Narrow UVB-Emitted YBO₃ Phosphor Activated by Bi³⁺ and Gd³⁺ Co-Doping

Abstract

Y_0.9(Gd_xBi_1−x)_0.1BO₃ phosphors (x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0, YGB) were obtained via high-temperature solid-state synthesis. Differentiated phases and micro-morphologies were determined by adjusting the synthesis temperature and the activator content of Gd³⁺ ions, verifying the hexagonal phase with an average size of ~200 nm. Strong photon emissions were revealed under both ultraviolet and visible radiation, and the effectiveness of energy transfer from Bi³⁺ to Gd³⁺ ions was confirmed to improve the narrow-band ultraviolet-B (UVB) (⁶P_J→⁸S_7/2) emission of Gd³⁺ ions. The optimal emission was obtained from Y_0.9Gd_0.08Bi_0.02BO₃ phosphor annealed at 800 °C, for which maximum quantum yields (QYs) can reach 24.75% and 1.33% under 273 nm and 532 nm excitations, respectively. The optimal QY from the Gd³⁺-Bi³⁺ co-doped YGB phosphor is 75 times the single Gd³⁺-doped one, illustrating that these UVB luminescent phosphors based on co-doped YBO₃ orthoborates possess bright UVB emissions and good excitability under the excitation of different wavelengths. Efficient photon conversion and intense UVB emissions indicate that the multifunctional Gd³⁺-Bi³⁺ co-doped YBO₃ orthoborate is a potential candidate for skin treatment.

1. Introduction

Skin treatment using artificial sources of ultraviolet (UV) radiation in controlled conditions is well established, and narrow-band ultraviolet-B (UVB) therapy has been demonstrated to be effective against skin diseases and disorders such as psoriasis, vitiligo and hyperbilirubinemia (commonly known as infant jaundice) [1,2,3,4]. Phototherapy with narrow-band UVB (310–313 nm) as photosensitizers is believed to result from the direct interaction between the light of certain frequencies and tissues, causing a change in immune response [5,6,7]. Furthermore, during phototherapy investigations, it was observed that light belonging to longer wavelengths of the UVB region was more effective, while that of the shorter wavelengths was much less effective or even harmful [8,9]. Rare-earth (RE) orthoborates (RE-BO₃, RE = lanthanide, yttrium, and scandium) have aroused considerable interest due to their wide range of applications in plasma display panels and mercury-free fluorescent lamps [10,11]. In particular, YBO₃ is an excellent host for UV phosphors due to its high-vacuum UV transparency, exceptional optical damage thresholds, strong absorption in the UV range, and good chemical inertness [12,13,14]. Additionally, the YBO₃ phosphors exhibit a wide bandgap and high host-to-activator energy transfer efficiency at moderate RE³⁺ concentrations [15]. Therefore, it is of great interest to investigate RE-doped YBO₃ orthoborates for UVB treatments.

Among the RE ions, lanthanide gadolinium (Gd³⁺) is of particular interest because of its ubiquitous nature (well known as U-spectrum) and the characteristic narrow-band UVB emission from ⁶P_J→⁸S_7/2 transitions [16,17]. The optical properties of Gd³⁺ ions have been widely studied, and many Gd-doped compounds can be used as efficient phosphors in the new generation of UV fluorescent lamps. Moreover, as a promising activator or sensitizer, the Bi³⁺ ion shows excellent emission and absorption ability in the UV region. Furthermore, the transitions of ⁸S_7/2→⁶I_J (emission at ~270 nm) and ⁸S_7/2→⁶P_J (at ~311 nm) of the Gd³⁺ ion overlap with the ³P₁→¹S₀ (at ~260 nm) transition of the Bi³⁺ ion in YBO₃ [18,19], which permits an efficient energy transfer from the Bi³⁺ to Gd³⁺ ions. Further research on improving the UVB emission has also been reported [18]. From a practical point of view, UV-emitting phosphors in well-defined regions are required for various applications. Keeping this in mind, we prepared a UVB-emitting Gd³⁺-Bi³⁺ co-doped YBO₃ phosphor in this work, which can effectively achieve light conversion and UVB emissions. When UV fluorescence is irradiated on the surface of a skin wound, the activity of the mitochondrial catalase can increase in cells, which could promote the synthesis of proteins and the decomposition of adenosine triphosphate (ATP), ultimately healing the wound. A schematic representation of healing, which adopts phosphors as light conversion layers, is conceived in Figure 1.

