Combustion-Synthesized BaAl₂O₄: Eu²⁺, Nd³⁺, Pr³⁺ Triple-Co-Doped Long-Afterglow Phosphors: Luminescence and Anti-Counterfeiting Applications

Abstract

Solution combustion-synthesized BaAl₂O₄: Eu²⁺, Nd³⁺, and Pr³⁺ blue–green long-afterglow phosphors are prepared and systematically investigated. First, XRD confirms the BaAl₂O₄ host and screens for trace residual features. SEM reveals the agglomerated granular morphology typical of combustion products. XPS verifies the valence states (Eu²⁺, Nd³⁺, Pr³⁺) and the chemical environment of the host lattice. UV-Vis diffuse reflectance spectra, transformed via the Kubelka–Munk function and analyzed using Tauc plots (indirect-allowed), indicate a wide band gap of the BaAl₂O₄ host with small, systematic shifts upon Nd³⁺/Pr³⁺ co-doping. PL measurements show Eu²⁺ 4f–5d emission and co-dopant-assisted excitation/defect pathways without altering the Eu²⁺ emission band shape. Afterglow lifetime and decay analyses correlate trap depth/distribution with the extended persistence. Finally, we demonstrate anti-counterfeiting by (i) snowflake printing and (ii) a binary 3 × 3 grid printed with two afterglow inks of different lifetimes to realize multi-level authentication. The sequential evidence links structure, chemistry, optical absorption, carrier trapping, and practical readout, providing a coherent basis for performance enhancement and application.

1. Introduction

Persistent luminescent phosphors (PLPs) store carriers in metastable traps and release them thermally at room temperature, enabling delayed emission for safety signage, bioimaging, information storage, and anti-counterfeiting [1,2,3,4,5]. Among oxide hosts, SrAl₂O₄: Eu²⁺, Dy³⁺ is the green benchmark, achieving minute- to hour-scale persistence through trap engineering (rare-earth co-dopants, defect chemistry, atmosphere control) [6,7,8,9]. However, blue-emitting PLPs remain comparatively under-developed, despite advantages for multiplexed/covert security channels, reduced spectral overlap with ambient lighting, and straightforward integration into RGB printing workflows [10,11,12].

BaAl₂O₄ is a wide-band-gap aluminate that supports Eu²⁺ 5d–4f emission in the cyan/blue region (~490 nm) and allows trap tailoring via rare-earth co-dopants [13,14,15]. Prior studies have explored Eu²⁺ activation and co-dopants (e.g., Nd³⁺, Pr³⁺), yet gaps persist: (i) limited low-temperature combustion routes that yield fine powders directly compatible with screen-printing inks; and (ii) a shortage of structure–optics–application chains that connect phase/chemistry → optical absorption/band gap (Kubelka–Munk + Tauc) → PL/PLE pathways → afterglow kinetics → device-level readout for authentication [16,17,18]. Moreover, while Sr-aluminate maximizes persistence duration in green emission, it is not necessarily optimal for blue, time-gated, printable security features where rapid “reset” and dual-lifetime encoding are desirable.

This work targets that application space by (i) synthesizing BaAl₂O₄: Eu²⁺ and BaAl₂O₄: Eu²⁺, Nd³⁺, Pr³⁺ via a 600 °C combustion process, producing ink-ready powders; (ii) establishing a sequential evidence chain—XRD/SEM/XPS → UV-Vis diffuse reflectance (Kubelka–Munk + Tauc) → PL/PLE → afterglow lifetime/decay—to clarify how Nd³⁺/Pr³⁺ modulates trap depths/distributions while preserving the Eu²⁺ emission band; and (iii) demonstrating screen-printed, time-gated anti-counterfeiting with a snowflake motif and a binary 3 × 3 grid using dual-lifetime inks to realize multi-level authentication. We acknowledge that the best persistence here (~34 s to visual threshold) is shorter than optimized SrAl₂O₄: Eu²⁺, Dy³⁺ systems (minutes–hours). Our aim is not to surpass Sr-aluminate in absolute duration but to provide a complementary blue channel with low-temperature, printing-compatible processing and time-gated readout suited to dynamic, multilayer security encoding.

2. Experiment Content

2.1. Materials and Methods

BaAl₂O₄: xEu²⁺, yNd³⁺, zPr³⁺ (x = 0, 0.01, 0.02, 0.03, 0.04, 0.05; y = 0.01, 0.02, 0.03, 0.04, 0.05; z = 0.0005, 0.001, 0.0015, 0.0020, 0.0025) series of fluorescent powders. Eu₂O₃ (purity: 99.99%), Al₂O₃ (purity: 99.99%), BaCO₃ (purity: 99.99%), Nd₂O₃ (purity: 99.99%), Pr(NO₃)₃ (purity: 99.99%), HNO₃ (purity: 80%), and urea (purity: 99.99%) were used as raw materials. All chemicals were sourced from Tianjin Chemical Reagent Factory (Tianjin, China) and employed as received without further drying or purification.

2.2. Material Synthesis

First, high-purity Eu₂O₃, Al₂O₃, BaCO₃, Nd₂O₃, and Pr(NO₃)₃ were dissolved in concentrated nitric acid and deionized water. By carefully controlling the molar concentration, the following nitrate solutions were prepared: Ba(NO₃)₂ (0.5 mmol/mL), Al(NO₃)₃ (1.0 mmol/mL), Eu(NO₃)₃ (0.1 mmol/mL), Nd(NO₃)₃ (0.5 mmol/mL), and Pr(NO₃)₃ (0.5 mmol/mL). The solutions were then combined in stoichiometric proportions corresponding to the target composition BaAl₂O₄: xEu²⁺, yNd³⁺, zPr³⁺.

During this process, 2.2 g of urea (CO(NH₂)₂) was added as both a reducing agent and combustion promoter. The mixture was magnetically stirred until a homogeneous transparent solution was obtained. The solution was transferred into an alumina crucible and placed in a preheated muffle furnace, where a spontaneous combustion reaction occurred. The reaction was maintained for 5–8 min until completion, producing a loose, porous, foamy white solid. After cooling to room temperature, the product was ground for 20–30 min to obtain a uniform fine powder. The final powders were sealed in test tubes for later use.

