Synthesis and Upconversion Luminescence Properties of BaBiO₂Cl:Yb³⁺,Er³⁺ Phosphor

Abstract

The interaction of near-infrared (NIR) light with matter that produces high-energy visible light emissions is known as photon upconversion, which has shown promising applications in different fields, including optoelectronics, biomedicine and photovoltaics. In this paper, a novel BaBiO₂Cl:Yb³⁺,Er³⁺ upconversion phosphor was successfully synthesized through a simple high-temperature solid-state reaction route. The crystal structure, phase purity, microstructure and upconversion luminescence properties of the as-prepared phosphor were characterized comprehensively. The XRD and SEM results clearly demonstrate the successful synthesis of the target phosphors with high purity. When excited by a 980 nm NIR laser, the as-prepared BaBiO₂Cl:Yb³⁺,Er³⁺ phosphor exhibited intense red upconversion luminescence due to the Er^3+ 4F_9/2→⁴I_15/2 transition, which enabled this phosphor to have high promise for important applications, such as anti-counterfeiting and advanced photonics.

1. Introduction

Upconversion phosphors are an important part of inorganic luminescent materials, which have the appealing ability to produce light emissions with higher energy than that of the excitation light [1,2]. Typical examples of upconversion phosphors are crystalline matrixes that contain upconversion ion pairs of a sensitizer (e.g., Yb³⁺) and an activator (e.g., Er³⁺, Tm³⁺ or Ho³⁺) [3,4,5]. The upconversion luminescence of these materials involves an anti-Stokes photophysical process, that is, the sequential absorption of two or more low-energy NIR photons by the sensitizer and energy transfer to the activator leads to the emission of a high-energy photon [6,7,8]. Due to their fascinating optical features, such as narrow emission spectrum, long luminescence lifetime, larger Stokes shift and high luminescence stability, upconversion luminescence phosphors have demonstrated a broad range of technological applications, including solar energy utilization [9,10], anti-counterfeiting [11,12,13], photocatalysis [14] and optical thermometry [15,16,17]. In particular, excitation with low-energy NIR light can greatly minimize the autofluorescence background when compared with high-energy ultraviolet light excitation and enable a high excitation penetration depth in biological tissues in consideration of the reduced light absorption and scattering; thus, upconversion luminescence nanoparticles have promising potential for bioanalytical and biomedical applications, including in vivo bioimaging, biosensing and photodynamic therapy [18,19,20].

Until now, efficient upconversion luminescence has been reported by incorporating typical upconverting ion pairs into bulk fluoride-based or oxide-based compounds [21,22]. More importantly, the significant achievements in the controllable synthesis of monodisperse upconverting nanoparticles via a bottom-up synthesis route have boosted the emerging technological applications of upconversion luminescent nanomaterials [23,24,25]. Recently, bismuth-based inorganic matrixes have been recognized as promising host candidates for the development of next-generation upconversion luminescence materials owing to the inexpensive bismuth-related raw materials and the appealing optical features of these compounds, such as low photon energy and high refractive index [26]. For example, Lei et al. reported a highly facile and ultrafast synthesis route to prepare novel hexagonal-phase NaBiF₄ upconversion nanoparticles, which exhibited good monodispersity and excellent photoluminescence performance upon 980 nm NIR laser excitation [27]. Just recently, they synthesized hollow NaBiF₄:Yb,Er upconversion nanoparticles by a template-free route under solvothermal conditions [28]. An et al. reported the production of novel K_0.3Bi_0.7F_2.4:Yb³⁺,Ln³⁺ upconverting nanoparticles by a solvothermal strategy and demonstrated their applications for dual-modal imaging [29]. Back et al. demonstrated the preparation of lanthanide-doped Bi₂SiO₅ upconversion nanocrystals via the impregnation of mesoporous silica nanoparticles with a bismuth precursor followed by heat treatment at a high temperature [30]. Chen et al. reported on Bi₂SiO₅:Yb³⁺,Ln³⁺ (Ln = Er, Ho, Tm) upconversion nanophosphors with a uniform core–shell structure by an in situ chemical reaction between the bismuth precursor nanospheres and the outer SiO₂ layer at a suitable temperature [31]. Back et al. also reported a similar method to synthesize Bi₂SiO₅:Nd³⁺@SiO₂ nanoparticles for optical thermometry [32]. Soon afterward, Chen et al. reported an ultrafast and facile aqueous-phase synthesis route to prepare K_0.3Bi_0.7F_2.4-based upconversion nanocrystalline particles at a very low temperature [33,34]. Despite enormous efforts, developing new bismuth-based inorganic matrixes for efficient upconversion luminescence is still highly desired.

