Wide-Band White-Light Emission of CaWO₄:Eu³⁺/g-C₃N₄ Composite Phosphor Under Near-Ultraviolet Excitation

Abstract

The development of efficient, single-phase-excitable white-light phosphors remains a critical challenge for solid-state lighting applications. In this work, white-light-emitting CaWO₄:Eu³⁺/g-C₃N₄ composites were successfully developed by integrating red-emitting CaWO₄:7%Eu³⁺ with blue-emitting graphitic carbon nitride (g-C₃N₄). Under 365 nm near-UV excitation, the composite exhibits dual-band emission originating from the ⁵D₀ → ⁷F₂ transition of Eu³⁺ (~616 nm) and the intrinsic band-edge luminescence of g-C₃N₄ (~460 nm). The optimal white-light performance is achieved at a g-C₃N₄ content of 0.5 wt%, yielding CIE chromaticity coordinates of (0.294, 0.324) and a correlated color temperature (CCT) of 7673 K. This sample demonstrates a photoluminescence quantum yield (PLQY) of 3.25%. Moreover, the CaWO₄:Eu³⁺/g-C₃N₄ composite shows enhanced thermal stability, retaining 78% of its initial emission intensity at 175 °C, with an activation energy of 0.41 eV—significantly higher than that of the pristine CaWO₄:Eu³⁺ (0.22 eV). These results indicate that the CaWO₄:Eu³⁺/g-C₃N₄ heterostructured phosphor is a promising candidate for single-phase-excitable white-light applications.

1. Introduction

In the field of optics, rare-earth-doped phosphors have attracted extensive attention due to their unique luminescence intensity, fluorescence lifetime, and other characteristics. Light-emitting diodes (LEDs), the fourth-generation lighting source, are widely used in lighting and display fields because of their high energy efficiency and conversion rate [1,2,3,4,5]. However, current white LEDs suffer from a low color-rendering index and a high correlated color temperature, mainly due to the lack of red emission [6]. Developing efficient red-emitting phosphors is therefore urgent [7]. The CaWO₄ host can effectively transfer the absorbed UV energy to doped RE³⁺ (RE = Eu, Tb, Sm, Dy, Er, and Pr) ions via resonant energy transfer, generating characteristic RE³⁺ emission [8,9]. Among oxysalt systems, tungstates (AW₄, A = Pb, Zn, Ca, etc.) are particularly attractive. Their unique structure, comprising tetrahedral [WO₄]²⁻ units and dodecahedral Ca²⁻ coordination environments, enables O²⁻ → W⁶⁺ charge-transfer transitions, realizing effective self-activated luminescence in the UV-to-blue spectral region [10]. This charge transfer endows tungstates with strong and broad absorption bands in the near-UV and visible regions, and they have been widely applied in X-ray intensifying screens, optical fiber devices, lasers, and medical instruments [11].

Currently, tungstate phosphors are mostly activated by Eu³⁺, Sm³⁺, Tb³⁺, Ho³⁺, etc. Eu³⁺, possessing a 4f electron configuration, exhibits 4f → 4f transitions that yield high-color-purity red emission independent of the host lattice; it can be efficiently excited by UV or blue light and is therefore the preferred red center [12]. Studies have shown that the luminescence efficiency of Eu³⁺-doped tungstate red phosphors can be twice that of commercial Y₂O₂S:Eu³⁺, indicating excellent prospects. Cai et al. [13] synthesized CaWO₄:Eu³⁺ via a high-temperature solid-state method and optimized the Eu³⁺ concentration to achieve high-purity red emission. Liu Zheng-wei et al. [14] first observed long-persistent red luminescence in CaWO₄:Eu³⁺. F B Xiong et al. [15] demonstrated that Eu³⁺- and Li⁺-co-doped CaWO₄ phosphors can be effectively excited by 270 nm ultraviolet (UV) or 394 nm near-ultraviolet chips, and exhibit a red emission derived from the ⁵D₀ → ⁷F_J (J = 1 and 2) transition of Eu³⁺. Although CaWO₄:Eu³⁺exhibits dual emission from the WO₄²⁻ groups (blue–green) and Eu³⁺ ions (red), the spectral balance is inherently difficult to optimize [16]. The energy transfer from WO₄²⁻ to Eu³⁺ is highly efficient, which often leads to complete quenching of the host emission at moderate-to-high Eu³⁺ doping levels. Consequently, the resulting luminescence is dominated by red emission, lacking sufficient green components for high-quality white light. Even at low Eu³⁺ concentrations where host emission persists, the overall spectrum typically shows poor color rendering and limited tunability under single-wavelength excitation.