Using this solution combustion method, three series of phosphors were synthesized: BaAl₂O₄: Eu²⁺, BaAl₂O₄: Eu²⁺, Nd³⁺, and BaAl₂O₄: Eu²⁺, Nd³⁺, Pr³⁺. The Eu²⁺ concentration was first varied systematically (0–5%) to determine the optimal doping level. Based on this value, Nd³⁺ was introduced in a gradient (0–5%) to investigate its effect. Finally, with the Eu²⁺/Nd³⁺ ratio fixed at the optimized level, Pr³⁺ was added in small amounts (0.05–0.25%) to study its synergistic role.

To further explore the influence of thermal treatment, samples with the optimized dopant concentrations were calcined at temperatures ranging from 500 to 900 °C (in 100 °C intervals). The resulting products were analyzed to evaluate their phase composition, photoluminescence (PL) spectra, and afterglow decay curves. These measurements enabled the determination of the optimal ion doping levels and calcination temperature parameters, and the overall stepwise optimization procedure is summarized in

Table 1. Summary of the stepwise optimization procedure for doping concentrations of Eu²⁺, Nd³⁺, and Pr³⁺ in BaAl₂O₄.

Step	Ion	Concentration Range (x, y, z)	Optimization Approach
Step 1	Eu2+	x = 0, 0.01, 0.02, 0.03, 0.04, 0.05	Optimized for maximum photoluminescence
Step 2	Nd3+	y = 0.01, 0.02, 0.03, 0.04, 0.05	Optimized for afterglow and emission properties
Step 3	Pr3+	z = 0.0005, 0.001, 0.0015, 0.0020, 0.0025	Optimized for further enhancing afterglow performance
Temperature Range	Eu2+, Nd3+, Pr3+	500–900 °C	After determining the optimal doping concentration, use this temperature range for sample synthesis at the optimum temperature.

.

2.3. Preparation of Ink

The BaAl₂O₄: Eu²⁺, Nd³⁺, Pr³⁺ fluorescent powder synthesized by the combustion method was introduced into a mixed solvent system composed of anhydrous ethanol and polyacrylic acid (PAA). The fluorescent powder was colloidally dispersed in the ethanol-PAA system by stirring it with a glass rod, ensuring its rheological properties met the process requirements for screen printing. In this process, PAA acts as a binder, enhancing the ink’s film adhesion and inhibiting fluorescence quenching through the coordination of carboxyl groups with rare-earth ions. The fluorescent ink appears colorless and transparent under visible light but exhibits corresponding colors under UV excitation. The flowchart in Figure 1 systematically analyzes the entire process chain from fluorescent powder synthesis, ink homogenization, and dispersion to screen printing pattern formation.

2.4. Characterization of Materials

The crystallographic framework of the as-prepared samples was examined with a Rigaku D/max 2200PC powder X-ray diffractometer (Cu Kα radiation, λ = 1.5406 Å) in the 10–70° (2θ) angular range. The microstructure of the long-afterglow phosphorescent materials was characterized using a field emission scanning electron microscope (Hitachi Quanta 250 FEG, Tokyo, Japan). Elemental composition was analyzed semi-quantitatively using a scanning electron microscope energy dispersive spectrometer (SEM-EDS, Thermo Fisher Scientific, Waltham, MA, USA). Binding energy was determined using an X-ray photoelectron spectrometer (Thermo Scientific ESCALAB 250 XI, Waltham, MA, USA), with the excitation source being monochromatic Al Kα radiation (hν = 1486.6 eV). UV-visible diffuse reflectance spectra were collected using a UV-3600 spectrophotometer (Shimadzu Corporation, Kyoto, Japan). Photoluminescence (PL) spectra and their excitation spectra (PLE) were measured using a fluorescence spectrometer (F4700, Hitachi, Japan). The fluorescence lifetime and afterglow decay curves of long-afterglow phosphors were quantitatively characterized using a transient-steady-state fluorescence spectrometer (Hamamatsu Photonics Quantaurus-Tau C16361-2, Chiyoda, Japan).

3. Results and Discussion

3.1. XRD Structural Analysis

The effects of rare-earth ion (Eu²⁺, Nd³⁺, Pr³⁺) doping on the crystal structure of BaAl₂O₄ fluorescent powders were systematically analyzed using X-ray powder diffraction (XRD, Cu-Kα radiation, λ = 1.5406 Å). As shown in Figure 2a, all samples exhibit significant diffraction peaks at 2θ = 19.602° (d = 4.53 Å), 28.282° (d = 3.15 Å), 34.317° (d = 2.61 Å), 40.115° (d = 2.25 Å), and 45.042° (d = 2.01 Å). These peak positions are consistent with the characteristic diffraction peaks of the hexagonal phase BaAl₂O₄ standard card (PDF#00-017-0306, space group P6₃22) [19]. Figure 2a indicates that the diffraction peak positions exhibit no observable shift, indicating that the doping of Eu²⁺, Nd³⁺, and Pr³⁺ ions has not altered the crystalline phase of the BaAl₂O₄ matrix.