In this work, narrow-band UVB-emitting phosphors of Gd³⁺-Bi³⁺ co-doped Y_0.9(Gd_xBi_1−x)_0.1BO₃ (YGB, x = 0, 0.2, 0.4, 0.6, 0.8 and 1.0) were fabricated by high-temperature solid-state synthesis. The samples with the hexagonal phase and well-dispersed particles were characterized by XRD and SEM techniques, manifesting a microsize of ~200 nm. The responses to UV and the visible (VIS) radiation of these YGB phosphors were compared, and sharp UVB luminescence was recorded with the adjustment of Gd³⁺ content. The sintering temperature indicated that co-doped Bi³⁺ ions enhanced the characteristic UVB luminescence from Gd³⁺ ions. The spectroscopic intensity parameters of YGB phosphors were derived from relative spectral power distributions, and the maximum quantum yields (QYs) at 313 nm were calculated at 24.75% and 1.33% under 273 nm and 532 nm excitations, respectively. YGB orthoborate phosphors with intense UVB emission could provide a viable approach for developing multifunctional composite materials for skin treatments.

2. Materials and Methods

The powders of Y_0.9(Gd_xBi_1−x)_0.1BO₃ (x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0, marked as YGB-0, YGB-0.2, YGB-0.4, YGB-0.6 YGB-0.8, and YGB-1.0, respectively) phosphors were prepared using high-purity reagents Y₂O₃ (99.9%), Gd₂O₃ (99.9%), Bi₂O₃ (A.R.), and H₃BO₃ (A.R.) as raw materials. The original chemicals for YGB with different Bi³⁺ contents as the designed sensitizer were mixed by grinding them in an agate mortar according to the stoichiometric ratio. The raw powders were transferred into alumina crucibles and pre-sintered at 500 °C for 1 h and then sintered at 700 °C, 800 °C, 900 °C and 1000 °C for 5 h. Afterwards, the samples were ground thoroughly after cooling.

The phase and the crystal structure of powders were identified by an X-ray diffractometer (XRD, MiniFlex 600, Rigaku, Tokyo, Japan) using Cu Kα radiation. Morphologies of the powders were analyzed by a field emission scanning electron microscope (SEM, JSM-7800F, JEOL, Tokyo, Japan) equipped with energy dispersive spectroscopy (EDS, X-MaxN 50, Oxford, Oxford, UK) using an accelerating voltage of 15 kV. Particle size distributions were measured in a Nanoparticle Analyzer (Zetasizer Nano-ZS, Malvern, UK). Photoluminescence (PL) spectra were recorded using a Keithley 2010 multimeter and the monochromator (λ500, Zolix, Beijing, China) equipped with a Si detector (DSi200, Zolix, Beijing, China). A commercial Xe lamp and two solid-state lasers emitting at 266 nm and 532 nm were used as the excitation sources for different excitation wavelengths. A standard PTFE diffuse reflective white plate (reflectivity greater than 99.9%) was used as a reference. The schematic diagram of the experimental setup is depicted in Figure 2. The relative spectral power distribution was obtained and calibrated by the Optical Power Meter (1830-C, Newport, Newport County, RI, USA). All measurements were performed at room temperature.