To address this spectral limitation, we herein construct a heterocomposite between CaWO₄:7%Eu³⁺—optimized for strong red emission—and graphitic carbon nitride, a metal-free semiconductor with intense and broad blue–green photoluminescence (450–550 nm). g-C₃N₄ possesses a suitable bandgap (~2.7 eV) [17], high thermal stability (>500 °C) [18], and a two-dimensional layered structure composed of triazine/heptazine units linked by amino groups, forming a highly π-conjugated network [19,20,21]. Its conduction-band minimum (−1.1 V vs. Normal Hydrogen Electrode) and valence-band maximum (+1.6 V vs. Normal Hydrogen Electrode) enable the formation of Type-II heterojunctions with CaWO₄ [22], promoting spatial separation of photogenerated carriers and suppressing non-radiative recombination. Surface functional groups (–NH₂, –NH, –C≡N) can interact with [WO₄]²⁻ tetrahedra via hydrogen bonding, van der Waals forces, or electrostatic interactions [23,24], enhancing interfacial coupling. The flexible 2D framework also mitigates lattice–mismatch stress, thereby improving the emission uniformity of Eu³⁺ ions [25].

CaWO₄/g-C₃N₄ heterostructures have been extensively investigated for photocatalytic environmental remediation. Tuna et al. [26] fabricated CaWO₄/tubular g-C₃N₄ hybrids via ultrasonication combined with thermal mixing, achieving a significant bandgap reduction from 3.60 eV to 1.85 eV. Their optimized composite exhibited 79.3% degradation of Allura red within 120 min under visible light, with a reaction rate constant 31.3-fold higher than pristine CaWO₄, and additionally demonstrated bifunctional potential for thermal-energy storage applications. In parallel, Vadivu et al. [27] synthesized CaWO₄/g-C₃N₄ nanocomposites via a simple ultrasonication method for rhodamine B degradation under visible light. Their optimal composition (3 wt% g-C₃N₄) achieved nearly 98% dye removal within 150 min, exhibiting a narrowed bandgap of 2.47 eV and following an S-scheme charge-transfer mechanism with excellent cyclic stability. While these studies established the efficacy of g-C₃N₄ heterojunctions for photocatalytic pollutant degradation through enhanced charge separation and broadened light absorption, the potential of such composites for lighting applications—particularly white-light generation via spectral tuning of rare-earth phosphors—remains unexplored [28,29,30,31].

Building on our earlier work on the Lu₂MoO₆:Eu³⁺/g-C₃N₄ phosphor system, white-light generation from rare-earth phosphors under near-UV excitation is a key route for energy-saving w-LEDs. Recently, we demonstrated that coupling g-C₃N₄ with Lu₂MoO₆:Eu³⁺ produces white light via blue–red dual emission. However, the high cost of lutetium oxide and significant thermal quenching hinder its commercial viability. This study investigates a tungstate-based heterostructure, CaWO₄:Eu³⁺/g-C₃N₄. The tungstate lattice provides a self-activating blue-emission component that complements the broadband luminescence of g-C₃N₄ and the characteristic red emission of Eu³⁺. This combination expands the spectral functionality of the composite while maintaining the benefit of a single-composite phosphor that is excitable under near-UV light.

2. Experimental Section

2.1. Experimental Raw Materials and Preparation Methods

CaWO₄:xEu³⁺ phosphors were prepared by high-temperature solid-state reaction. The raw materials were melamine (99.0%), CaCO₃ (99.9%), Eu₂O₃ (99.9%), and WO₃ (99.9%). Melamine was calcined at 550 °C for 4 h (5 °C·min⁻¹) to obtain g-C₃N₄. Stoichiometric amounts of CaCO₃, WO₃, and Eu₂O₃ were thoroughly ground, placed in alumina crucibles, and calcined at 900 °C for 8 h in air (5 °C·min⁻¹). After natural cooling, CaWO₄:xEu³⁺ powders were obtained.

For the fabrication of CaWO₄:Eu³⁺/g-C₃N₄ composites, the as-prepared CaWO₄:Eu³⁺ red phosphor (specifically the optimal 7% doped sample) was dispersed in de-ionized water and ultrasonicated at 70 °C for 10 min. A designed amount of g-C₃N₄ was then added to the suspension.

The mixture was stirred at room temperature for 12 h, followed by filtration and drying at 70 °C for 12 h to yield the final composites. In this study, the pristine phosphor CaWO₄:7%Eu³⁺ is denoted as CW:7%Eu, and pure g-C₃N₄ is denoted as CN. The composite samples with g-C₃N₄ loadings of 5.0 wt%, 1.0 wt%, and 0.5 wt% are labeled as CW-CN-5, CW-CN-1, and CW-CN-0.5, respectively.