We conducted Rietveld refinement on the XRD patterns for BaAl₂O₄: Eu²⁺, Nd³⁺, Pr³⁺ long-afterglow phosphors synthesized via calcination at 600 °C employing the General Structure Analysis System-II (GSAS-II, v5.3.3, Revision #5806) software. Presented in Figure 2b, the experimental data exhibit high consistency with the fitted curve, with no major unidentified reflections. However, a small number of faint residual reflections can still be identified, indicating the presence of trace impurity phases. For the BaAl₂O₄: Eu²⁺, Nd³⁺, Pr³⁺ sample, the Rietveld refinement results show that the measured Rwp value is 8.53%. The Rp value is 5.5%, which is less than 10%, indicating that the sample has good physical and mathematical agreement with the standard card data, further confirming the hexagonal phase structure (PDF#00-017-0306) and purity of the sample. Eu²⁺ and Nd³⁺ ions tend to occupy the Ba²⁺ positions in the BaAl₂O₄ main lattice [20,21], thereby introducing a doping effect. However, the doping concentration of Pr³⁺ is low (0.15%), and it may also exert a particular influence on the lattice structure. In the full-range pattern (Figure 2a), an impurity peak is observed at 2θ = 31.0°. Based on JADE analysis, Figure 2a overlays Al₂O₃ (PDF #00-012-0539); three residual reflections—including the ~31° line—match the Al₂O₃ sticks within ± 0.1–0.2° (2θ), and the feature is therefore assigned to a trace Al₂O₃ secondary phase. Quantitative Rietveld analysis in GSAS-II yields an Al₂O₃ fraction of ~1.3 wt.%, confirming that the impurity level is minor. For clarity, Figure 2b has been updated to show Bragg reflection markers for Al₂O₃. This Al₂O₃ most likely results from incomplete reaction inherent to the combustion-synthesis process. Figure 2c shows the XRD patterns of BaAl₂O₄: 0.03Eu²⁺, 0.03Nd³⁺, 0.0015Pr³⁺ nanophosphors obtained by calcination at different temperatures. The diffraction peaks’ positions are essentially unchanged in comparison to Figure 2a, suggesting that the crystal phase of the BaAl₂O₄ host is not affected by the co-doping of Eu²⁺, Nd³⁺, Pr³⁺ ions.

Based on the XRD spectrum data, we used Origin 2024 software to perform Gaussian fitting on the diffraction peaks to extract key parameters such as peak position (expressed in radians) and full width at half maximum (FWHM). The grain size was calculated using the Scherrer formula [22]:

$$\mathrm{D}={\displaystyle \frac{\mathrm{k}\mathsf{\lambda }}{\mathsf{\beta }\cos \mathsf{\theta }}}$$

(1)

Among these, k is the Scherrer constant (set to 0.89), λ is the X-ray wavelength (0.15406 nm), and β and θ correspond to the full width at half maximum (FWHM) (in radians) and the diffraction angle of the corresponding peak, respectively. All calculations were performed using Origin software. We further determined the sample’s lattice spacing, microstrain, and dislocation density, with corresponding results tabulated in

Table 2. XRD data of BaAl₂O₄: Eu²⁺, Nd³⁺, Pr³⁺ phosphors.

2θ (°)	hkl	Intensity (I) (a.u.)	FWHM (β) (°)	Lattice Spacing (d) (Å)	Crystallite Size (D) (nm)	Dislocation Density (δ) (m−2)	Micro Strain (ε) (Dimensionless)
19.602	200	45	0.191	0.453	41.740	0.574	4.824
28.282	202	100	0.180	0.315	45.008	0.494	3.117
34.317	220	40	0.338	0.261	24.325	1.690	4.777
40.115	222	25	0.277	0.225	30.192	1.097	3.310
45.042	402	19	0.315	0.201	26.999	1.372	3.315

.

3.2. SEM Analysis

SEM images (Figure 3a–e) reveal temperature-dependent morphology of BaAl₂O₄: Eu²⁺, Nd³⁺, Pr³⁺. At 500–600 °C, irregular nanoparticles form a mesoporous network. With rising temperature (700–800 °C), particles coalesce and locally densify, and by 900 °C submicron agglomerates dominate with reduced porosity. This morphological evolution is attributed to vigorous gas release (NO_x, CO₂) during combustion, which generates transient escape channels and leaves residual pores [23]. The particle size exhibits non-uniformity, which is attributed to the thermal gradient effect at the combustion front leading to localized nucleation, imbalance in solute diffusion caused by changes in the precursor solution [24], and uneven distribution of mechanical stress during grinding [25]. At elevated temperature, the hexagonal BaAl₂O₄ lattice (P6₃22) promotes bulk agglomeration, while the remaining pores can act as diffusion pathways for rare-earth ions, affecting their lattice incorporation efficiency [26,27,28].

The detection of Ba, Al, O, Eu, Nd, and Pr has been confirmed through EDS analysis of the BaAl₂O₄: 0.03Eu²⁺, 0.03Nd³⁺, 0.0015Pr³⁺ sample (Figure 3f). Local micro-area analysis further revealed the elemental composition of the sample and its corresponding characteristic X-ray intensity signals [29]. Although the chemical composition of the sample generally aligns with the EDS spectral intensity, there is a certain deviation between the actual doping ratios of Eu, Nd, and Pr and the expected stoichiometric ratios [30]. This deviation may stem from the solution combustion synthesis method employed, which completes the doping process through rapid redox reactions of the precursor solution in an extremely short time, leading to intense exothermic reactions. This could lead to non-uniform elemental distribution within the crystal lattice, causing segregation phenomena.

Due to the low doping concentration of Pr³⁺ (only 0.15%), its characteristic peak intensity in the EDS spectrum (Figure 3f) is relatively weak, with significant overlap with the Nd³⁺ peak. The intensity difference between the two is slight, which, to some extent, increases the difficulty of quantitative analysis of low-concentration elements. Nevertheless, EDS can still clearly detect the characteristic peaks of Eu²⁺, Nd³⁺, and Pr³⁺, indicating that all three have successfully entered the crystal lattice structure of BaAl₂O₄. As an orthogonal check, SEM–EDS elemental maps (Figure 3g) show a reproducible Pr distribution above background, co-localized with the Ba/Al/O host and without RE-rich segregation at the micron scale. Eu and Nd maps exhibit similar uniformity. Given the Nd/Pr L-line overlap, conventional EDS is unsuitable for quantitative Pr/Nd partitioning. We therefore use EDS only for above-background presence and colocalization, while XPS provides chemically specific identification and oxidation states.

3.3. XPS Analysis

In this study, to systematically evaluate the chemical valence states of Eu²⁺, Nd³⁺, and Pr³⁺ ions in a BaAl₂O₄ matrix and their effects on the material’s luminescent properties, we characterized of samples using X-ray photoelectron spectroscopy (XPS) technology. XPS is a surface-sensitive analytical method that provides information on elements’ chemical composition, oxidation state, and chemical environment [31]. Through full-spectrum scanning and high-resolution narrow-band scanning, we successfully identified the characteristic binding energy peaks of the main elements (Ba, Al, O, Eu, Nd, Pr) in the samples, as shown in Figure 4.

