Eu²⁺-Activated Ba_0.5Sr_0.5Al₂O₄ Phosphors for Screen Printing and Anti-Counterfeiting Flexible Film

Abstract

Herein, a series of Ba_0.5Sr_0.5Al₂O₄: xEu²⁺ (x = 0.01, 0.02, 0.03, 0.04, 0.06) nanophosphors were synthesized by a combustion method. The investigation encompassed the characterization of the phase purity, morphology, elemental composition, and photoluminescence behavior of Ba_0.5Sr_0.5Al₂O₄: xEu²⁺ nanoparticles. Under excitation by 303 nm and 365 nm ultraviolet light, the nanoparticles exhibited blue-green emission arising from the 4f⁶5d→4f⁷ transition of Eu²⁺ ions. The optimal doping concentration was determined to be 2%. Notably, the nanoparticles demonstrated fluorescence lifetimes and quantum yields of 1010 ns ($\lambda$_ex = 303 nm), 112 ns ($\lambda$_ex = 365 nm), 10.5%, and 10.3%, respectively. Additionally, a comprehensive analysis of the band structure and electronic density of states was conducted, revealing a theoretical direct band gap of 4.05 eV for the Ba_0.5Sr_0.5Al₂O₄ host. In addition, the prepared fluorescent powder can be used to prepare fluorescent flexible films. This film does not change the characteristic emission of Eu²⁺ ions and has more stable physicochemical properties, which may be more suitable for use in harsh environments. Also, the fluorescent powder can be blended with polyacrylic acid to form colorless anti-counterfeiting ink that can be applied to banknotes as an anti-counterfeiting mark. A clover pattern was successfully printed using screen-printing technology, proving its potential application in the field of anti-counterfeiting.

1. Introduction

Nowadays, all kinds of counterfeit products, such as fake banknotes, fake drugs, and counterfeit food, have penetrated every aspect of our life, posing a great threat to our personal property and life and health as well as the reputation and profit of brand owners [1,2,3]. And with the development of economic globalization, the proliferation of these problematic products will also affect the image of the country’s products, thus losing its advantage in international competition [4]. To maintain the healthy development of the market and the safety of people’s life and property, in addition to using legal means to crack down on the production and sale of counterfeits, countries around the world have been continuously exploring various advanced anti-counterfeiting technologies [5], for example, multiple laser anti-counterfeiting, 1D barcode anti-counterfeiting, 2D barcode anti-counterfeiting, temperature change anti-counterfeiting, watermark anti-counterfeiting, fluorescent anti-counterfeiting, etc. [6,7,8,9,10,11]. Among them, fluorescent materials with rare-earth doped inorganic substances are favored because of their photochemical stability, rich energy levels, and ability to achieve multi-color emission from UV to NIR [12,13,14,15]. For example, Xue Bai et al. prepared CaWO₄: Yb³⁺, Er³⁺, Bi³⁺ photochromic phosphors showing reversible photochromic and dual-mode upconversion (UC) and downshifting (DS) luminescence properties, which have potential applications as composite anti-counterfeits and optical storage media [16]. Mengxiao Li et al. synthesized dual-mode luminescent NaYF₄: Er, Yb(Tm) carbon quantum dots by a facile solvothermal method and successfully printed a variety of dual-mode fluorescent patterns using a mixture of a fluorescent material and an aqueous poly(acrylic acid) solution as a colorless anti-counterfeit ink, showing good dual-mode fluorescent properties [17].

Although these materials have achieved many remarkable results, these materials are simply phosphors or fluorescent inks, which have many problems, such as the single excitation mode, not being easy to preserve, not being recyclable, etc. [18,19]. Moreover, as the counterfeiting technology continues to iterate and update, the possibility of its being cracked or counterfeited remains high. In recent years, the application of a rare earth anti-counterfeiting film in anti-counterfeiting has become increasingly widespread. These films usually appear colorless and transparent in daylight but show specific anti-counterfeiting patterns when exposed to UV light, etc. This design combines the properties of both thin film and rare-earth light-emitting materials, enabling more advanced anti-counterfeiting. Polydimethylsiloxane (PDMS) is known for its high chemical and thermal stability, corrosion resistance, flexibility, and repeatability, and it is widely used in electronic components, sensors, fiber-optic waveguide coatings, and circuit board packaging [20,21,22,23]. The composite of rare-earth fluorescent material and PDMS elastic substrate can prepare an excellent flexible anti-counterfeiting material by combining the luminescent properties of rare-earth fluorescent material with the compressible and stretchable physical properties of PDMS.

