Enhanced 311 nm (NB-UVB) Emission in Gd₂O₃-Doped Pb₃O₄-Sb₂O₃-B₂O₃-Bi₂O₃ Glasses: A Promising Platform for Photonic and Medical Phototherapy Applications

Abstract

A novel series of Gd₂O₃-doped Pb₃O₄–Sb₂O₃–B₂O₃–Bi₂O₃ glasses was synthesized via the conventional melt-quenching technique to explore their structural, thermal, and optical properties for potential photonic and medical phototherapy applications. X-ray diffraction and SEM analyses confirmed the amorphous and homogeneous nature of the samples, while their FTIR spectra revealed characteristic Pb–O, Sb–O, Bi–O, and B–O vibrational bands indicative of a stable glass network. Differential scanning calorimetry (DSC) demonstrated good thermal stability, suitable for high-temperature optical applications. Optical absorption and emission studies indicated the presence of prominent Gd³⁺ ion transitions, with a strong and sharp ultraviolet emission at 311 nm (⁶P_7/2→⁸S_7/2) when excited at 274 nm. The emission intensity and lifetime increased with Gd₂O₃ concentrations of up to 1.0 mol%, beyond which concentration quenching was observed. The optimized composition exhibited a reduced optical band gap and enhanced NB-UVB emission efficiency, suggesting efficient energy transfer with minimal non-radiative losses. These results establish the designed glass system as a promising multifunctional material for NB-UVB-based phototherapy, UV-laser generation, scintillation, and other next-generation photonic devices.

1. Introduction

Rare-earth-doped glasses have attracted significant attention as functional optical materials due to their ability to exhibit sharp, stable, and host-sensitive emission features. Among various dopants, gadolinium oxide (Gd₂O₃) has emerged as a valuable activator due to its notable luminescent and magnetic properties. The half-filled 4f⁷ electronic configuration of Gd³⁺ ions provides well-defined energy levels and stable ultraviolet (UV) emissions. In particular, the narrowband ultraviolet-B (NB-UVB) emission around 311 nm (⁶P_7/2→⁸S_7/2 transition) is highly suitable for medical phototherapy applications, especially for treating skin disorders such as psoriasis and vitiligo [1,2,3,4,5,6]. Beyond phototherapy, Gd2O3-based glasses are also promising for radiation shielding (γ, X-ray, neutron radiation) and advanced photonic devices such as luminescent materials, optical amplifiers, and white light-emitting diodes (W-LEDs). Incorporation of Gd₂O₃ enhances neutron attenuation, luminescence efficiency, optical efficiency, and structural compactness, thereby making the glass system multifunctional for optical, biomedical, and nuclear technologies [7,8,9,10].

Incorporating Gd₂O₃ into suitable glass matrices provides an effective route to develop transparent, thermally stable, and optically active materials. The host glass matrix strongly influences the local coordination environment, clustering tendency, and overall luminescence efficiency of Gd³⁺ ions. Among various hosts, antimonite, borate, and heavy-metal oxide glasses offer distinct advantages, including high rare-earth solubility, low phonon energy, and favorable structural flexibility, which make them excellent platforms for enhancing UV emission [11,12].

Furthermore, the incorporation of heavy metal oxides such as Pb₃O₄ significantly improves the structural integrity and physical stability of the glass network compared with other heavy metal oxide systems, including PbO–B₂O₃, TeO₂–WO₃ and Bi₂O₃–ZnO–B₂O₃ glasses [13,14,15]. The present glass matrix, Pb₃O₄–Sb₂O₃–B₂O₃–Bi₂O₃, provides an optimized combination of high optical density, broad UV transparency, and superior thermal stability, leading to reduced non-radiative losses and enhanced optical performance, NB-UVB (311 nm) emission intensity, and lifetime of Gd³⁺ ions. This multicomponent configuration synergistically integrates the unique benefits of Pb₃O₄, Sb₂O₃, B₂O₃, and Bi₂O₃, making the system highly versatile for applications in UV lasers, display devices, radiation shielding, and medical phototherapy.

The novelty of this work lies in the design of a multifunctional heavy-metal oxide glass system (Pb₃O₄–Sb₂O₃–B₂O₃–Bi₂O₃) tailored for efficient Gd₂O₃ incorporation, enabling remarkable enhancement of a narrowband 311 nm (NB-UVB) emission intensity and lifetime compared to conventional borate and antimonite glasses. The presently developed system demonstrates exceptional versatility, combining optical, thermal, and biomedical functionalities for next-generation photonic applications such as targeted phototherapy, scintillation, and UV-laser sources.