As shown in Figure 4a, XPS measurement results reveal binding energy peaks corresponding to Ba 3d, Al 2p, O 1s, Eu 3d, Nd 3d, and Pr 3d, indicating that these elements are present in the sample [32,33]. Further high-resolution scanning analysis revealed that the binding energy peak of Eu 3d was 1125.85 eV, consistent with the characteristic binding energy of Eu²⁺ (3d_5/2), indicating that Eu²⁺ exists in the sample in its divalent form [34,35,36]. Additionally, the binding energy peak of Nd 3d is located at 977.78 eV and 1003.8 eV, consistent with the typical binding energy range of Nd³⁺, indicating that Nd³⁺ also maintains its expected trivalent state [37,38,39]. Low-concentration Pr³⁺ doping (0.15%) results in a weak characteristic peak, with a binding energy peak at 933.34 eV. High-resolution scanning confirms the presence of the Pr 3d characteristic peak, demonstrating that Pr³⁺ has been doped into the BaAl₂O₄ lattice and maintains its trivalent state [40,41]. XPS confirmed that all doped ions are in the target valence state, a result that directly supports subsequent studies on the luminescent properties of the material.

3.4. Ultraviolet Diffuse Reflectance Characterization

UV-Vis diffuse reflectance spectroscopy (UV-Vis) was employed to characterize the long-afterglow phosphors BaAl₂O₄: Eu²⁺, BaAl₂O₄: Eu²⁺, Nd³⁺, and BaAl₂O₄: Eu²⁺, Nd³⁺, Pr³⁺ synthesized at 600 °C, with a measurement range of 200–800 nm, to determine their optical bandgap values. Figure 5a–c show the UV-Vis diffuse reflectance spectra of the three samples, respectively. According to the Tauc formula [42,43]:

$$(\mathsf{\alpha }\mathrm{hv}{)}^{1/2} = \mathrm{A}(\mathrm{h}\mathsf{\nu } - {\mathrm{E}}_{\mathrm{g}})$$

(2)

Among these, A is a material-related constant, E_g is the bandgap value, hν is the photon energy, and α is the light absorption coefficient. By plotting (αhν)^1/2 against hv and extrapolating to (αhν)^1/2 = 0, the bandgap values for BaAl₂O₄: Eu²⁺, BaAl₂O₄: Eu²⁺, Nd³⁺, and BaAl₂O₄: Eu²⁺, Nd³⁺, Pr³⁺ are determined to be 4.31 eV, 4.39 eV, and 4.5 eV, respectively. The incorporation of Eu²⁺, Nd³⁺, and Pr³⁺ does not significantly alter the host band gap, indicating that the electronic structure of BaAl₂O₄ is largely preserved. We note that these numbers represent the Tauc onset rather than a rigid shift of the host band edges. The modest widening upon Nd³⁺ and Nd³⁺/Pr³⁺ co-doping is attributed to a reduction in sub-gap/tail-state absorption and edge sharpening due to defect re-distribution and charge-compensation by trivalent co-dopants, together with fewer non-radiative centers; effectively, the co-dopants clean the absorption edge, yielding a slightly larger apparent Tauc gap while leaving the Eu²⁺ emission band shape essentially unchanged. This stability is beneficial for sustaining its long afterglow luminescence performance.

Figure 5. (a–c) UV-Vis absorption spectra of (a) BaAl₂O₄: Eu²⁺; (b) BaAl₂O₄: Eu²⁺, Nd³⁺, and (c) BaAl₂O₄: Eu²⁺, Nd³⁺, Pr³⁺. Insets: Tauc plots for indirect-allowed transitions.

Figure 5. (a–c) UV-Vis absorption spectra of (a) BaAl₂O₄: Eu²⁺; (b) BaAl₂O₄: Eu²⁺, Nd³⁺, and (c) BaAl₂O₄: Eu²⁺, Nd³⁺, Pr³⁺. Insets: Tauc plots for indirect-allowed transitions.

Upon Nd³⁺/Pr³⁺ co-doping, the absorbance in the 270–280 nm segment of the Eu²⁺ 4f → 5d envelope decreases. This behavior is attributed to the suppression of sub-gap/Urbach-tail and defect-related absorption by charge-compensated defect complexes, together with a slight redistribution of oscillator strength due to local crystal-field modulation at Eu²⁺ sites. The cleaner near-edge response is consistent with the modest blue shift of the apparent edge (4.31 → 4.39 → 4.5 eV), while the wide band gap of the host remains essentially preserved.

Diffuse reflectance R(λ) was converted to the Kubelka–Munk function F(R) = (1−R)²/(2R). To quantitatively characterize the edge steepness within this section, we further employed the Urbach-tail formalism on the near-edge exponential region: linear fits of lnA (absorbance used as a surrogate of α) versus photon energy hν were performed below the respective Tauc onsets to avoid interference with the Tauc linear segment. The Urbach energy was extracted as E_U = 1/slope. For each composition we report the fit window (eV), data-point count, least-squares R², and 95% confidence interval (CI) of E_U; the dimensionless steepness index S = kT/E_U (300 K) is also listed. The corresponding data is shown in

Table 3. Urbach-tail quantification.

Composition	EU (eV)	95% CI (eV)	Fit Window (eV)	Points (n)	R2	S = kT/EU (300 K)
BaAl2O4: Eu2+	0.656	0.643–0.669	3.71–4.01	25	0.9977	0.0394
BaAl2O4: Eu2+, Nd3+	0.38	0.373–0.386	2.85–3.15	42	0.997	0.0681
BaAl2O4: Eu2+, Nd3+, Pr3+	0.491	0.485–0.497	2.79–3.09	43	0.9984	0.0527

.

All Urbach fits were carried out below the corresponding Tauc onsets (4.31/4.39/4.50 eV); therefore, these model-based metrics quantify edge steepness without altering or contradicting the bandgap values reported above. Within this framework, Nd co-doping yields the steepest/cleanest edge (smallest E_U), while additional Pr slightly increases the tail (E_U rises).