2.2. Materials Characterization and Performance Testing

The powder X-ray diffraction patterns of the synthetic products were monitored by a Rigaku SmartLab SE X-ray diffractometer (Rigaku Corporation, Tokyo, Japan). The morphology and particle size of the samples were observed by scanning electron microscopy (ZEISS Sigma 300, Carl Zeiss AG, Oberkochen, Germany). The Fourier-transform infrared spectra in the range of 400–4000 cm⁻¹ were collected by an infrared spectrophotometer (Thermo Fisher Scientific, Waltham, MA, USA). The ultraviolet diffuse reflection spectra ranging from 200 to 800 nm were measured using a Shimadzu UV-3600 from Japan(Shimadzu Corporation, Kyoto, Japan). The excitation spectrum, emission spectrum, quantum efficiency, and fluorescence lifetime were determined by the Edinburgh FLS1000 fluorescence spectrophotometer(Edinburgh Instruments Ltd., Livingston, UK). For lifetime measurements, each sample was measured three times independently; the reported lifetimes represent the average ± standard deviation (n = 3).

3. Results and Discussion

3.1. Crystal Structure and Compositional Characterization

CaWO₄ crystallizes in a regular and ordered scheelite-type structure, in which W⁶⁺ and O²⁻ form tetrahedral polyhedra (Figure 1), while Ca²⁺ occupies specific interstitial sites surrounded by eight oxygen atoms in a dodecahedral coordination environment, endowing CaWO₄ with certain stability and unique physicochemical properties. Upon the introduction of Eu³⁺ as an activator dopant, it partially substitutes for Ca²⁺ sites, because Eu³⁺ and Ca²⁺ are non-isovalent cations with different valence states. Such non-isovalent partial substitution inevitably induces local lattice distortion, thereby affecting the electronic environment and luminescence-transition characteristics of Eu³⁺ ions [32].

CaWO₄:xEu³⁺ samples with different Eu³⁺ doping levels (x = 0.03, 0.07, 0.10, 0.15, 0.20) were synthesized and characterized to investigate the effects of Eu³⁺ incorporation on phase purity, elemental composition, and microstructural evolution. XRD patterns reveal that all diffraction peaks match well with the standard card of tetragonal CaWO₄ (PDF#97-019-4079), indicating the formation of a pure scheelite-type phase without detectable impurities across the entire doping range (Figure 2a). The elemental composition of CaWO₄ doped with different Eu³⁺ concentrations (x), as determined by EDS analysis, is listed in

Table 1. CaWO₄ with different Eu³⁺ doping amounts (x): xEu³⁺ sample determined by EDS analysis.

x (wt%)	Ca	Eu	W	O
0.03	27.88	3.71	20.62	39.43
0.07	25.92	8.64	19.98	36.46
0.1	24.36	12.35	19.66	36.38
0.15	21.92	17.88	19.43	34.21
0.2	19.48	23.41	19.19	32.12

.

The consistent peak positions demonstrate that Eu³⁺ doping does not alter the main crystal structure of CaWO₄, suggesting that Eu³⁺ successfully substitutes for Ca²⁺ sites in the lattice due to their comparable ionic radius [33]. This is further corroborated by EDS analysis, which clearly reveals the presence of C, Ca, Eu, W, and N elements in the samples (Figure 2b). Notably, the Eu peak intensity gradually increases with increasing doping level, while the relative contents of other elements change accordingly, confirming the successful incorporation of Eu³⁺ into the host lattice.

The microstructural morphology and spatial distribution of constituent elements were further characterized by SEM and EDS mapping, with the representative data presented in Figure 2c and Figure 2d, respectively [34].