Recently, Eu²⁺-activated Ba/Sr-based aluminate phosphors have been extensively studied, and the phosphors have unique luminescence properties through Ba/Sr mutual substitution. Daisuke Nakauchi et al. synthesized Eu-doped Ba_xSr_1−xAl₂O₄ (x = 0, 0.1, 0.3, and 0.5) single crystals by the floating zone method [24]. They investigated their scintillation properties, observing the high optical yield of Eu: Ba_0.5Sr_0.5Al₂O₄. Max Volhard synthesized Sr_0.97−xBa_xEu_0.03Al₂O₄ (x = 0–0.97) phosphor by the solid-phase reaction and studied the phase change and luminescence characteristics with the changing Ba content [25]. Marcos et al. investigated the X-ray excited optical luminescence (XEOL) properties of Eu-doped Ba_1−xSr_xAl₂O₄ phosphors. They found that the XEOL yield was higher in the Ba-rich samples and the decay rate of long-lasting phosphorescence (LLP) was slower in the Sr-rich samples [26].

In this study, Ba_0.5Sr_0.5Al₂O₄: xEu²⁺ (x = 0.01, 0.02, 0.03, 0.04, 0.06) phosphor was synthesized by a simple combustion method. To study the optical properties of the phosphor, the synthesized samples were measured and characterized by scanning electron microscope (SEM), photoluminescence (PL), X-ray diffraction (XRD), etc. Additionally, the optimal doping concentration of Eu ion x = 0.02, and the optimal reaction temperature was 700 °C. Finally, fluorescent flexible films and anti-counterfeiting inks were prepared using the optimized samples, showing their potential applications in the field of anti-counterfeiting.

2. Experimental Section

2.1. Materials and Method

A series of Ba_0.5Sr_0.5Al₂O₄: xEu²⁺ (x = 0.01, 0.02, 0.03, 0.04, 0.06) samples were prepared by the combustion method. The raw materials used were BaCO₃ (purity: 99.99%), SrCO₃ (99.99%), Al₂O₃ (99.99%), Eu₂O₃ (99.99%), HNO₃ (80%), and urea (99.99%), which were all purchased from Tianjin Chemical Reagent Factory, Tianjin, China. The deionized water was homemade. All reagents were directly used as received without further purification.

2.2. Synthesis of Nanomaterials

First, appropriate amounts of HNO₃ and deionized water were added to BaCO₃, SrCO₃, Al₂O₃, and Eu₂O₃ to prepare 0.2 mmol/mL of Ba(NO₃)₂, 0.5 mmol/mL of Sr(NO₃)₃, 1 mmol/mL of Al(NO₃)₃, and 0.1 mmol/mL of Eu(NO₃)₂ solutions, respectively. Then, Ba(NO₃)₂, Sr(NO₃)₃, Al(NO₃)₃ and Eu(NO₃)₂ solutions were measured in stoichiometric ratios (0.5 − 0.5x:0.5 − 0.5x:2:x) and placed in a crucible, then 2.2 g of urea for the combustion agent was added and mixed homogeneously, and placed in a preheated muffle furnace. After waiting 3 to 5 min, a loose white and porous solid powder is obtained. A series of samples can be obtained by adjusting the Eu ion doping concentration (x = 0.01, 0.02, 0.03, 0.04, 0.06) and reaction temperature (500, 600, 700, 800, 900 °C). Ultimately, the obtained samples were naturally cooled to room temperature and ground into powder for a test.