3.5. Photoluminescence Analysis

Figure 6 shows the influence of ion concentration and temperature upon the luminescence characteristics of BaAl₂O₄ phosphors. Figure 6a displays the emission spectrum of BaAl₂O₄: xEu²⁺ (x = 0–0.05) samples sintered at 600 °C under 365 nm excitation, exhibiting a broad blue-green emission peak at 490 nm, originating from the Eu²⁺ 4f⁶5d¹ → 4f⁷ transition [44,45,46,47]. As the Eu²⁺ concentration increases, the luminescence intensity enhances, reaching a peak at x = 0.03 before undergoing concentration quenching. The emission peak position (490 nm) remains unchanged across different x values, indicating a stable lattice field environment.

Figure 6b investigates the effect of Nd³⁺ co-doping (Ba_0.97-yAl₂O₄: 0.03Eu²⁺, yNd³⁺, y = 0–0.05). When y = 0.03, the emission intensity at 490 nm is maximum. Nd³⁺ ions introduce trap centers in the BaAl₂O₄ matrix. These centers enhance the afterglow by facilitating energy transfer to Eu²⁺ and serve as reservoirs for controlled carrier release [48,49,50].

Figure 6c shows the effect of Pr³⁺ doping (Ba_0.94-zAl₂O₄: 0.03Eu²⁺, 0.03Nd³⁺, zPr³⁺, z = 0.0005–0.0025). The best luminescence effect is observed at z = 0.0015, where Pr³⁺ introduces additional traps and facilitates more efficient energy transfer between Eu²⁺ and Nd³⁺. This synergistic role enhances the afterglow and suppresses non-radiative losses [49,51], thereby improving the overall PL performance [49,50]. Deviating from this concentration leads to insufficient defects or concentration quenching [48,50].

Based on the optimized dopant concentrations (Eu²⁺: 0.03; Nd³⁺: 0.03; Pr³⁺: 0.0015), Figure 6d depicts the effect of sintering temperature (500–900 °C) on the luminescence performance. The sample sintered at 600 °C exhibits the highest emission intensity at 490 nm. However, the luminescence intensity progressively decreases when the temperature exceeds 800 °C. This reduction is likely due to increased lattice distortion at elevated temperatures, destabilizing the luminescent centers and reducing energy transfer efficiency.

Figure 6e presents the photoluminescence excitation (PLE) spectrum of the sample, showing a broad excitation band centered around 365 nm, corresponding to the 4f⁷ → 4f⁶5d¹ transition of Eu²⁺ ions. This indicates that 365 nm ultraviolet light is an effective excitation source, consistent with the emission spectra in Figure 6a–d.

The CIE 1964 Supplementary Standard Colorimetric System (10° field of view) defines a complete geometric model for color quantification in visible light colorimetry. The associated chromaticity diagram exhibits a horseshoe-shaped spectral locus, comprising chromaticity coordinates of monochromatic light (380–780 nm) with boundary points representing saturated spectral colors. The central white point (x₁₀ = 0.3333, y₁₀ = 0.3333) corresponds to equi-energy white light. Color saturation progressively diminishes toward this central point. At the same time, hue differences are quantified azimuthally—radial lines extending from the white point intersect the spectral locus at coordinates corresponding to dominant wavelengths. Chromaticity coordinates (x₁₀, y₁₀) are derived from normalized tristimulus values (X₁₀, Y₁₀, Z₁₀) as follows [52]:

$$\mathrm{x}={\displaystyle \frac{\mathrm{X}}{\mathrm{X}+\mathrm{Y}+\mathrm{Z}}}$$

(3)

$$\mathrm{y}={\displaystyle \frac{\mathrm{Y}}{\mathrm{X}+\mathrm{Y}+\mathrm{Z}}}$$

(4)

Among these, X, Y, and Z are the CIE three-stimulus values, obtained by integrating the color stimulus function with the color matching function of the CIE 1964 color space system. This model converts human color perception into quantitative parameters, improving accuracy as the field of view increases. It is the core standard for material luminescence analysis and industrial color difference detection. This study calculated spectral data for samples with different Pr³⁺ doping concentrations, yielding corresponding CIE 1964 xy chromaticity coordinates (Figure 6f). The chromaticity coordinates of all five concentration samples are concentrated in the blue-green light region, indicating that Pr³⁺ doping has not altered the material’s emission color, consistent with the 490 nm emission peak position.

Figure 6. PL/PLE of BaA₂O₄-based phosphors: (a) Eu²⁺ concentration (λex = 365 nm); (b) Nd³⁺ co-doping; (c) Pr³⁺ co-doping; (d) calcination temperature (500–900 °C); (e) PLE; (f) CIE map; (g,h) normalized/raw PL; (i) normalized PLE.

Figure 6. PL/PLE of BaA₂O₄-based phosphors: (a) Eu²⁺ concentration (λex = 365 nm); (b) Nd³⁺ co-doping; (c) Pr³⁺ co-doping; (d) calcination temperature (500–900 °C); (e) PLE; (f) CIE map; (g,h) normalized/raw PL; (i) normalized PLE.

To clarify the excitation pathways and the effects of co-doping, we uniformly processed the PL/PLE data for the three compositions. Figure 6i presents the normalized PLE spectra monitored at the Eu²⁺ emission (490 ± 10 nm). After normalization to the maximum, the overall response follows Eu > Eu + Nd > Eu + Nd + Pr, indicating that Nd³⁺/Pr³⁺-related traps compete for excited carriers and thereby divert part of the excitation from being immediately fed to Eu²⁺ under steady-state monitoring. In contrast, the normalized PL spectra (λex = 365 nm) in Figure 6g nearly overlap in λ_max and FWHM, demonstrating that the Eu²⁺ emission band is preserved and that co-doping mainly affects excitation/trapping and non-radiative channels, rather than altering the local crystal field at the Eu²⁺ site. For independent verification, the raw (non-normalized) PLE and PL spectra corresponding to Figure 6e,h are shown here for absolute-intensity comparison.