3.2. Spectral Analysis of CaWO₄:xEu³⁺ Phosphor

The influence of Eu³⁺ doping concentration on the photoluminescence performance of CaWO₄:xEu³⁺ materials was systematically investigated. Under excitation at 394 nm, the emission intensity exhibits a trend of initial enhancement followed by attenuation as the Eu³⁺ content increases from 3% to 20% (Figure 3b), reaching a maximum at x = 0.07. The integrated emission intensity plotted as a function of Eu³⁺ concentration (Figure 3c) provides quantitative evidence for this concentration-quenching behavior and corroborates the selection of 7% as the optimal doping level. At low doping levels, the increased concentration of Eu³⁺ raises the number of luminescent centers, thereby enhancing the emission intensity; at elevated doping levels, the reduced interionic distance between Eu³⁺ ions facilitates more frequent interionic energy transfer, increases the probability of non-radiative transitions, and induces concentration quenching [35]. This phenomenon is consistent with the literature reports, as the Eu³⁺ content increases, the decreased separation between luminescent centers enhances energy migration to defects, impurities, or multi-phonon relaxation, ultimately leading to luminescence quenching. According to the Judd–Ofelt theory [36,37], the ⁵D₀ → ⁷F₁ transition is a magnetic-dipole transition, while the ⁵D₀ → ⁷F₂ transition is an electric-dipole transition (Figure 3b), with the latter being highly sensitive to the crystal-field environment. The optimal doping concentration is determined to be 7%, at which the ⁵D₀ → ⁷F₂ transition displays the strongest emission peak at 616 nm.

The photoluminescence excitation and emission spectral characteristics and chromaticity performance of CaWO₄:7%Eu³⁺ further elucidate its optical properties. The excitation spectrum monitored at an emission wavelength of 616 nm exhibits multiple characteristic peaks (Figure 3d), wherein intense peaks include the ⁷F₀ → ⁵L₆ transition at 394 nm and the ⁷F₀ → ⁵D₂ transition at ~465 nm, while the weaker component at ~365 nm is assigned to the ⁷F₀ →⁵D₄ transition; other transitions appear with relatively lower intensity [38]. The emission spectrum displays the characteristic electric-dipole ⁵D₀ → ⁷F₂ transition at ~616 nm as the dominant emission band. This multi-wavelength excitation characteristic broadens the excitation spectral range, enabling the material to match different light sources such as UV and blue LEDs. Chromaticity analysis reveals that the CIE 1931 chromaticity coordinates of samples with different doping concentrations determine the position of their emission color in the color space (Figure 3a). This shift originates from changes in the relative intensities of characteristic emission peaks, and the emission color of the material can be tuned by regulating the Eu³⁺ doping concentration [39]. The photographic image in Figure 3e shows CaWO₄:7%Eu³⁺ phosphor in daylight (presenting white) and 365 nm ultraviolet light (showing bright red emission), confirming its potential for practical lighting and anti-counterfeiting applications.

3.3. Energy-Level Transition Diagram of g-C₃N₄ and Eu³⁺

The photoluminescence mechanism of the CaWO₄:Eu³⁺/g-C₃N₄ heterostructure under near-ultraviolet excitation can be elucidated through the energy-level diagram depicted in Figure 4. Under 365 nm excitation, both constituents of the composite are simultaneously activated, generating dual-band emission through distinct optical transition pathways.

For the g-C₃N₄ semiconductor component, the 365 nm photons promote electrons from the valence band (VB) to excited states above the conduction-band (CB) minimum via the intrinsic n → π* electronic transition. Subsequent non-radiative (NR) relaxation to the CB edge leads to radiative recombination, producing the broad blue–green emission centered at ~460 nm. This broadband luminescence originates from the surface radiative recombination of charge carriers in the π-conjugated triazine/heptazine units, serving as the essential blue component for white-light generation.

Concurrently, the Eu³⁺ activator ions are excited via direct f–f transitions, including the ⁷F₀ → ⁵D₄ transition at 365 nm and the ⁷F₀ → ⁵L₆ transition at 394 nm, as well as indirectly through [WO₄]²⁻ host absorption in the near-UV region followed by efficient host-to-Eu³⁺ energy transfer. Rapid non-radiative relaxation populates the metastable ⁵D₀ emitting level, from which characteristic red emission at 616 nm occurs via the electric-dipole ⁵D₀ → ⁷F₂ transition. The magnetic-dipole ⁵D₀ → ⁷F₁ transition at ~591 nm is also observed with lower intensity, consistent with the Judd–Ofelt theory predictions for Eu³⁺ in a low-symmetry crystal-field environment.