2.3. Preparation of Anti-Counterfeit Flexible Film and Ink

Polydimethylsiloxane (PDMS, Sylgard184, Dow Corning, Midland, MI, USA) was used to assemble the elastomer for Ba_0.5Sr_0.5Al₂O₄: Eu²⁺. The milled phosphor was mixed with the uncured PDMS material and put into a mold, and then the molds were dried at 100 °C for 1 h to obtain the “ML” pattern composite phosphor. The final flexible, stretchable anti-counterfeit film was obtained by placing the resulting “ML” composite in an uncured PDMS matrix and curing it at 120 °C for 40 min.

The fluorescent powder was introduced into a mixed solution of ethanol and polyacrylic acid, followed by vigorous stirring. The amounts of polyacrylic acid and ethanol were adjusted to attain a viscosity suitable for screen printing, resulting in the final fluorescent ink.

2.4. Characterizations

The crystalline phases were collected on an X-ray diffraction (Rigaku D/Max-2400, Rigaku, Tokyo, Japan) with Cu Kα radiation (λ = 1.54 Å), operating at 40 kV and 150 mA. The PLE and PL spectra were measured by a fluorescence spectrometer (FluoroMax-4, Horiba, Ltd., Kyoto, Japan). The morphology of phosphors was observed through scanning electron microscopy (Quanta 250 FEG, Thermo Fisher Scientific, Waltham, MA, USA). The fluorescence lifetime and quantum yield of phosphors were measured by a transient steady-state fluorescence spectrometer (FLS1000, Edinburgh Instruments, Edinburgh, UK). The UV-Vis absorption spectra were performed using a UV-3600 UV-Vis Near Infrared Spectrophotometer (Shimadzu Corporation, Kyoto, Japan). The energy-dispersive X-ray spectroscopy (EDS) spectrum was obtained with a desktop scanning electron microscope energy dispersive spectrometer (SU8020, Hitachi, Ltd., Hitachi, Japan).

2.5. Theoretical Calculations

The structural optimization and electronic structure calculations of Ba_0.5Sr_0.5Al₂O₄ were performed using density functional theory (DFT) based on first-principles calculations. The structure consists of a unit cell. All DFT calculations were performed using the Vienna ab initio Simulation Package (VASP) [27]. The Perdew–Burke–Ernzerhof (PBE) exchange–correlation functional and projector-augmented wave (PAW) pseudopotentials were employed for spin-polarized calculations [28,29,30]. During the structural optimization process, a convergence criterion of 10⁻⁶ eV for the total energy was set, and atomic positions were relaxed until the forces acting on each atom were less than 0.01 eV/Å. Gaussian smearing with a width of 0.05 eV was used for the occupation of the electronic orbitals. A plane-wave cutoff energy of 500 eV was applied in all calculations. The Brillouin zone integration was performed using the Monkhorst–Pack (MP) special k-point mesh with a spacing of 0.03 Å⁻¹.

3. Results and Discussion

3.1. Structure and Morphology Analysis of BSAO: Eu²⁺

The phase composition and purity of Ba_0.5Sr_0.5Al₂O₄ (abbreviated as BSAO) phosphor were investigated by X-ray powder diffraction. Figure 1a shows the XRD patterns of the BSAO host and BSAO: xEu²⁺ (x = 0, 0.01, 0.02, 0.03, 0.04, 0.06) phosphors at a reaction temperatures of 600 °C. Most of the diffraction peaks of the samples can be indexed to the standard data of PDF#97-015-3164. However, for the BSAO sample, some byproducts can be observed, as noted by the rhombus. The impurity phase is inferred to be BaO, and the weight ratio to be approximately 0.4%, by a simple quantitative analysis, which can be almost ignored on the effect of the luminescence properties. According to the XRD standard cards, it is identified that the BSAO belongs to the P6₃(173) space group of the hexagonal phase. The cell parameters a = b = 8.972 Å, c = 8.606 Å calculated for BSAO are between BaAl₂O₄ (a = b = 10.449 Å, c = 8.793 Å) and SrAl₂O₄ (PDF#97-015-3164) [31,32]. Generally, it is reasonable to assume that the Eu²⁺ dopants tend to occupy the Sr²⁺ or Ba²⁺ sites based on similar effective ionic radii (IR) of the cation with varying coordination numbers (CNs) and valences [33,34,35]. Moreover, the main diffraction peak of (112) shifts toward a higher angle with the introduction of Eu ions as shown in Figure 1b. It means that the lattice parameters and the cell volume of the substituted samples will be reduced to some extent, which is in line with the shift of the diffraction peaks, which could be well accepted based on Bragg’s law (2d sin $\theta$ = n$\lambda$) [36,37]. It confirms again that the Eu ions with smaller ionic radii are constantly replacing the potentially substitutable ions. Moreover, the presence of additional peaks at about 20° of the 2$\theta$ angle may be due to the unstable crystalline phase of the sample caused by the insufficient reaction temperature.