3. Results

3.1. Structure and Morphology

Figure 3a shows the XRD patterns of YGB phosphors with different compositions annealed at 800 °C, the main peaks of which are in agreement with the JCPDS Card of YBO₃ (PDF#16-0277). The phosphors were confirmed as polycrystalline materials that possess a hexagonal crystal structure with space group P63/m and the cell parameters of a = b = 3.778 Å and c = 8.81 Å, similarly to what has been previously reported [20]. Moreover, the well-defined sharp diffraction peaks imply that these samples have high crystallinity, illustrating that Gd³⁺ and Bi³⁺ ions are substituted within the host. Some small impurity peaks are identified as Bi₆B₁₀O₂₄ (PDF#29-0228), which are attributed to the interaction of Bi₂O₃ and excess H₃BO₃ during the fabrication process [21]. The corresponding reaction equation is as follows: 2Bi₂O₃ + B₂O₃→Bi₄B₂O₉ and 3Bi₄B₂O₉ + 7B₂O₃→2Bi₆B₁₀O₂₄ [22]. Here, the XRD peaks (2θ = ~27.2°) of YGB samples with slightly smaller angles, in comparison with the standard YBO_3, should be attributed to the larger radius of Gd³⁺ (1.053 Å, 8-coordination) and Bi³⁺ (1.170 Å, 8-coordination) relative to that of Y³⁺ (1.019 Å, 8-coordination), according to Bragg’s law [23,24,25], while the shift in diffraction peaks resulted from the different Gd³⁺-Bi³⁺ contents in these phosphors. The slight change is also attributed to the surface charge redistribution of the crystal nucleus, induced by an inner-electron charge transfer between the doped ions and lattice cations [26,27]. Therefore, these results show that doped Gd³⁺ and Bi³⁺ ions do not affect the main crystal structure of YBO₃ and should be completely dissolved into the host lattice.

As shown in Figure 3b, the impurities decrease, and the relative intensities of diffraction peaks initially increase and then decrease with the annealing temperature; this is attributed to the melting point of Bi₆B₁₀O₂₄ and the selectivity of the growth in the solid-state synthesis process [25,28]. In addition, the YGB crystal is composed of 8-coordinated Y³⁺ and 4- coordinated B³⁺ ions, which is illustrated in Figure 3c. Here, the Y³⁺ ions are 8- coordinated with two nonequivalent environments, while the B³⁺ ions and two interconnected BO₄ tetrahedral coordination form (BO₃)³⁻ groups [29]. Moreover, due to the similar ionic radii of the 8-coordinated Y³⁺, Bi³⁺, and Gd³⁺ ions, Gd³⁺ and Bi³⁺ ions can easily substitute the Y³⁺ sites and form a solid solution of (Y,Gd,Bi)BO₃ crystals.

Take the Y_0.9Gd_0.08Bi_0.02BO₃ (YGB-0.8) phosphor as an example. The SEM images in Figure 4a–d show the typical morphologies of particles annealed at 700, 800, 900, and 1000 °C, revealing that the powders annealed at 800 °C and below possess an average size of ~200 nm and the regular morphology. To show this, the particle size distributions of the YGB-0.8 phosphor annealed at 800 °C are shown in Figure 4i. Here, the inset shows the macroscopic appearance of the sample exhibited under natural light irradiation. It can be observed that the particle size is mainly concentrated at ∼200 nm, which is consistent with the SEM images. As shown in the SEM images, the powders with a narrow particle size distribution have been synthesized at lower sintering temperatures, and they possess a large effective surface area and weak atomic binding energy, resulting in the lower local symmetry of the YO₈ polyhedron and the surface defects of nanoparticles. When the annealing temperature exceeds a certain value, the crystal phase is gradually purified together with grain growth. Obviously, the morphology becomes more irregular in angularity, heterogeneity and compactness with the increase in sintering temperature, which is due to the changes in van der Waals attractions, while the small particle size may be caused by the distortion of anionic groups on the particle’s surface [30,31,32]. The size of spherical particles is significantly larger after sintering at 1000 °C, while compositional particles lose their spherical shape and undergo significant aggregation, which is attributed to the higher activity of atoms on the particle’s surface caused by the further decomposition of precursors. Under higher temperature annealing, the atoms could diffuse and combine with adjacent ones to form stable chemical bonds, leading to agglomeration [33,34]. No obvious changes in the morphology or particle size with various Gd³⁺ contents were observed at the same sintering temperature (not shown here), indicating that the doped Bi³⁺ and Gd³⁺ ions do not impact crystallization and grain growth. For the YGB-0.8 sample annealed at 800 °C, the homogeneous distributions of Gd, Bi, O, and Y elements are clearly observed by EDS, as shown in Figure 4e–h,j. The B element is undetected since its corresponding energy in the X-ray spectrum falls outside the scope. Moreover, high-packing densities, good slurry properties, and well-distributed particles in YGB systems are conducive to photon release.