3.4. Structural and Optical Properties of CaWO₄:7%Eu³⁺/(y wt%) g-C₃N₄ Composites

Upon incorporation of graphitic carbon nitride (g-C₃N₄) into the CaWO₄:7%Eu³⁺ matrix, the elemental composition analysis by EDS confirms the successful fabrication of the heterostructured composites with tunable carbon and nitrogen contents increasing from 0.145 wt% C and 0.226 wt% N (CW-CN-0.5) to 1.45 wt% C and 2.26 wt% N (CW-CN-5), as summarized in

Table 2. Elemental composition (wt%) of CaWO₄:7%Eu³⁺/(y wt%) g-C₃N₄ samples determined by EDS analysis.

g-C3N4 Content (wt%)	Ca	Eu	W	O	C	N
5.0	24.32	8.64	19.2	34.73	1.45	2.26
1.0	25.6	8.64	19.7	36.11	0.29	0.45
0.5	25.76	8.64	19.9	36.27	0.15	0.23

. The Ca content gradually decreased from 25.76 wt% to 24.32 wt% with the increase in g-C₃N₄ loading, which was consistent with the expected dilution effect. In contrast, the content of Eu³⁺ remained statistically unchanged in all samples (8.64 wt%). Systematic structural and spectroscopic investigations reveal profound modulations in both the crystallographic integrity and photophysical behavior. X-ray diffraction (XRD) analysis demonstrates that all diffraction peaks of the composite samples (CW-CN-0.5, CW-CN-1, and CW-CN-5) align precisely with the tetragonal scheelite structure of CaWO₄ (PDF#97-019-4079), confirming that g-C₃N₄ integration does not induce phase segregation or structural decomposition of the host lattice [40,41,42,43,44] (Figure 5a). Nevertheless, subtle peak shifts and variations in relative intensity are observed with increasing g-C₃N₄ content, which is particularly evident in the magnified view of the (112) reflection region (2θ = 28–30°), indicating localized lattice distortion and modified interatomic interactions at the heterointerface. Such structural perturbations originate from the electrostatic and van der Waals interactions between the terminal functional groups (–NH₂, –NH) of g-C₃N₄ and the [WO₄]²⁻ tetrahedra, which induce microscopic strain fields without compromising the long-range crystallographic order of the CaWO₄ framework.

Photoluminescence spectroscopic analysis under near-ultraviolet excitation (λ_ex = 365 nm) reveals a dual-band emission profile [45] (Figure 5d). The emission spectrum exhibits emission comprising the characteristic ⁵D₀ → ⁷F₂ transition of Eu³⁺ ions (~616 nm) and the broad blue–green luminescence of g-C₃N₄ centered at ~465 nm; the characteristic blue emission of [WO₄]²⁻ expected at ~430 nm is not distinctly observed due to efficient energy transfer under a high Eu³⁺ doping concentration (7%).

Critically, quantitative comparison of the emission spectra reveals that the intensity of the characteristic ⁵D₀ → ⁷F₂ red emission from Eu³⁺ (~616 nm) in CW-CN-0.5 is significantly attenuated relative to pristine CaWO₄:7%Eu³⁺ under identical measurement conditions (Figure 5f). This observation conclusively refutes the possibility of g-C₃N₄-to-Eu³⁺ sensitization. Instead, the apparent enhancement in integrated emission intensity arises exclusively from the superposition of two independently excited emission centers: (i) the residual red emission from Eu³⁺ (attenuated but still present) via [WO₄]²⁻ → Eu³⁺ energy transfer within the CaWO₄ host; and (ii) the direct blue–green luminescence (450–550 nm) from photoexcited g-C₃N₄.

To verify the absence of energy transfer from g-C₃N₄ to Eu³⁺, photoluminescence excitation (PLE) spectra of the constituent materials and composite were compared (Figure 5e). The PLE spectrum of pristine CaWO₄:7%Eu³⁺ monitored at 616 nm exhibits characteristic Eu³⁺ excitation peaks at 394 nm (⁷F₀ → ⁵L₆) and 465 nm (⁷F₀ → ⁵D₂). In CW-CN0.5, these Eu³⁺ peaks persist but with markedly reduced intensity, accompanied by a broad absorption band below 400 nm attributed to Eu–O/W–O charge-transfer states and the low-energy absorption tail of the [WO₄]²⁻ host matrix, rather than to g-C₃N₄. Notably, the characteristic absorption edge of g-C₃N₄ near ~350 nm is absent in the PLE spectrum, indicating that no g-C₃N₄ → Eu³⁺ energy transfer is observed. Crucially, if sensitization were operative, excitation into the g-C₃N₄ absorption band (365 nm) should enhance the Eu³⁺ emission relative to direct excitation; however, the opposite is observed, confirming that g-C₃N₄ and Eu³⁺ function as isolated emitters.

