spectrum of   Y₂O₃:Tb³⁺ nanoparticles (NPs) measured at an emission wavelength of 545 nm. (b) The emission spectra measured with an excitation wavelength of 307 nm.

Figure 6 (a) The excitation spectrum for Y₂O₃:Tb³⁺, Bi³⁺    nanoparticles (NPs) recorded at an emission wavelength of 545 nm. (b) The emission spectra obtained using an excitation wavelength of 309 nm.

Figure 7: Schematic Energy transfer mechanism in Bi³⁺ ions for Y₂O₃:Tb³⁺ nanophosphor.

Calculation of CIE parameters       

The analysis of CIE coordinates, and correlated color temperature (CCT) was conducted for Y₂O₃:Tb³⁺   and   Bi³⁺    co-doped Y₂O₃:Tb³⁺ phosphors. Fig. 8 shows the evaluation of the CIE coordinates and the resulting color.  The nature of colour emission was examined by computing the Correlated Color Temperatures (CCT).  CCT was calculated using the following McCamy empirical formula (5).

         (5)

Where x_e = 0.3320 and y_e = 0.1858.6 and n is the ratio of (x−x_e) and (y−y_e) [24, 25]. Figure 9.   Presents the CCT graphs for Y₂O₃:Tb³⁺ (1mol %)   and   Bi³⁺   (1mol %) co-doped Y₂O₃:Tb³⁺ (1mol %)   , with the CCT values for the phosphor. The CCT values range from 6689 to 6875K, As a result, the CCT values for all the prepared samples indicate a warm light emission.  Table 2 lists the calculated CIE coordinates and CCT values.

Figure 8. (a) Presents the CIE coordinates for Y₂O₃:Tb³⁺ and   Y₂O₃:Tb³⁺, Bi³⁺.

Figure 9. Diagram of u' and v' for Y₂O₃:Tb³⁺and Y₂O₃:Tb³⁺, Bi³⁺.

Table 2. CIE, u' and v' and CCT data for Y₂O₃:Tb³⁺and Y₂O₃:Tb³⁺, Bi³⁺.

XPS analysis

The chemical composition of the Y₂O₃:Tb³⁺, Bi³⁺ nanocomposite is determined using X-ray photoelectron spectroscopy (XPS).  The XPS survey produced from Y₂O₃ is shown in the Figure 10. The Tb³⁺, Bi³⁺ nanocomposite forms the O(1s), Bi(4f), Y(3d), C(1s), and Tb(4d) peaks.  The high resolution peaks of the O(1s) spectra at 525.1 eV and 527.8 eV, which correlate to the regular Y-O bond of the Y₂O₃ nanoparticles [26]. The high resolution peak of the C(1s) spectra at 281 eV and 296 eV.  The peak at 308 eV represents the Y (3p) spectra. The sharp peak observed in the high-resolution Y (3d) spectra occurs at 160.9 eV, while the peak at 165.5 eV is attributed to the Bi (4f) of the sample. Additionally, the peaks at 153.02 eV and 155.08 eV represent a doublet for Tb (4d), resulting from spin-orbit coupling [27-28].

Figure 10. High-resolution XPS spectra for the (a) survey,   (b) Bi4f, Y3d, Tb4d,   (c) O 1s   and (d)   Y 3p, C1s   peaks.

In conclusion, Y₂O₃:Tb³⁺    and   Bi³⁺   co-doped Y₂O₃:Tb³⁺   nano-phosphors emitting green light was synthesized using a urea-assisted solution-combustion technique. The produced phosphor’s phase purity was validated by XRD. Successful nanophosphor production was indicated by the consistently dispersed, primarily spherical crystallizes between 80 and 120 nm that were found by SEM analysis. Co-doping did not significantly impact the morphology, indicating that the inclusion of Bi³⁺ and Tb³⁺ does not change the microstructure. The elemental composition was further confirmed by EDS analysis, which supported efficient dopant integration without phase separation and confirmed the existence of Yttrium, Oxygen, Terbium, and Bismuth. Green emission is seen in the PL excitation spectra of Y₂O₃:Tb³⁺ from ⁵D₄ → ⁷F_J transitions. The ¹S₀ → ³P₁ transition of Bi³⁺ causes extra greem emission in Bi³⁺ co-doped Y₂O₃: Tb³⁺. By effectively transmitting energy to Tb³⁺, Bi³⁺ acts as a sensitizer, increasing excitation efficiency in the 300–400 nm region. There is a great chance that the excitation spectra of Y₂O₃: Bi³⁺ and Y₂O₃:(Tb³⁺, Bi³⁺) will be used in UV-LED-pumped white solid-state illumination. Warm-white light emission is indicated by the matching correlated color temperatures of 6689 K and 6875 K. These findings imply that Bi³⁺ co-doped Y₂O₃:Tb³⁺ phosphors are good options for warm -white components needed in near-UV-excited white LEDs.