The effect of the reaction temperature on the phase and morphology of the final sample will be further investigated. Figure 1c shows the XRD patterns of BSAO: 0.02Eu²⁺ synthesized at different reaction temperatures. With the increase in temperature, the strength of the major diffraction peak progressively grows, and the position of its diffraction peak is basically unchanged compared with Figure 1a. It means that the crystals of the BSAO: 0.02Eu²⁺ samples are formed successfully at different reaction temperatures and crystallized better with increasing temperature. In addition, the XRD pattern of the sample shows a decrease in the impurity peak when the reaction temperature is 700 °C. Furthermore, the intensity of the extra diffraction peak at the 2$\theta$ angle of about 20° gradually decreases with the increase in the reaction temperature. When the reaction temperature reaches 900 °C, the extra peaks disappear, and the crystalline phase starts to stabilize. The influence of the reaction temperature on the microscopic morphology of the phosphor is investigated using SEM pictures. Figure 2 depicts the crystal morphology of BSAO: 0.02Eu²⁺ produced at different reaction temperatures. In all five of these samples, the phosphor particles are aggregated into tiny clusters of varied shapes and sizes. In addition, there are lamellar structures with fractures and air pores. At 500–600 °C, the gaps in the sample structure progressively become smaller and gradually tend to be united, and at 700 °C, the gaps are negligible, showing a laminar structure, and the structure shows no obvious fracture symptoms. At 800–900 °C, the lamellar structure disappears gradually, and the gaps in the sample structure keep rising again, which may be caused by the violent outward burst of the gas created during the burning of the sample [38]. The irregularities in the shape, size, and pores of these samples may be related to the irregular mass flow and unequal temperature distribution of the samples during combustion [39]. Moreover, the existence of the elements Ba, Sr, Al, O, and Eu is demonstrated by the EDS spectrum of BSAO: 0.02Eu²⁺ as shown in Figure 2f. The ratio of Ba to Sr can be observed to be close to 1:1, and the successful doping of Eu ions into the host material is achieved.

3.2. UV-Vis Absorption and Photoluminescence Properties of BSAO: Eu²⁺

UV-vis absorption spectroscopy was used to study the absorption performance of BSAO: 0.02Eu²⁺ (Figure 3a). The absorption spectrum shows two absorption peaks from 230 to 400 nm. Absorption near 230 nm is attributed to the host band absorption. The broad absorption band covering the spectral range of 300–400 nm is assigned to the 4f→5d electronic transitions of Eu²⁺. To experimentally estimate the bandgap scale, the E_g of the BSAO: 0.02Eu²⁺ can be estimated according to the following equation [40]:

$${\left(\alpha h\nu \right)}^{1/n}=A(h\nu -E_g)$$

(1)

where hν is the photon energy, $\alpha$ is the absorption coefficient, E_g is the band gap energy (eV), and A is a constant. The value of n is determined according to the type of interband conversion. It is equal to 2 for indirect transitions and 1/2 for direct transitions [41]. In addition, the intermediate optical band gap ($E_g^{\prime }$) can be obtained by the linear fit in the flat part in Figure 3b. Then, the value of n is calculated using the following equation [42]:

$$n=ln\left(\alpha h\nu \right)/ln(h\nu -E_g^{\prime })$$

(2)

As shown in Figure 3c, the value of n is approximated by 1/2. Thus, the plot of (αhν)² versus (hν − E_g) is shown in Figure 3d. The absorption Tauc plot shows that the band gap is 4.96 eV.