The luminescence mechanism of BaAl₂O₄-based phosphors is similar to that of Sr/CaAl₂O₄ systems, especially in the presence of Eu²⁺ and rare-earth ions. The luminescence of Eu²⁺ originates from the transition from the ground state 4f⁶ to the excited state 5d¹. When Nd³⁺, Pr³⁺, and other ions are incorporated, these ions provide deep-level traps in the system, which delay electron recombination and significantly extend the afterglow time [53,54,55,56].

In the BaAl₂O₄: Eu²⁺, Nd³⁺, Pr³⁺ system, Nd³⁺ and Pr³⁺ not only provide luminescent centers but also create additional electron trapping sites. These traps slow down the electron recombination process, thereby enhancing the afterglow properties. Furthermore, the energy transfer mechanism between Eu²⁺ and Nd³⁺ explains how these materials can achieve prolonged afterglow luminescence [57,58].

The luminescence mechanism of BaAl₂O₄-based phosphors primarily relies on the 5d–4f transition of Eu²⁺ and its interaction with deep-level traps. The incorporation of Nd³⁺ and Pr³⁺ ions further improves the efficiency of electron trapping and release, thus achieving long-lasting afterglow luminescence.

3.6. Fluorescence Lifetime and Afterglow Decay

The fluorescence decay of BaAl₂O₄: Eu²⁺, BaAl₂O₄: Eu²⁺, Nd³⁺, and BaAl₂O₄: Eu²⁺, Nd³⁺, Pr³⁺ samples under UV excitation was tested. Figure 7 shows these samples’ decay curves and fitting results under 365 nm excitation. All samples exhibited double-exponential decay, consisting of fast and slow components. The double-exponential function fitting using Equation (5) was employed, where I₀ is the background constant, A₁ and A₂ are constants, t is the decay time, and τ₁ and τ₂ are the decay times of the exponential components [49]. These parameters were calculated using Origin software, with specific values in

Table 4. Average fluorescence lifetimes of BaAl₂O₄: Eu²⁺, BaAl₂O₄: Eu²⁺, Nd³⁺, and BaAl₂O₄: Eu²⁺, Nd³⁺, Pr³⁺ phosphors.

BaAl2O4	Decay Lifetimes (μs)
	A1	τ1	A2	τ2	τ*
Eu2+	16,847	60	73	712	92
Eu2+, Nd3+	15,046	10	32	480	54
Eu2+, Nd3+, Pr3+	15,880	10	10	500	25

Note: “τ*” denotes the average decay time calculated using Equation (6).

. The average decay time τ * of the samples can be calculated using Equation (6) [59].

$$\mathrm{I}(\mathrm{t})={\mathrm{I}}_0+{\mathrm{A}}_1\exp (-\mathrm{t}/{\mathsf{\tau }}_1)+{\mathrm{A}}_2\exp (-\mathrm{t}/{\mathsf{\tau }}_2)$$

(5)

$${\mathsf{\tau }}_{\mathrm{ave}}={\displaystyle \frac{{\mathrm{A}}_1{\mathsf{\tau }}_1^2+{\mathrm{A}}_2{\mathsf{\tau }}_2^2}{{\mathrm{A}}_1{\mathsf{\tau }}_1+{\mathrm{A}}_2{\mathsf{\tau }}_2}}$$

(6)

The fast component is primarily attributed to the intrinsic radiative relaxation of the Eu²⁺ centers (5d → 4f transition) and the rapid detrapping of carriers from shallow traps. In contrast, the slow component reflects population stored in deeper traps, which are released slowly either by thermally activated detrapping or by tunneling through the traps before recombination at Eu²⁺ centers. This dual decay process—rapid release from shallow traps and slower release from deep traps—is consistent with the known models for Eu²⁺-activated aluminates.

The average decay times calculated using the above formula are as follows: BaAl₂O₄: Eu²⁺ is 92 μs, BaAl₂O₄: Eu²⁺, Nd³⁺ is 54 μs, and BaAl₂O₄: Eu²⁺, Nd³⁺, Pr³⁺ is 25 μs. It can be observed that the introduction of dopant ions significantly reduces the fluorescence lifetime of the samples. This may be attributed to Nd³⁺ replacing some Ba²⁺ sites in the matrix, forming trap centers with suitable energy levels that effectively capture some excited-state electrons. Pr³⁺ further enhances the trap effect, increasing electron capture efficiency and exacerbating concentration quenching. The dopant ions effectively regulate the fluorescence lifetime of the material.

The afterglow decay of BaAl₂O₄: Eu²⁺, Nd³⁺ and BaAl₂O₄: Eu²⁺, Nd³⁺, Pr³⁺ samples was measured under 365 nm excitation. The experimental data were fitted using a double exponential function, with the results shown in

Table 5. Afterglow decay times of BaAl₂O₄: Eu²⁺, Nd³⁺ and BaAl₂O₄: Eu²⁺, Nd³⁺, Pr³⁺ phosphors.

BaAl2O4	Decay Lifetimes (s)
	A1	τ1	A2	τ2	τ*
Eu2+, Nd3+	2159	2	126	29	14
Eu2+, Nd3+, Pr3+	181	2	25	44	34

Note: “τ*” denotes the average decay time calculated using Equation (6).

. The afterglow decay consists of two stages: a fast decay dominates the initial intensity decline, while the slow decay corresponds to extended afterglow emission [60]. As shown in Figure 8, the average afterglow time τ for BaAl₂O₄: Eu²⁺, Nd³⁺ is 14 s, while that for BaAl₂O₄: Eu²⁺, Nd³⁺, Pr³⁺ is extended to 34 s.

Pr³⁺ doping enhances trap density and optimizes the electron capture and release process, improving afterglow performance. The deeper energy level traps formed extend the electron storage time, significantly prolonging the afterglow lifetime. Pr³⁺ doping plays a critical role in modifying the trap properties by introducing deeper energy level traps. These traps effectively capture more electrons, thereby prolonging the afterglow. Additionally, the increased trap density in the system, due to Pr³⁺ codoping, enhances the overall electron capture and release efficiency, contributing to the extended afterglow times observed in BaAl₂O₄: Eu²⁺, Nd³⁺, Pr³⁺ samples. Anti-counterfeiting labels based on such phosphors enhance reliability and ease of identification and also have application potential in the fields of safety emergency indication and optical information storage [61].