Time-resolved photoluminescence decay measurements monitored at the ⁵D₀ → ⁷F₂ emission wavelength (616 nm) reveal distinct luminescence kinetics for the constituent materials and composites. The decay curves for pristine g-C₃N₄, CaWO₄:7%Eu³⁺, and the CW-CN-0.5 composite are presented in Figure 5b,c, revealing that g-C₃N₄ exhibits significantly faster decay kinetics (τ = 0.156 ± 0.012 ms) compared to the millisecond-scale lifetimes of the Eu³⁺-doped samples (τ = 1.245 ± 0.032 ms for CaWO₄:7%Eu³⁺, and τ = 0.832 ± 0.028 ms for CW-CN-0.5; mean ± SD, n = 3). Furthermore, systematic variation in g-C₃N₄ loading (CW-CN-5, CW-CN-1, and CW-CN-0.5) demonstrates a progressive modulation of the fluorescence lifetime (Figure 5c), wherein the decay rates become more rapid with increasing g-C₃N₄ content (τ decreases from 1.245 ± 0.032 ms to 0.521 ± 0.029 ms), signifying an increased probability of non-radiative deactivation pathways. Consequently, the intrinsic quantum efficiency of the Eu³⁺ sublattice experiences marginal suppression, consistent with the observed lifetime shortening. However, despite this localized quenching effect, the integrated photoluminescence quantum yield (PLQY) of the CW-CN0.5 composite achieves a substantial enhancement to 3.25%, representing approximately a 10-fold improvement over the 0.32% value recorded for the CaWO₄:7%Eu³⁺ host (Figure 5g), reflecting the dominant contribution of the broadband g-C₃N₄ emission to the total photon output.

3.5. FT-IR and UV–Vis DRS Analysis

The molecular-level structural and electronic characteristics of the CW-CN series composites were systematically investigated using Fourier-transform infrared (FT-IR) spectroscopy and UV–Vis diffuse reflectance spectroscopy, providing critical insights into the interfacial interactions and optoelectronic properties arising from the integration of g-C₃N₄ with CaWO₄-based phosphors. The FT-IR spectrum of CW-CN0.5 (Figure 6a) reveals distinct vibrational modes characteristic of both components: the broad absorption bands centered at 1240–1420 cm⁻¹ are attributed to C–N–C stretching vibrations of g-C₃N₄, while the peak near 1630 cm⁻¹ corresponds to C=N or C=O stretching modes [46,47,48,49,50], which are typical signatures of graphitic carbon nitride. Additionally, a weak N–H bending vibration is observed around 3150 cm⁻¹, and a broad band near 3400 cm⁻¹ is assigned to O–H stretching from adsorbed water molecules [51]. Concurrently, the presence of CaWO₄ is confirmed by the characteristic absorption features of the [WO₄]²⁻ tetrahedron: the ν₁ symmetric stretching mode at ~808 cm⁻¹ and the lattice vibrations below 600 cm⁻¹, consistent with the tetragonal scheelite structure. These observations collectively confirm the coexistence of both g-C₃N₄ and CaWO₄ phases within the composite.

To assess the influence of g-C₃N₄ loading on the composite composition, the FT-IR spectra of the CW-CN series (Figure 6b) were compared. As the g-C₃N₄ content decreases from CW-CN5 to CW-CN0.5, the intensity of the C=N stretching vibration near 1645 cm⁻¹ progressively diminishes and exhibits a slight redshift, indicating a reduction in the density of conjugated nitrogen-containing units. Similarly, the N–H bending mode at ~3150 cm⁻¹ and the O–H stretching band near 3400 cm⁻¹ show systematic attenuation, reflecting a decreasing contribution of g-C₃N₄-related functional groups. This trend corroborates the controlled modulation of the composite’s chemical environment and supports the hypothesis that the interfacial interaction strength may be tuned via g-C₃N₄ concentration.

The optical absorption behavior was further probed using UV–Vis diffuse reflectance spectroscopy (Figure 6c). The diffuse reflectance spectra display the reflectance (%) in the extended wavelength range of 350–800 nm, clearly revealing the absorption edges corresponding to the bandgap energies. The reflectance profile of CW-CN0.5 lies intermediate between those of pure g-C₃N₄ and CW:7%Eu³⁺, demonstrating enhanced visible-light harvesting capability compared to the bare tungstate host. A slight redshift in the absorption edge relative to CW:7%Eu³⁺ suggests a modification of the electronic structure due to the formation of a heterostructure.