The luminescence properties of the BSAO: Eu²⁺ phosphors are investigated in detail by photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy. It is found that the peak positions and spectral shapes of emission spectra do not vary with Eu²⁺ concentration for the BSAO: Eu²⁺ nanophosphor, but the intensity of emission peaks for BSAO: xEu²⁺ (x = 0.01, 0.02, 0.03, 0.04, and 0.06) nanophosphor strongly depends on the doping concentration of Eu²⁺ ions. As shown in Figure 4a, the PLE spectrum of BSAO: 0.02Eu²⁺ encompasses the UV to the blue area and exhibits two broad absorption bands centered at 365 and 303 nm, which is attributable to the 4f⁷→4f⁶5d transition of Eu²⁺ [43]. Figure 4b,c show the emission spectra of the samples under UV excitation at 303 and 365 nm, and their emission peaks are 443 and 501 nm, respectively. The PL spectrum shows a broad asymmetric band with a maximum at 443 nm under 303 nm excitation, corresponding to the allowed 4f⁶5d→4f⁷ transition of the Eu²⁺. As shown in Figure 4c, the narrow-band green emission, correlating to the 5d-4f transition of Eu²⁺, is observed under 365 nm excitation. The full width at half-maximum (FWHM) of the emission spectrum is around 62 nm. Furthermore, concentration quenching typically occurs in the emission center when certain structural defects and nonradiative transition rates reach a saturation state. In general, the critical value of the concentration (having intense emission) is determined as the optimal doping concentration. Therefore, the strongest PL emission intensities of the BSAO: Eu²⁺ phosphors are recorded when x = 0.02, revealing that the optimal doping concentrations of Eu²⁺ ions in the BSAO host are 2% as illustrated in Figure 4d. Moreover, it should be clarified whether the mechanism of concentration quenching is the critical reason for their different optimal concentrations. Indeed, the energy transfer concentration quenching mechanism can be investigated by calculating the interaction coefficient (Q) as shown below [44,45,46]:

$$log\left(I\right./x)=A-\left(Q\right./3)logx$$

(3)

where x denotes the Eu²⁺ ion concentration, and I is the luminescence intensity. In this respect, the fitting slope (−(Q/3)) is determined to be −1.72 and −2.25, respectively. The interaction coefficient (Q) values are analyzed to be about 5.16 ($\lambda$_ex = 303 nm) and 6.75 ($\lambda$_ex = 365 nm) as shown in Figure 5. All of them are close to 6, suggesting that the dipole–dipole interaction (Q = 6 ) is the main mechanism to control concentration quenching rather than the dipole–quadrupole (Q = 8) or quadrupole–quadrupole (Q = 10) interaction.

The reaction temperature is a highly essential aspect that directly impacts the structure and luminescent properties of the sample. The produced solutions are sintered at 500, 600, 700, 800, and 900 °C, and the emission spectra of the synthesized products are given in Figure 6. When the reaction temperatures are different, the characteristics of the spectra do not change significantly under the same wavelength of excitation, except for the emission intensity. Figure 6a,b show the emission spectra of the samples under the excitation of the UV light at 303 and 365 nm. It shows a broad emission band centered at 442 and 500 nm, respectively. The luminescence intensities of the samples all reach the highest when the reaction temperature is 700 °C. These blue-green emission bands are attributed to the 4f⁶5d→4f⁷ electron transition of the Eu²⁺ ion. In addition, the emission bandwidth of the samples becomes narrower when the reaction temperature is 800 °C. It may be due to the improved crystallinity of the material at higher reaction temperatures, which reduces the defects and impurities that may lead to broader emission bands. Figure 6c provides a thorough illustration of this procedure.

Figure 6d depicts the coherent infrared energy (CIE) chromaticity coordinate positions of the BSAO: 0.02Eu²⁺ phosphor under the excitation wavelength of 303 nm and 365 nm. The corresponding CIE chromaticity coordinates are (0.166, 0.208) and (0.176, 0.365). It is observed from Figure 6d that the CIE chromaticity coordinate varies with the excitation wavelength and the corresponding colors are changed from blue to green.