2. Experimental

In this work, glass samples with the compositional formula 10Pb₃O₄–20Sb₂O₃–10Bi₂O₃–(60–x)B₂O₃–x Gd₂O₃ (x = 0.1, 0.5, 1.0, 1.5, 2.0 mol%) were synthesized via the traditional melt-quenching method, as shown in Figure 1. The raw chemicals, viz., Pb₃O₄ (99.99% pure, Thermo Fisher Scientific, Mumbai, India), Sb₂O₃, H₃BO₃, Bi₂O₃ (99.99% pure, Loba, Mumbai, India), and Gd₂O₃ (Metall Rare Earth Ltd., Hong Kong, China, 99.99% pure), were used for the preparation of the glass samples. These samples were labeled as Gd1, Gd5, Gd10, Gd15, and Gd20, based on the content of Gd₂O₃. The sample preparation process involved accurately weighing the high-purity raw materials according to the desired molar composition and then uniformly mixing them in an agate mortar. The homogenized batch was then melted at 1350 °C for 30 min in a silica crucible, followed by casting in a preheated brass mold. Finally, the samples were annealed at 420 °C to relieve internal thermal stresses.

The amorphous nature of the glass samples was verified through X-ray diffraction (XRD) analysis carried out using a Rigaku D/Max ULTIMA III diffractometer (Rigaku Corporation, Tokyo, Japan) utilizing CuKα radiation. For optical characterization, the specimens were polished to dimensions of 1.0 cm × 1.0 cm in area with a thickness of 0.25 cm.

The density of the prepared glass samples was determined using Archimedes’ principle, with o-xylene as the immersion liquid to serve as the immersion medium, and measurements taken with an Ohaus digital balance with an accuracy of ±0.002 g. The refractive index (n_d) was assessed at a wavelength of 589.3 nm using an Abbe refractometer, with mono-bromonaphthalene applied as the contact fluid. By using a Tescan scanning electron microscope (Model VEGA3 LMU), the surface morphology of the glass samples was analyzed. A Mettler TGA/DSC 3+ thermo balance was used to record DSC (Differential scanning calorimetry) traces. The surface features of the samples were investigated using a Tescan VEGA3 LMU scanning electron microscope (SEM).

Optical absorption spectrum measurements were carried out across a 200–2000 nm wavelength range using a JASCO UV–Vis–NIR spectrophotometer. Photoluminescence spectra were recorded using a SP-2357 monochromator with a Thorlabs DET10C InGaAs detector. Luminescence decay characteristics were recorded with a 250 Hz pulsed laser diode and a fast response system (0.1 μs rise time). Fourier-transform infrared spectra (300–2000 cm⁻¹) were recorded using a Shimadzu IR Affinity-1S spectrophotometer.

3. Results and Discussion

The physical and optical properties of the Pb₃O₄–Sb₂O₃–Bi₂O₃–B₂O₃–Gd₂O₃ glass system, incorporating varying concentrations of Gd³⁺ ions (0.1 to 2.0 mol%), were assessed using established methods [16,17,18,19]. These methods considered the average molecular weight (M), density (d), and refractive index (n_d) of the glass samples, with the resulting values presented in

Table 1. Physical parameters of Gd³⁺ ion-doped Pb₃O₄–Sb₂O₃–B₂O₃–Bi₂O₃ glasses.

Glass Sample	Density d (g/cm3)	Gd3+ Ion Conc. (Ni) (×1019/cm3) ± 0.001	Gd3+ Inter Ionic Distance (Ri) nm ± 0.001	Polaron Radius RP (nm)	Field Strength, Fi (×1014, cm−2)	Refractive Index, n	E0 (eV)
Gd1	4.231	1.184	4.387	0.918	0.346	1.521	4.45
Gd5	4.233	1.297	2.569	1.586	0.325	1.523	4.43
Gd10	4.238	1.474	2.043	1.972	0.231	1.535	4.05
Gd15	4.235	1.752	2.387	2.254	0.177	1.527	4.20
Gd20	4.233	2.323	2.426	2.477	0.146	1.525	4.40