The afterglow times observed in BaAl₂O₄: Eu²⁺, Nd³⁺ (14 s) and BaAl₂O₄: Eu²⁺, Nd³⁺, Pr³⁺ (34 s) are relatively short compared to the long afterglow times seen in SrAl₂O₄-based phosphors (such as SrAl₂O₄:Eu²⁺, Dy³⁺), where afterglow persists for tens of minutes. The shorter afterglow times observed in BaAl₂O₄-based phosphors highlight the influence of the host matrix and the trap distribution on the afterglow performance. However, by optimizing trap properties through codoping with Nd³⁺ and Pr³⁺, the afterglow duration in BaAl₂O₄-based phosphors can be significantly improved.

The Eu²⁺ 5d → 4f fluorescence lifetime decreases upon Nd³⁺ and Pr³⁺ co-doping (92 → 54 → 25 µs), whereas the afterglow time increases (14 → 34 s). This behavior is consistent with a branching-kinetics picture. Co-dopants introduce efficient trap-capture channels that compete with prompt emission on the microsecond timescale; hence the observed lifetime follows

$${\tau }_{\mathrm{obs}}^{-1}={\tau }_r^{-1}+{\tau }_{nr}^{-1}+k_{\mathrm{E}\mathrm{T}\to traps}$$

(7)

and shortens as k_ET→traps increases. In contrast, persistent luminescence is governed by the release of carriers from traps on much longer timescales,

$$I_{\mathrm{a}\mathrm{f}\mathrm{t}\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{l}\mathrm{o}\mathrm{w}}(t)\propto \sum _i{n}_i(0)\mathrm{e}\mathrm{x}\mathrm{p}(-t/{\tau }_{t,i})$$

(8)

A back-of-the-envelope estimate using the measured lifetimes yields incremental fast-capture rates of Δk ≈ 7.6 × 10³ s⁻¹ for Eu → Eu + Nd (92 → 54 µs) and Δk ≈ 2.9 × 10⁴ s⁻¹ for Eu → Eu + Nd + Pr (92 → 25 µs), evidencing enhanced diversion of carriers into traps. While this quenches the prompt Eu²⁺ channel, it increases the initial trapped population n_i(0) and, with Pr³⁺, optimizes the distribution of thermally addressable traps at 300 K, thereby extending the macroscopic afterglow. Prompt PL lifetime and persistent luminescence thus probe different segments of the relaxation network and need not exhibit parallel trends.

To provide a clearer comparison with the existing materials, we have compared the fluorescence lifetimes and afterglow decay times of our BaAl₂O₄: Eu²⁺, Nd³⁺, Pr³⁺ phosphor with SrAl₂O₄: Eu, Nd system, as shown in

Table 6. Comparison of fluorescence lifetimes and afterglow decay times for BaAl₂O₄-based phosphors and SrAl₂O₄-based phosphors.

Material	Doping Concentration	Fluorescence Lifetime (μs/ns)	Afterglow Decay Time (s)
BaAl2O4: Eu2+	Eu2+ = 0.03	92 μs	0 s
BaAl2O4: Eu2+, Nd3+	Eu2+ = 0.03, Nd3+ = 0.03	54 μs	14 s
BaAl2O4: Eu2+, Nd3+, Pr3+	Eu2+ = 0.03, Nd3+ = 0.03, Pr3+ = 0.0015	25 μs	34 s
SrAl2O4: Eu2+	Eu2+ = 0.02	404 ns	0 s
SrAl2O4: Eu2+, Nd3+	Eu2+ = 0.02, Nd3+ = 0.01	46 ns	13 s

.

As shown in the table, our BaAl₂O₄: Eu²⁺, Nd³⁺, Pr³⁺ samples exhibit fluorescence lifetimes of 92 μs, 54 μs, and 25 μs, and afterglow decay times of 0 s, 14 s, and 34 s for Eu²⁺, Eu²⁺, Nd³⁺, and Eu²⁺, Nd³⁺, Pr³⁺, respectively. In comparison, SrAl₂O₄: Eu, Nd has fluorescence lifetimes of 404 ns and 46 ns, with afterglow decay times of 0 s and 13 s, respectively [62].

While the BaAl₂O₄: Eu²⁺, Nd³⁺, Pr³⁺ system demonstrates shorter afterglow times compared to SrAl₂O₄-based materials, the afterglow times in our system still show promising results, demonstrating the effectiveness of the trap-assisted luminescence mechanism. Moreover, our system offers the advantage of lower synthesis temperatures and more controlled emission wavelengths, making it more versatile for specific applications, including anti-counterfeiting and security labeling.

To probe the role of traps in the afterglow, we performed 2D thermoluminescence (2D-TL) on BaAl₂O₄: Eu²⁺, BaAl₂O₄: Eu²⁺, Nd³⁺, and BaAl₂O₄: Eu²⁺, Nd³⁺, Pr³⁺ (pre-irradiation 365 nm; 300–600 K; heating rate β = 3 K s⁻¹; detection 300–580 nm, spectrally integrated to 1D glow curves). The TL envelopes change markedly upon co-doping. We quantified the dominant peak of each curve using the high-temperature half-width (δ) method (single-rate, first-order effective estimate), obtaining trap depths of 0.296 eV for BaAl₂O₄: Eu²⁺, 0.106 eV for BaAl₂O₄: Eu²⁺, Nd³⁺, and 0.109 eV for BaAl₂O₄: Eu²⁺, Nd³⁺, Pr³⁺. In the same 300–600 K window, the high-T half-widths increase from 35.25 K (Eu²⁺) to 73.14 K (Eu²⁺, Nd³⁺) and 71.07 K (Eu²⁺, Nd³⁺, Pr³⁺), while the integrated TL areas rise from 5.07 × 10³ (Eu²⁺) to 2.90 × 10⁴ (Eu²⁺, Nd³⁺) and 1.06 × 10⁵ a.u.·K (Eu²⁺, Nd³⁺, Pr³⁺). The dominant-peak temperatures are 348 K (Eu²⁺) and 300 K (Nd/Pr-containing), with local maxima in the 314–341 K range [63,64].