In this work, we report the synthesis of a novel BaBiO₂Cl:Yb³⁺,Er³⁺ upconversion phosphor using a high-temperature solid-state reaction approach. The crystal structure, phase composition, microstructure and upconversion luminescence properties of the as-prepared phosphors were investigated in detail. The advanced anti-counterfeiting application has been demonstrated by making use of the intense red upconversion luminescence from the BaBiO₂Cl:Yb³⁺,Er³⁺ phosphor.

2. Experimental

2.1. Materials Synthesis

BaBiO₂Cl:Yb³⁺,Er³⁺ phosphors were synthesized via a simple high-temperature solid-state reaction method. Stoichiometric amounts of Bi₂O₃, Yb₂O₃, Ln₂O₃ (Ln = Er, Ho, Tm) and NH₄Cl powders were thoroughly ground in an agate mortar. Then, the mixed powders were calcined at 500 °C in air for 3 h. After being mixed with Ba(NO₃)₂ and ground thoroughly, the powder samples were sintered at 900 °C in air for 12 h to obtain the final phosphors.

2.2. Characterization

The phase composition of the phosphor was identified by using a DMAX-2500PC powder X-ray diffractometer with Cu Kα₁ radiation (λ = 1.5406 Å). The microstructure and elemental composition of the phosphor were measured using a field-emission scanning electron microscope (SEM, JSM-7800F, JEOL). The spectroscopic investigations were conducted using an Edinburgh FLS1000 fluorescence spectrophotometer equipped with a 980 nm laser diode as the excitation source and a photomultiplier tube (measurement range: 200–900 nm). The upconversion luminescence decay curve was measured using a Horiba DeltaFlex fluorescence lifetime spectrometer.

3. Results and Discussion

Structure Characterization of the BaBiO₂Cl:Yb³⁺,Ln³⁺ Phosphors

BaBiO₂Cl crystallizes in the orthorhombic structure and adopts a Cmcm space group with a = 5.880, b = 12.945, c = 5.677 Å, V = 432.0 ų and Z = 4 [35,36]. Figure 1 depicts the simulated crystal structure of the BaBiO₂Cl compound. It is a member of the Sillen layered family that was first discovered and described by Sillén [37]. As shown in Figure 1, the separate oxygen and chlorine layers stack alternatively along the b axis. Meanwhile, the cations of Bi³⁺ and Ba²⁺ are located between the oxygen and chlorine layers with two distinct coordination environments. The metal–oxygen layers composed of [BaO₄] and [BiO₄] tetrahedrons are separated by the chlorine layers.

Figure 2 depicts the XRD patterns of the BaBiO₂Cl:Yb³⁺,Er³⁺ upconversion phosphors with varying Yb³⁺ doping concentrations. All diffraction peaks of the as-prepared BaBiO₂Cl:Yb³⁺,Er³⁺ phosphors coincide well with those of the BaBiO₂Cl crystal (JCPDS No. 83-0442), indicating that BaBiO₂Cl:Yb³⁺,Er³⁺ phosphors were synthesized with a pure phase. Moreover, introducing Yb³⁺-Er³⁺ ion pairs did not cause the formation of the second phase, even when the doping content of Yb³⁺ ions reached 10%. These results demonstrate that the added Yb³⁺ and Er³⁺ dopants were completely dissolved in the crystal lattice of the BaBiO₂Cl host lattice.

Analyzing the microstructures of the prepared BaBiO₂Cl:Yb³⁺,Er³⁺ phosphor particles using SEM, presented in Figure 3, the sample showed an irregular morphology with the average particle size ranging from 1–5 μm. Mapping the elemental distribution of a selected particle using EDS revealed that the elements of Ba, Bi, O, Cl, Yb and Er existed in the sample and these elements were distributed uniformly throughout the whole particle, as shown in Figure 3c,d.

Figure 4a shows the upconversion luminescence spectra of the BaBiO₂Cl:x%Yb³⁺,2%Er³⁺ (x = 0.5, 1, 3, 5, 7 and 10) phosphors with varying Yb³⁺ ion doping contents. As presented in Figure 4a, the dominant red emission band in a wavelength range of 625–725 nm was observed upon 980 nm laser excitation, which originated from the Er^{3+ 4}F_9/2→⁴I_15/2 electron transition. Moreover, the weak green upconversion bands with peak maxima at ~525 nm and ~545 nm were attributed to the ²H_11/2→⁴I_15/2 and ⁴S_3/2→⁴I_15/2 electron transitions of Er³⁺, respectively. As the doping content of Yb³⁺ increased from 0.5% to 5%, the integral upconversion luminescence intensity showed a significant increasing trend. However, the intensity of the upconversion emission monotonously decreased with a further increase in the doping concentration of Yb³⁺ up to 10%, which was probably caused by the concentration quenching mechanism. The fluorescence decay curve of the BaBiO₂Cl:5%Yb³⁺,2%Er³⁺ phosphor by monitoring 676 nm emission of Er³⁺ is also presented in Figure 4b. The average lifetime was determined to be 49 μs, which coincided with the lifetime of Er³⁺ in other studies [33].