. By observing the physical values of the glasses, it is evident that the sample doped with 1.0 mol% of Gd³⁺ ions exhibits the highest values among all compositions. Figure 2 shows the variation of density d (g/cm³) and refractive index n_d as a function of Gd₂O₃ concentrations. The decrease in both density and refractive index at higher dopant (1.5 and 2.0 mol% of Gd₂O₃) levels can be attributed to the formation of non-bridging oxygen (NBO) sites or possible clustering, reducing the glass compactness and altering its local field effects, leading to a lower density and refractive index. The X-ray diffraction (XRD) patterns of both the undoped and Gd₂O₃-doped glass samples exhibit only a broad diffuse hump without any sharp diffraction peaks, confirming their fully amorphous nature. The incorporation of Gd³⁺ ions does not induce any crystalline phase formation, indicating that the dopant is uniformly accommodated within the glass network without altering its structural amorphousness (see Figure 3). Figure 4 represents the SEM picture for Gd10 glass, and it does not show any grains, confirming the amorphous nature of the glass. The EDS analysis performed on the Gd³⁺ ion-doped Pb₃O₄–Sb₂O₃–B₂O₃–Bi₂O₃ glass matrix (Figure 4b) offers clear information on the elemental composition and spatial distribution of the dopant ions. The EDS elemental mapping confirms that the Gd³⁺ ions are uniformly dispersed throughout the glass, which supports the attainment of stable and consistent optical characteristics. The absence of any detectable phase separation indicates that the Gd³⁺ ions are well accommodated within the glass network, and it demonstrates their effective solubility in the host matrix.

Thermal analysis DSC (Differential scanning calorimetry) was employed to investigate the thermal behavior of the prepared glass samples. Figure 5 shows the DSC traces for all the glass samples recorded at temperatures of up to 1000 °C, revealing an endothermic glass transition temperature (T_g) in the range of 350–400 °C, an exothermic crystallization temperature (T_c) between 450 and 470 °C, and an endothermic peak observed at about T_m ≈ 800 °C (melting temperature). The difference between T_c and T_g reflects the thermal stability of the glass. The inset in Figure 5 shows that the Gd10 glass exhibits the highest T_c-T_g (°C) value compared to the other glass samples. This enhanced thermal stability can be attributed to its optimal concentration of Gd₂O₃ (1.0 mol%), which strengthens the glass network through increased Gd–O bond formation and improved structural compactness. At this level, Gd³⁺ ions act as effective intermediate/conditional network formers, reducing the mobility of structural units and delaying the onset of crystallization. However, at higher Gd₂O₃ concentrations (more than 1.0 mol%), Gd³⁺ ions begin to distort the network, creating non-bridging oxygens and reducing overall stability. Thus, the Gd10 composition represents an optimal balance between network reinforcement and structural homogeneity, resulting in the highest thermal stability. All finalized samples remain homogeneous and fully amorphous with good resistance against crystallization.

Figure 6 shows that the FTIR spectra of the glasses containing various concentrations of gadolinium ions exhibit several characteristic absorption bands, corresponding to the vibrational modes of the structural units within the glass network. A band at about 580 cm⁻¹ is associated with the PbO₄ and SbO₃ structural units [20,21]. The peak near 755 cm⁻¹ is attributed to the stretching vibrations of Sb–O/Pb–O/Bi–O units [22,23,24,25]. Additionally, a band is present at 960 cm⁻¹ that is associated with B–O–B linkages in borate groups of symmetric stretching vibrations. Another band observed at approximately 1330 cm⁻¹ is related to O–B–O (BO₄) stretching vibrations [26]. A band observed at 1680 cm⁻¹ is attributed to O–H (H₂O), and the deformation mode of the water molecule. The broad bands observed at 2360 cm⁻¹ and 3295 cm⁻¹ are symmetric stretching vibrations of the hydrogen band and the O–H groups. It is noteworthy that the bands associated with asymmetric vibrations exhibit their highest intensity in the glass sample doped with 1.0 mol% of Gd³⁺ ions, indicating a greater extent of network depolymerization in this composition [27,28,29].

Figure 7 represents a schematic demonstration of the chemical composition of the Pb₃O₄–Sb₂O₃–B₂O₃–Bi₂O₃–Gd₂O₃ glass system, highlighting the structural arrangement and bonding interactions among the constituent oxides. The multicomponent glass system demonstrates that a complex network is formed by Pb, Sb, B, and Bi linked through oxygen bridges, while Gd³⁺ ions are incorporated into the mixture through the surrounding oxygen atoms. This structural arrangement highlights the stabilization of the glass framework and tailors its optical and photonic properties. The following diagram serves as a visual framework for understanding the micro-structural organization and the contribution of individual components to the overall glass network.