Taken together, these broad-feature descriptors (broader half-widths and much larger areas) and the δ-based trap depths are consistent with Nd/Pr co-doping producing a broader and more effective trap manifold, with only modest changes in the dominant-peak depth but a substantial increase in trap population/retrapping effectiveness. This picture accounts for the observed trend in persistence (Eu²⁺ < Eu²⁺, Nd³⁺ < Eu²⁺, Nd³⁺, Pr³⁺) via trap-assisted luminescence, without overstating what can be inferred from the present single-rate, 300–600 K dataset.

As illustrated in Figure 9b, under 365 nm excitation electrons are promoted to the conduction band (CB) and partially relax non-radiatively to the Eu²⁺ 4f⁶5d level, from which radiative recombination to the Eu²⁺ 4f⁷ ground state yields the ≈490 nm emission. In the co-doped samples, Nd³⁺ creates shallower traps whereas Pr³⁺ introduces deeper traps below the CB. Electrons captured by these traps are thermally released and repopulate the Eu²⁺ 5d state, sustaining the persistent luminescence. Consistent with this picture, the average afterglow time increases from ≈14 s (Eu²⁺, Nd³⁺) to ≈34 s (Eu²⁺, Nd³⁺, Pr³⁺), while the BaAl₂O₄ host bandgap remains ≈4.3–4.5 eV.

3.7. Anti-Counterfeiting Applications

BaAl₂O₄: Eu²⁺, BaAl₂O₄: Eu²⁺, Nd³⁺, and BaAl₂O₄: Eu²⁺, Nd³⁺, Pr³⁺ phosphors were mixed with polyacrylic acid and ethanol to prepare screen-printed anti-counterfeiting ink, which was used to print a snowflake pattern (Figure 10). Under daylight, the pattern shows no obvious fluorescence, but it exhibits clear luminescence under 365 nm UV light. Upon removal of the UV light source, the fluorescence of BaAl₂O₄: Eu²⁺ disappears rapidly, while the BaAl₂O₄: Eu²⁺, Nd³⁺ and BaAl₂O₄: Eu²⁺, Nd³⁺, Pr³⁺ samples exhibit persistent afterglow. The doping of Nd³⁺ and Pr³⁺ significantly enhances the material’s luminescent properties, increasing its application value in anti-counterfeiting printing.

We further demonstrate a 3 × 3 lifetime-encoded grid using two inks (≈14 s vs. ≈34 s) (Figure 11). Readout is equation-free: inspect cells in fixed order (weights 1–256) to obtain a decimal code at two time windows (e.g., 365 → 63). Camera exposure and binarization thresholds were kept constant.

3.8. Comparative Landscape and Application Positioning

Table 7. Comparative summary of aluminate persistent phosphors for printing/security.

Host System	Representative Activators/Co-Dopants	Typical Synthesis Route & T	Dominant Emission (Qualitative)	Persistence Window (Qualitative)	Ink/Printing Compatibility	Application Notes/Positioning
BaAl2O4	Eu2+ (emitter); Nd3+/Pr3+ (trap engineering)	Combustion (this work): ~600 °C; also solid-state reported	Cyan/blue (Eu2+ 5d–4f)	Second scale (~14 s → ~34 s via co-doping)	High (fine powders, low-T processing, screen-print inks)	Time-gated; print-ready
SrAl2O4	Eu2+ (emitter); Dy3+ (traps)	Solid-state, typically > 1200 °C; flux/atmosphere tuning common	Green (~520–530 nm)	Minutes–hours under optimized conditions	Moderate (higher-T process;)	Long-duration; signage
CaAl2O4	Eu2+ (emitter); Nd3+/Dy3+ (traps)	Solid-state/combustion (composition- dependent)	Blue (~440–460 nm)	Minute-class in literature (composition- dependent)	Moderate (depends on particle)	Blue; tunable

summarizes key features of BaAl₂O₄, SrAl₂O₄, and CaAl₂O₄ persistent phosphors relevant to printing and anti-counterfeiting. In brief, SrAl₂O₄:Eu²⁺, Dy³⁺ remains the green, long-duration benchmark under optimized high-temperature solid-state synthesis, whereas Ca-aluminates offer blue emission with routes ranging from solid-state to combustion. Our BaAl₂O₄: Eu²⁺, Nd³⁺, Pr³⁺ targets a blue/cyan channel produced at 600 °C by combustion, yielding ink-compatible powders and a time-gated, dual-lifetime readout (365 → 63 code pair) suited for multilayer authentication. Rather than maximizing absolute duration, the present work emphasizes low thermal budget, printing compatibility, and lifetime-encoded authentication [65,66,67,68].

4. Conclusions

This work elucidates how Nd³⁺/Pr³⁺ co-doping reshapes trap distributions in BaAl₂O₄: Eu²⁺ without shifting the Eu²⁺ 4f–5d emission. Indirect-allowed (n = 1/2) Tauc plots give band gaps of 4.31, 4.39, and 4.50 eV for Eu²⁺, Eu²⁺/Nd³⁺, and Eu²⁺/Nd³⁺/Pr³⁺, respectively. Prompt PL lifetimes shorten (92 → 54 → 25 μs), whereas macroscopic afterglow extends from 14 s (Eu²⁺,Nd³⁺) to 34 s with Pr³⁺, consistent with deeper/more numerous traps at 300 K. Printed dual-lifetime inks enable time-gated authentication using a binary 3 × 3 grid with two non-overlapping readout windows, illustrating a print-compatible blue/cyan security channel.

Although this study has made significant progress in the synthesis and application of BaAl₂O₄-based long afterglow materials, further optimization of the trap structure and stability of the materials is needed. Future research will focus on further improving the afterglow performance and exploring more doping combinations to meet a wider range of application requirements.