Moreover, the luminescence decay curves of the BSAO: 0.02Eu²⁺ phosphor ($\lambda$_ex = 303 nm, $\lambda$_em = 442 nm; $\lambda$_ex = 365 nm, $\lambda$_em = 501 nm) are shown in Figure 7, and the decay curves are well fitted by a second-order and triple-order exponential function expressed as follows [47]:

$$I\left(t\right)=I_0+A_{1}exp(-t/{\tau }_1)+A_{2}exp(-t/{\tau }_2)+\dots$$

(4)

where I₀ represents the background constant, A₁ and A₂ are constants, t is the decay time, $\tau$₁ and $\tau$₂ refer to the decay times of the exponential components and the average lifetimes of all BSAO with different Eu²⁺ doping concentrations are calculated by the following equation:

$${\tau }^{*}=(A_1\times {\tau }_1^2+A_2\times {\tau }_2^2+\dots )/(A_1\times {\tau }_1+A_2\times {\tau }_2+\dots )$$

(5)

According to the formula, the average lifetime of BSAO: 0.02Eu²⁺ is 1010 ns ($\lambda$_ex = 303 nm) and 112 ns ($\lambda$_ex = 365 nm), respectively. Furthermore, the photoluminescence quantum yield (PLQY) is an important parameter for luminescent materials. The QY of the phosphors can usually be measured according to the percentage of the photon number emitted by the sample ($\epsilon$) versus the photon number absorbed by the sample ($\alpha$). The PLQY spectrum of the BSAO: 0.02Eu²⁺ phosphor is presented in Figure 7c,d. The QY can be determined by the following equation [48]:

$$PLQY=\epsilon /\alpha =S_{em}/(S_0-S)$$

(6)

where S_em is the integrated intensity of the emission light of the phosphor, S₀ and S is integrated with the intensity of the scattered light of the reference background plate and the phosphor, respectively. Thus, the absorbed photon number of the BSAO: 0.02Eu²⁺ phosphor is obtained by subtracting the scattered light intensity (S) of the BSAO: 0.02Eu²⁺ phosphor from the scattered light intensity (S₀ ) of the reference background plate. Therefore, it is clear that the QY could reach up to 10.5% ($\lambda$_ex = 303 nm) and 10.3% ($\lambda$_ex = 365 nm). The detailed parameters of the lifetimes and QY are shown in

Table 1. Parameters of fluorescence lifetime decay curves of BSAO: 0.02Eu²⁺ and PLQY.

Excitation	PLQY	Decay Lifetimes (ns)
		${\mathbf{\tau }}_{\mathbf{1}}$	${\mathbf{\tau }}_{\mathbf{2}}$	${\mathbf{\tau }}_{\mathbf{3}}$	${\mathbf{\tau }}_{\mathbf{avg}}$
303 nm	10.5% ± 0.01%	407.71	1330.3	null	1010 ± 0.03
365 nm	10.3% ± 0.05%	36.94	368.6	1.4	112 ± 0.02

.

3.3. Electronic Structure

The band structure and density of states (DOSs) of the BSAO host are calculated using density functional theory (DFT). Considering the mutual substitution between Ba and Sr and their atomic ratios close to 1:1 (Figure 2f), we design a structural model of Ba_0.5Sr_0.5Al₂O₄. The structural model is based on the substitution of 50% of Sr atoms with Ba in the SrAl₂O₄ system. As shown in Figure 8a,b, the valence band maximum (VBM) and the conduction band minimum (CBM) are located at the same point, indicating that BSAO exhibits a direct band gap (E_g). The predicted value of E_g is approximately 4.05 eV. In addition, as shown in Figure 8d, the experimental band gap of the BSAO host can be obtained by Equation (1). The experimental E_g (5.18 eV) is slightly larger than the calculated E_g (4.05 eV), which can be attributed to the general limitations of the generalized gradient approximation scheme in the DFT calculations [49,50]. Figure 8c displays the total density of states (TDOS) and partial density of states (PDOS) of the BSAO host. It is observed that the d orbitals of the Ba and Sr atoms predominantly contribute to the DOS at the CBM, while the VBM is primarily derived from the p orbitals of the O atoms.