The optical absorption (OA) spectra of Pb₃O₄–Sb₂O₃–B₂O₃–Bi₂O₃ glasses with different concentrations of Gd³⁺ ions added to them are recorded in the spectral range from 200 to 2000 nm, as shown in Figure 8. The spectra revealed multiple absorption bands originating from the ground state ⁸S_7/2 to various excited states of Gd₂O₃ corresponding to ⁸S_7/2→⁶D_7/2, ⁶D_9/2, ⁶I_15/2,_13/2,_11/2, ⁶I_9/2,_17/2, ⁶I_7/2, ⁶P_7/2, ⁶I_{9/2, 17/2}, and ⁸S_7/2→⁶L_J transitions, respectively. With an increase in the concentration of Gd₂O₃ of up to 1.0 mol%, a significant increase in the intensity of the absorption peaks was observed in the Gd₁₀ glass. However, at higher doping contents (1.5 and 2.0 mol%) of Gd³⁺ ions, a noticeable decrease in the band intensities was observed. This reduction is attributed to concentration-quenching effects, which become predominant at elevated Gd³⁺ ion concentrations. Such effects arise from non-radiative energy transfer, cross-relaxation processes, and ion clustering, which hinder the effective participation of Gd³⁺ ions in optical absorption [30,31].

The characteristics of the Gd³⁺ ions in the absorption bands in the UV region not only validate the successful incorporation of Gd³⁺ ions into the host lattice but also highlight the potential of these glasses for ultraviolet photonic devices, UV lasers, scintillators, and advanced optoelectronic and biomedical applications [31,32].

The optical band gap (E₀) is essential for understanding the electronic structure and optical behavior of glass materials. The host glasses doped with Gd³⁺ ions with band gaps can be estimated from their UV-Vis spectra using Tauc’s plot (Figure 9). Incorporating rare-earth ions of up to 1.0 mol% reduces the band structure, resulting in a decrease in the E₀ value and an increase in the Gd³⁺ ion content [33]. These results suggest that the incorporation of Gd³⁺ ions increases the number of non-bridging oxygen (NBO) sites and enhances the structural disorder within the glass network. By studying E₀ values confirms these structural changes indicating that Gd³⁺ ions doped glasses possess promising characteristics for medical phototherapy and next-generation photonic applications [34].

The relation between the absorption coefficient α and E₀ is

α(hν) ∝ B (hν − E_opt)^1/2

In the above equation, where B is a band-tailing parameter, and hν is the photon energy. The low value of E₀ observed for the Gd₁₀ glass (Table 1) suggests a high degree of disorder in the glass network.

The ultraviolet emission from Gd³⁺ ions in the Pb₃O₄–Sb₂O₃–B₂O₃–Bi₂O₃ glass originates from 4f to 4f electronic transitions. Although these transitions are normally parity forbidden, they become partially allowed due to the asymmetric local ligand field in the amorphous glass network, which causes a slight mixing of the 4f energy level. The excitation spectra (Figure 10) of all glasses recorded were monitored at λ_em = 311 nm, corresponding to the following exhibited transitions: ⁸S_7/2→⁶D_9/2 (253 nm), ⁶I_11/2 (274 nm), ⁶I_7/2 (280 nm), ⁶P_5/2 (307 nm), and ⁶P_7/2 (312 nm). Out of these, ⁸S_7/2→⁶I_11/2 had the most intense peak, and the same method was used to record the emission spectra in the UV-visible region.

The emission spectra (Figure 11) recorded (λ_ex = 274 nm) for all the glass samples revealed a sharp emission band in the ultraviolet spectral region corresponding to the ⁶P_7/2→⁸S_7/2 (~311 nm) transition of the Gd³⁺ ions [12,35]. The emission intensity increased significantly with Gd³⁺ ion concentrations of up to 1.0 mol%, beyond which a gradual decrease was observed with an increasing concentration of Gd³⁺ ions. This process is governed by minimal multi-phonon relaxation due to the low-phonon-energy environment created by heavy metal oxides (Pb₃O₄, Bi₂O₃, Sb₂O₃), which suppresses non-radiative energy losses through cross-relaxation and multi-phonon interactions and enhances ultraviolet emission efficiency. The mechanism is consistent with the transition scheme reported in similar Gd³⁺ ion-activated heavy metal oxide systems [36].

Thus, the strong emission transition of ⁶P_7/2→⁸S_7/2 (~311 nm) not only confirms the successful incorporation of Gd³⁺ ions into the Pb₃O₄–Sb₂O₃–B₂O₃–Bi₂O₃ glass matrix but also highlights their potential as candidates for medical UV lasers (especially UV near ~311 nm, which is invisible to the eye, but biologically significant), phototherapy devices, scintillators, and energy transfer-based next-generation photonic applications.