To study the underlying upconversion luminescence mechanism in depth, the excitation power-dependent emission spectra were also recorded. Figure 5a presents the emission spectra of the BaBiO₂Cl:5%Yb³⁺,2%Er³⁺ phosphor with variations in the output power of a 980 nm laser. Upon 980 nm excitation, the BaBiO₂Cl:Yb³⁺,Er³⁺ phosphor could emit bright and intense red upconversion luminescence. It is noted that the red upconversion luminescence intensity (I) of the BaBiO₂Cl:5%Yb³⁺,2%Er³⁺ sample gradually increased with the increase in excitation power (P) frp, 1.78–21.32 W/cm² (50–600 mW). The fitting was also drawn in log(I)–log(P) coordinates, as depicted in the inset of Figure 5a. The fitting result showed that the slope of the straight line was 1.29 for the BaBiO₂Cl:5%Yb³⁺,2%Er³⁺ phosphor, revealing that the upconversion luminescence in the BaBiO₂Cl:Yb³⁺,Er³⁺ phosphor involved a two-photon excitation process (See Supplementary Materials Figure S1). As shown in Figure 5b, a 980 nm NIR photon could promote the Yb³⁺ sensitizer from the ²F_7/2 ground state to the ²F_5/2 excited state. Then, the excited Yb³⁺ transferred its energy to the Er³⁺, promoting the Er³⁺ ion from the ⁴I_15/2 ground state to the ⁴I_11/2 state. The excited Er³⁺ ion could absorb another 980 nm NIR photon and move to a ⁴F_7/2 excited state. When the ⁴F_7/2 state relaxed non-radiatively to the ⁴F_9/2 emitting state, bright red upconversion luminescence was observed in the BaBiO₂Cl:Yb³⁺,Er³⁺ phosphor.

In view of the intense red upconversion luminescence upon invisible 980 nm NIR excitation, white photoluminescence under 365 nm UV excitation (Figure S2) and the white body color of the prepared BaBiO₂Cl:Yb³⁺,Er³⁺ powders, their potential anti-counterfeiting application is shown in Figure 6. A schematic diagram of the anti-counterfeiting application is presented in Figure 6 (Top). A dual-mode flexible luminescence panel of “8888” was designed by making use of the BaBiO₂Cl:5%Yb³⁺,2%Er³⁺ powders, BaBiO₂Cl host materials and non-emissive white powders, which were used to fill different parts of the number “8888”, respectively. The designed panel exhibited a white body color under indoor lighting conditions. Upon 365 nm UV lamp excitation, the number “6839” is presented in white in the dark. However, only the BaBiO₂Cl:5%Yb³⁺,2%Er³⁺ powders in the panel could produce bright-red upconversion luminescence when excited by a 980 nm NIR beam, and the truthful number of “1234” could be clearly observed by the naked eye in the dark. The above results show that the as-prepared BaBiO₂Cl:Yb³⁺,Er³⁺ upconversion phosphors showed excellent upconversion luminescence performance; thus, they have promising applications as an inorganic pigment for advanced anti-counterfeiting.

4. Conclusions

In summary, a new BaBiO₂Cl:Yb³⁺,Er³⁺ upconversion phosphor was synthesized by a high-temperature solid-state reaction method. Not only could the BaBiO₂Cl host matrix accommodate a high content of trivalent lanthanides (Yb³⁺ and Er³⁺) without the introduction of a second phase, but they were also very easy to synthesize and air-stable once obtained. The prepared BaBiO₂Cl:Yb³⁺,Er³⁺ phosphor showed bright red upconversion luminescence upon 980 nm laser excitation. Considering the white body color of the phosphor powders under ambient light, the strong white photoluminescence upon excitation by a UV lamp and bright-red upconversion luminescence under 980 nm NIR laser excitation, the BaBiO₂Cl:Yb³⁺,Er³⁺ phosphor is expected to be used in advanced anti-counterfeiting applications. The outcomes of this work will lay the groundwork for the design and development of novel bismuth-based upconversion phosphors.