3.2. Fluorescence Behaviors of YGB Phosphor

Figure 5a,b show the typical emission spectra of YGB phosphors with different Bi³⁺/Gd³⁺ contents (x = 0, 0.2, 0.4, 0.6, 0.8 and 1.0) under UV (273 nm) and VIS (532 nm) excitations. Notably, strong UV emissions can also be obtained by up-conversion under the excitation of a 532 nm laser, and all samples show a narrow-band emission at 313 nm, which is attributed to the ⁶P_7/2→⁸S_7/2 transition of Gd³⁺ ions [6]. Emission intensity increases until the Gd³⁺ content exceeds x = 0.8, which results from the energy transfer between Bi³⁺ and Gd³⁺ ions; with the further increase in Gd³⁺ contents, concentration quenching occurs with the attenuation of emission intensity. In order to identify the change in spectral intensity more clearly, the dependence of the PL emission intensity at 313 nm on the Gd³⁺ content (x) in YGB phosphors under 273 nm and 532 nm excitations is illustrated in the inset of Figure 5b. Compared with the sample without Bi³⁺, the weaker wide emission located around 440 nm resulted from the 6s²→6s6p transitions of Bi³⁺ ions. According to the photoluminescence excitation (PLE) spectra of these YGB phosphors in Figure 5c, monitored at 313 nm, the strongest excitation band centered at 273 nm should be attributed to the ⁸S_7/2→⁶I_J transition of the Gd³⁺ ion, which well overlaps with the 253.7 nm line of mercury lamps [35], while the emission peak at 440 nm was derived from the ³P₁→¹S₀ transition of Bi³⁺ ions [36]. In particular, Figure 5d shows the PLE spectra of YGB samples with different Gd³⁺ contents in monitoring the 440 nm emission, the intensity of which decreases with the Gd³⁺ content, illustrating the energy transfer (ET) from Bi³⁺ to Gd³⁺ ions that consumes the excitation energy of Bi³⁺ ions.

The energy transfer depends on the overlap between the excitation band of the activator and the emission band of the sensitizer in the phosphors. Bi³⁺ ions have a 6s² outer electronic configuration with a ¹S₀ ground state, and the excited state has the configuration of 6s6p with ³P₀, ³P₁, ³P₂, and ¹P₁ splitting levels. Due to the forbidden transitions of ¹S₀→³P₀ and ¹S₀→³P₂ by the electronic selection rules, the ¹S₀→³P₁ and ¹S₀→¹P₁ transitions of Bi³⁺ ions are usually observed [37]. For the sample doped with only Bi³⁺ ions, it would first relax and transit into the lowest ³P₁ excited state and then return to the ¹S₀ ground state via radiation. However, when Bi³⁺ and Gd³⁺ ions were co-doped into the host, energy transfer would occur, since the ³P₁→¹S₀ emission of Bi³⁺ effectively overlapped with the energy levels of Gd³⁺ (⁶P_7/2, ⁶P_5/2, and ⁶P_3/2) [38]. For the Y_0.9Gd_0.08Bi_0.02BO₃ (YGB-0.8) sample annealed at 800 °C, the overlapped excitation spectra of Bi³⁺ and Gd³⁺ ions are shown in Figure 5e, confirming their efficient excitability in the short-wave UV region, which is advantageous for the resonance energy transfer from Bi³⁺ to Gd³⁺ ions. In order to further clarify the controversies over the ET from Bi³⁺ to Gd³⁺, Figure 5f presents the Gd³⁺ fluorescence intensity ratio between 800 °C annealed YGB-0 and YGB-0.8 phosphors, monitored at 313 nm, which demonstrates the sensitizing effect of Bi³⁺ on Gd³⁺ ions. Compared with the sample without Gd³⁺ ions, the excitation energy of Bi³⁺ in Bi³⁺-Gd³⁺ co-doped samples is transferred to the Gd³⁺ ion and leads to stronger fluorescence emissions from the Gd³⁺ ion in short-wave UVB radiation, resulting in increased excitability. In addition, the excitation peaks located at 258 nm are consistent with the characteristic excitation peaks of Bi³⁺ (¹S₀→¹P₁) [25,37], which further confirms the effectiveness of ET from Bi³⁺ to Gd³⁺ ions.