Figure 12 shows the decay profile of Gd³⁺ ion-doped Pb₃O₄–Sb₂O₃–B₂O₃–Bi₂O₃ glasses recorded at room temperature from the ⁶P_7/2 level (λ_ex = 274 nm and λ_em = 311 nm) and their different transitions to measure their experimental lifetimes. The decay curves were fitted by using a single exponential function, and the corresponding average lifetime (τ_avg) values were obtained and are presented in

Table 2. Summary of lifetimes evaluated from decay profiles of ⁶P_7/2 emitted level of Gd³⁺ ion-doped Pb₃O₄–Sb₂O₃–B₂O₃–Bi₂O₃ glasses.

Composition/Host Type	Gd3+ Conc. (mol%)	Emission λ (nm)	Lifetime τ (ns)	Optical Band Gap E0 (eV)
Pb3O4–Sb2O3–B2O3–Bi2O3 (Present work)	1.0	311	4.82	4.05
Sb2O3–SiO2 with Pb3O4 tuning [37]	1.0	311	~4–5	―
Borate glasses with modifiers [38]	Varied	311–312	~3–5	~4.0–5.0
Fluorophosphate/phosphate-based glasses [39]	Varied	311	~4–8	~3.5–4.5
Pb-containing HMO/lead-borate glasses [40]	0.5–2.0	311–312	~3.5–6	~3.8–4.5

.

To substantiate the novelty of the present Gd³⁺-doped Pb₃O₄–Sb₂O₃–B₂O₃–Bi₂O₃ glass system, a comparative analysis with previously reported Gd³⁺ ion-doped borate, phosphate, and heavy metal-oxide glasses was carried out.

Table 3. Comparison of emission characteristics (λ_em), radiative lifetime (τ), and optical band gap (E₀) of Gd³⁺ ion-doped glasses from different host systems.

Glass Sample	τ (ns)
Gd1	3.61
Gd5	3.83
Gd10	4.82
Gd15	4.36
Gd20	4.07

summarizes the key optical parameters, such as emission wavelength, lifetime, and band gap, as reported in the literature [37,38,39,40] and compares them with the optimized Gd10 sample. The data clearly show that the present system exhibits a lifetime that is comparable to, or higher than, many borate hosts and similar to phosphate/fluorophosphate systems, while simultaneously offering higher density, strong emission intensity, and better thermal stability. These results confirm that the proposed multicomponent matrix provides a distinct performance advantage and validates its designation as a promising platform for UV-emitting and NB-UVB (311 nm) applications.

The measured lifetimes and emission intensities surpass those reported for conventional antimonite and borate glasses, underlining the potential applicability of the present system for medical phototherapy, scintillators, and next-generation photonic applications [41,42,43,44]. Figure 13 represents the energy level diagram of the Gd10 glass. The diagram illustrates the electronic energy states of the Gd³⁺ ions within the glass matrix and highlights the absorption and emission transitions. The upward and downward arrows represent the radiative excitation and emission transitions, in which the ions return to lower energy states, releasing photons. This schematic helps in understanding the optical behavior of the glass and its potential suitability for next-generation photonic applications. Thus, our findings provide a new perspective in the design of rare-earth Gd2O3-doped glasses with tailored structural and optical characteristics.

4. Conclusions

Gd2O3-doped Pb₃O₄–Sb₂O₃–B₂O₃–Bi₂O₃ glasses were successfully synthesized using the conventional melt-quenching technique, and their structural, thermal, and optical properties were systematically investigated. XRD and SEM analysis confirmed the amorphous nature and homogeneous surface morphology of all the glass samples. FTIR spectra revealed the presence of Pb–O, Sb–O, Bi–O, and B–O structural units, indicating the development of a stable and interconnected glass network. Among all the compositions, the glass containing 1.0 mol% of Gd₂O₃ exhibited optimal physical parameters, enhanced UV absorption, and the most intense emission at 311 nm, corresponding to the ⁶P_7/2→⁸S_7/2 transition. The improvement in emission intensity and lifetime behavior up to this concentration demonstrates efficient energy transfer and minimal non-radiative losses, whereas higher Gd³⁺ ion concentrations resulted in concentration-quenching effects. The thermal analysis revealed good stability, confirming the suitability of this host matrix for high-temperature optical applications.

Overall, the combination of high density, broad optical transparency, and pronounced NB-UVB emission identifies this glass system as a strong candidate for medical phototherapy (psoriasis and vitiligo treatment), UV-based laser sources, radiation scintillators, and next-generation photonic devices. The present investigation establishes a reliable foundation for developing multifunctional rare-earth-activated glass materials integrating optical performance with structural robustness.