3.4. Morphology Control of BSAO: Eu²⁺ via Surfactants

Numerous factors, such as grain size, shape, and dimensionality can alter the luminescence of rare earth ions. Surfactants are extensively employed to modulate inorganic nanocrystal size, shape, and crystallinity [51]. To examine the effect of surfactants on the morphology of fluorescent materials, BSAO: 0.02Eu²⁺ phosphors were synthesized with the assistance of surfactants. Figure 9 shows the XRD patterns of BSAO: 0.02Eu²⁺ at a reaction temperature of 700 °C with the addition of different surfactants (polyvinyl alcohol (PVA), ethyl acetate (EA), and polyethylene glycol (PEG)). The results show that the impurity peaks decrease with the entry of the surfactant, and the position of the main diffraction peak is basically the same as that of the diffraction peak without the addition of the surfactant. In particular, the addition of PVA and PEG shows a significant reduction in the additional peaks at about 20° of the 2$\theta$ angle. Moreover, the intensity of the main diffraction peak of BSAO: 0.02Eu²⁺ with the addition of PVA is higher, which indicates that the samples with the addition of PVA are better crystallized. Compared with the samples without the surfactant addition, the development of the samples is significantly improved. Crystals with higher crystallinity generally have fewer trapping sites and exhibit stronger luminescence. Figure 10 shows the SEM images of BSAO: 0.02Eu²⁺ phosphor without and with different surfactants added. Figure 10b,c,d are the SEM images of the samples doped with surfactants PVA, EA, and PEG, respectively. By comparison, it is found that the samples with the addition of PVA are more compact and show an obvious lamellar structure. Therefore, adding surfactant PVA can improve the morphology of BSAO: Eu²⁺ phosphor.

3.5. Anticounterfeiting Application

To further investigate the anti-counterfeiting properties of phosphors, we fabricate PDMS composite films by embedding optimized Eu²⁺ doped BSAO phosphors in the polydimethylsiloxane (PDMS) polymer matrix. The fabricated luminescent film is well proportioned with flexible performance as presented in Figure 11a. As shown in Figure 11b,c, the BSAO: 0.02Eu²⁺/PDMS films emit blue and green emissions under 303 nm and 365 nm UV light excitation, respectively. Figure 11d,e show SEM images of the surface and a cross section of the composite film with the letter portion, respectively. It can be seen that the particles are well dispersed in the PDMS substrate. Also, the emission spectra of PDMS film are recorded to confirm that the featured transitions of Eu²⁺ ions are barely affected by the PDMS film as shown in Figure 12. In addition, the good physical and chemical stability and thermal stability of the phosphor-PDMS film are confirmed [52,53,54]. It means that the resulting luminescent film can meet the requirements of our daily lives and that the performance will not be compromised, even in more extreme environments. Furthermore, fluorescent inks are also prepared for anti-counterfeiting marks and screen-printing applications. As shown in Figure 13a, the ink is coated on the banknote, and the coated area is not significantly different from other parts of the substrate under daylight. However, fluorescence emission can be observed under UV light. Finally, as depicted in Figure 13b, the prepared ink is printed on white paper using screen-printing technology, forming a four-leaf clover pattern. The pattern is almost invisible under visible light but becomes clearly visible under UV light. From the above findings, we suggest that the Eu²⁺ ion-activated BSAO/PDMS composite films and ink are anticipated for high-level anti-counterfeiting applications.

4. Conclusions

In summary, BSAO: Eu²⁺ phosphors with the hexagonal phase are successfully synthesized via a simple combustion reaction from XRD patterns and EDS spectra. The optical band gap, concentration quenching mechanism, and optimal doping concentration are confirmed. Meanwhile, the optimal BSAO: 0.02Eu²⁺ has a blue-green emission band under the excitation of UV lamps due to the 4f-5d transition of Eu²⁺. In addition, the fluorescence lifetime and the quantum yield are determined as 1010, 111.98 ns, 10.49%, and 10.25%, respectively. Furthermore, an anti-counterfeiting strategy is presented, utilizing phosphors/PDMS films that offer flexibility and exhibit distinct colors when exposed to different UV wavelengths. Finally, by blending the fluorescent powder with ethanol and polyacrylic acid, a transparent ink is prepared, demonstrating the excellent anti-counterfeiting application potential of the fluorescent powder.