To further investigate the effect of sintering temperatures on luminous properties, the PL and PLE spectra measured at room temperature from the YGB-0.8 phosphors annealed at different temperatures are illustrated in Figure 6a–c. The intensity of the excitation peak at 313 nm increases with the annealing temperature until 800 °C. The grain size increases while the porosity decreases significantly with the increase in temperature, enhancing the luminous intensity. When the sintering temperature is higher than 800 °C, the decreased intensity is attributed to the accelerated volatilization of Bi³⁺ and the crystalline defects. Furthermore, the enhanced PL emission originates from the absorption of exciting UV light by co-doped Bi³⁺ ions, which transfer the energy to the Gd³⁺ ions [18,39]. This mechanism is schematically shown in Figure 6d. Firstly, phosphors absorb the UV light, which leads to the ¹S₀→³P₁ transition of Bi³⁺ ions. The Bi³⁺ ions then transfer the energy non-radiatively to Gd³⁺ ions and ultimately realize UVB emissions from Gd³⁺ ions. Moreover, the smaller electronegativity of Gd³⁺ (1.20), compared to that of Y³⁺ (1.22) and Bi³⁺ ion (~2.02), allows an easier charge transfer, thus promoting PL emissions [19,40].

In order to evaluate the optical property of Gd³⁺-Bi³⁺ co-doped YGB phosphors with different Gd³⁺ contents and annealing temperatures, the relative spectral power distributions and relative photon distributions were determined and compared, as in Figure 7a–d. Under the 273 nm excitation, the sharp narrow-band UVB emission at 313 nm that originates from the ⁶P_J→⁸S_7/2 transition increases significantly with the increase in Gd³⁺ content and annealing temperature, which reaches the maximum value while the Gd³⁺ content is x = 0.8 and is annealed at 800 °C. This should be attributed to the increased energy transfer caused by the reduced distance among Gd³⁺-Gd³⁺ ion pairs [41]. The relative photon distribution provides fundamental information with respect to optical fields and relevant applications. Depending on the relative spectral power distribution P(λ), photon distribution N(ν) can be deduced by $\mathit{N}\left(\mathsf{\nu }\right)=\frac{{\mathsf{\lambda }}^3}{\mathit{h}\mathit{c}}\mathit{P}\left(\mathsf{\lambda }\right)$, where ν, λ, h, c, and P(λ) represent wavenumber, wavelength, Planck constant, vacuum light velocity, and spectral power distribution, respectively [42]. Here, the abscissae of the distribution spectra were converted to a wavenumber (cm⁻¹) for accurate deconvolution. The net absorption and emission photon distribution curves of Gd³⁺-Bi³⁺ co-doped YBO₃ phosphors were derived, as presented in Figure 7b,d, and their net emission and absorption intervals were selected at 30,300–33,300 cm⁻¹ (corresponding to 313 nm) and 36,300–39,200 cm⁻¹ (corresponding to 273 nm).

In addition, upon the excitation under 532 nm VIS light, the samples still emit the up-conversion UVB emission at 313 nm. The spectral power distribution and the photon number distribution of all samples with different Gd³⁺ contents (x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0) are displayed in Figure 8. With the increase in Gd³⁺ contents, the emission intensity increases until Y_0.9Gd_0.08Bi_0.02BO₃, because the transfer probability is proportional to the interaction between the sensitizer (Bi³⁺) and the activator (Gd³⁺) in both non-radiative and radiative resonance energy transfers. When x > 0.8, the intensity significantly decreases due to prominent concentration quenching caused by the reduced distance among Gd³⁺-Gd³⁺ ions.