To extract the optical bandgap, Tauc plots were constructed based on the Kubelka–Munk function [52,53] (Figure 6d), where F(R) = (1 − R)²/(2R) was applied to convert the diffuse reflectance data into the absorption coefficient-related function. Using the Tauc relationship [F(R)hν]² versus photon energy (hν) for the direct-bandgap semiconductor, linear extrapolation of the [F(R)hν]² curves to the photon-energy axis yields bandgap energies of 3.54 eV for CW:7%Eu³⁺ and 3.41 eV for CW-CN0.5. This reduction in bandgap energy is attributed to the formation of a Type-II heterojunction or charge-transfer states at the interface between CaWO₄ and g-C₃N₄, which facilitates electron delocalization and lowers the effective energy barrier for excitation. The narrowed bandgap not only enhances light absorption in the visible range but also implies improved potential for energy transfer processes, particularly in the context of luminescent down-conversion applications.

In summary, the combined FT-IR and UV–Vis analysis demonstrates that the incorporation of g-C₃N₄ into the CaWO₄ matrix results in a well-defined heterostructured composite with preserved phase integrity and modified electronic properties. The observed changes in vibrational modes and optical bandgap provide direct evidence of interfacial coupling and electronic hybridization, laying the foundation for understanding the enhanced photophysical performance observed in these materials.

3.6. Thermal-Quenching Behaviour

The thermal stability of phosphors is a decisive factor for their practical deployment in solid-state lighting devices. Temperature-dependent emission spectra (Figure 7a,d,e) reveal that g-C₃N₄, CaWO₄:7%Eu³⁺, and CW-CN-0.5 exhibit typical thermal quenching: their characteristic 616 nm emission and integrated intensity decrease monotonically over 25–225 °C [54]. Pristine g-C₃N₄ demonstrates severe intrinsic thermal instability, retaining merely 35% of its room-temperature intensity at 175 °C. This arises from thermally enhanced lattice vibrations, which increase the phonon energy density around luminescent centers and thereby elevate non-radiative transition probability [55]. Notably, at 175 °C, CW-CN-0.5 retains 78% of its room-temperature intensity, surpassing the 74% retention of the pristine phosphor and demonstrating that g-C₃N₄ incorporation effectively improves high-temperature luminescence retention.

The thermal-quenching activation energy reflects the material’s ability to resist thermal quenching. The higher the activation energy, the less likely the material is to undergo thermal quenching [56]. At 175 °C, the composite retains 78% of its room-temperature intensity, compared to 74% for the pristine phosphor (Figure 7b), indicating that the composite phosphor displays a modest improvement in maintaining luminescence performance at elevated temperatures.

According to the Arrhenius equation, the relationship between the luminescence intensity I(T) and the temperature T is:

$$\mathrm{I}(\mathrm{T})={\displaystyle \frac{I_0}{1+Aexp(-\frac{E}{K_{B}T})}}$$

(1)

where I(T) is the luminescence intensity at temperature T; I₀ is the luminescence intensity at low temperature (usually taken as room temperature); A is a constant; k_B is the Boltzmann constant (k_B = 8.617 $\times$ 10⁻⁵ ev/K⁻¹); and T is the absolute temperature (T = t + 273.15, where t is the Celsius temperature).

Based on the fitting results shown in Figure 7c and Figure 7f, the activation energies are determined to be 0.22 eV for CW:7%Eu³⁺ and 0.41 eV for CW-CN-0.5, respectively. The increase in activation energy from 0.22 eV to 0.41 eV suggests that g-C₃N₄ incorporation may help mitigate non-radiative losses, though further optimization is needed. With increasing temperature, the relative luminescence intensities of both CW:7%Eu³⁺ and CW-CN-0.5 decrease, but the composite maintains higher intensity at the same temperature. This indicates that CW-CN-0.5 shows slightly better thermal stability, as reflected by its higher activation energy [57].

In the evaluation of thermal stability of luminescent materials, the Arrhenius model is commonly employed to quantitatively analyze the temperature-dependent decay of emission intensity. Taking the CW:7%Eu³⁺ sample as an example, its thermal-quenching behavior in the temperature range of 75–175 °C is well described by the following equation:

$$\mathrm{l}\mathrm{n}\left[\right({\mathrm{I}}_0/\mathrm{I})-1]=6.56-2.55\times (1000/\mathrm{T})(R^2=0.926)$$

(2)

as shown in Figure 7c, indicating that the system follows a typical thermally activated quenching mechanism. As the temperature increases, intensified lattice vibrations perturb the local crystal-field symmetry [58], thereby altering the energy-level structure and transition probabilities of Eu³⁺ ions. Consequently, a greater proportion of the excitation energy originally destined for radiative transitions is instead dissipated via non-radiative pathways, resulting in a progressive decrease in luminescence intensity.