According to Blasse [43,44], the energy would transfer from one activator to another until all energy is consumed. This phenomenon is regarded as concentration quenching in fluorescence, which is due to the non-radiative energy transfer among identical ions. Thus, the critical distance (R_C) is a parameter that is essential to understanding this phenomenon, which is calculated using the following equation: $R_c=2{\left[\frac{3V}{4\pi x_{c}N}\right]}^{\frac{1}{3}}$, where V is the volume of the unit cell (in ų), x_c is critical concentration, and N is the number of Y³⁺/Bi³⁺/Gd³⁺ ions in the unit cell. Herein, the values are x_c = 0.08, N= 6, and V = 108.90 ų, and the critical distance R_C of the YGB phosphor is calculated to be about 7.57 Å. Meanwhile, the corresponding spectral power distribution and photon number distribution of YGB-0.8 phosphors annealed from 700 °C to 1000 °C under the 532 nm excitation were also derived, and they are shown in Figure 9 to demonstrate the up-conversion emission monitored at 313 nm and the optimal annealing temperature of 800 °C. These results verify the effectiveness of Gd³⁺-Bi³⁺ co-doped phosphors in photon conversion and provide the theoretical basis for their application in skin treatments.

The spectral parameters could also provide external quantum yields (QYs) to assess luminescence and laser materials, which are used to calculate the utilization efficiency of the absorbed photons for desired emissions, defined as the photon number ratio of emission and absorption. Namely, QY = emitted photons/absorbed photons = N_em/N_abs. Here, the maximum QY is derived to be 24.75% in a Y_0.9Gd_0.08Bi_0.02BO₃ sample annealed at 800 °C under the 273 nm excitation, which is larger than that of other Gd³⁺ ions doped phosphors [45,46], and it is 75 times the single Gd³⁺-doped sample in this work. On the basis of these QYs, a higher photon release efficiency is achieved, which further exhibits the potential of Gd³⁺-Bi³⁺ co-doped YBO₃ phosphors for UVB skin treatment and reflects the energy transfer effectiveness between Bi³⁺ and Gd³⁺ ions in these phosphors. Moreover, this phosphor maintains a unique up-conversion excitability in the VIS region with a QY of 1.33% under the excitation of 532 nm. The QY values for the different contents and annealing temperatures of these co-doped YGB phosphors, under the excitation of the 273 and 532 nm, are listed in

Table 1. Quantum yields in Gd³⁺-Bi³⁺ co-doped phosphors with different Gd³⁺ contents and sintering temperatures under 273 and 532 nm excitation.

Excitation Wavelength (nm)	External Quantum Yield QY (%)
	Gd3+ Content (x) 800 °C Annealing						Sintering Temperature (°C)
	0	0.2	0.4	0.6	0.8	1.0	700	800	900	1000
273	0.33	7.53	14.80	21.81	24.75	3.91	13.70	24.75	14.02	12.83
532	0.01	0.51	0.91	1.25	1.33	0.23	0.81	1.33	0.77	0.76

. These results reveal that the Gd³⁺-Bi³⁺ activated YBO₃ phosphors with up/down-conversion excitability exhibit excellent UVB emission performance.

4. Conclusions

UVB-emitting Y_0.9(Gd_xBi_1−x)_0.1BO₃ phosphors (x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0) with the hexagonal phase and an average ~200 nm grain size were fabricated via the solid-state synthesis method. The enhanced PL emissions and the overlapped spectra verify the energy transfer between the Bi³⁺ and Gd³⁺ ions and a well-defined sharp and intense peak centered at 313 nm due to the ⁶P_7/2→⁸S_7/2 transitions of Gd³⁺ ions. The Y_0.9Gd_0.08Bi_0.02BO₃ phosphor annealed at 800 °C exhibits the highest QY values of 24.75% and 1.33% under the excitation of 273 nm and 532 nm, respectively, confirming that the system possesses excellent excitability in both UV and VIS regions. The optimal QY from the Gd³⁺-Bi³⁺ co-doped YBO₃ phosphor is 75 times the single Gd³⁺-doped sample. Bright and narrow UVB emissions resulting from efficient photon conversion demonstrate the multifunctional applications of Gd³⁺-Bi³⁺-activated YBO₃ phosphors and provide a new route for skin treatments.