Upon further incorporation of carbon nitride, the composite sample CW-CN-0.5 exhibits significantly enhanced resistance to thermal quenching. Its corresponding Arrhenius fitting equation is:

$$\mathrm{l}\mathrm{n}[{(\mathrm{I}}_0/\mathrm{I})-1]=11.5-4.80\times (1000/\mathrm{T})(R^2=0.978)$$

(3)

with a markedly increased slope (Figure 7f), reflecting a higher activation energy. This enhancement can be attributed to the modulation of the host lattice parameters and chemical bonding characteristics induced by carbon nitride, which collectively improve the overall lattice thermal stability [51]. Moreover, the weak van der Waals interactions between carbon nitride layers may effectively absorb a portion of the thermal vibrational energy, thereby buffering the perturbation of the local crystal-field environment around rare-earth ions caused by the rising temperature, and maintaining a relatively stable coordination environment for Eu³⁺ even at elevated temperatures [59].

The CW-CN-0.5 composite phosphor retains 78% of its room-temperature emission intensity at 175 °C, demonstrating excellent thermal stability. These characteristics highlight its potential as a red-emitting conversion material for white-light-emitting diode (w-LED) applications and provide a viable pathway for further optimization of its high-temperature luminescent performance.

3.7. Chromaticity Engineering Toward White-Light Emission

The strategic impetus underlying the heterostructuring of CaWO₄:7%Eu³⁺ with g-C₃N₄ originates from the inherent spectral deficiency of conventional europium-doped tungstate phosphors. Specifically, CaWO₄:Eu³⁺ systems typically exhibit dominant red emission centered at 616 nm (⁵D₀ → ⁷F₂) accompanied by weak host emission, resulting in high correlated color temperatures (CCTs) and compromised color-rendering indices (CRIs) due to the absence of sufficient green spectral components—a critical limitation for solid-state lighting applications [60]. To circumvent this constraint, the present work exploits the complementary spectral characteristics of g-C₃N₄, whose broadband blue–green luminescence (450–550 nm), arising from n–π* electronic transitions, ideally fills the spectral gap between the UV excitation source and the red emission of Eu³⁺ [61], as evidenced by its chromaticity coordinate (0.206, 0.281) positioned in the blue region of the CIE 1931 diagram (Figure 8a).

The chromaticity engineering proceeds via precise modulation of the g-C₃N₄ loading ratio to achieve balanced spectral integration. As depicted in the CIE 1931 chromaticity diagram, pristine CaWO₄:7%Eu³⁺ occupies the deep red region with coordinates of (0.656, 0.329), whereas pure g-C₃N₄ resides in the blue region at (0.206, 0.281). Through systematic compositional optimization, the CW-CN-0.5 heterocomposite successfully bridges these extremes, achieving chromaticity coordinates of (0.294, 0.324) that fall squarely within the white-light region. This coordinate shift from the red toward the blue–green region with increasing g-C₃N₄ content (CW-CN-1: 0.216, 0.317; CW-CN5: 0.240, 0.349) substantiates the efficacy of Type-II heterojunction-mediated energy transfer in redistributing spectral weights [62]. The underlying mechanism involves the partial energy funneling from photoexcited [WO₄]²⁻ groups and g-C₃N₄ to Eu³⁺ activators, concurrent with the direct radiative recombination within the g-C₃N₄ semiconductor, thereby generating simultaneous blue–green and red emissions from a single-phase phosphor matrix.

The complete CIE chromaticity data for all samples are summarized in

Table 3. CIE chromaticity coordinates of different samples.

Sample	CIE (x, y) Coordinates
CW-CN5	(0.240, 0.349)
CW-CN1	(0.216, 0.317)
CW-CN0.5	(0.294, 0.324)
CW:7%Eu3+	(0.656, 0.329)
g-C3N4	(0.206, 0.281)

. Quantitative assessment of the white-light quality reveals that the CW-CN-0.5 phosphor exhibits a correlated color temperature (CCT) of 7673 K, calculated via McCamy’s empirical formula T = −449n³ + 3525n² − 6823.3n + 5520.33, where n = ${\displaystyle \frac{\mathrm{x}-{\mathrm{x}}_{\mathrm{e}}}{\mathrm{y}-{\mathrm{y}}_{\mathrm{e}}}}$, (x_e = 0.3320) (y_e = 0.1858), categorizing it as a high-quality cold-white-light source suitable for display backlighting and medical illumination applications [63].To further quantify the color rendering capability, the general color rendering index (R_a) was calculated according to the CIE 13.3-1995 standard method based on the measured emission spectra. The special color rendering indices (R_i) for 14 selected Munsell test samples were obtained by the following equation:

R_i = 100 − 4.6ΔE_i (i = 1, 2, 3, …, 14)

(4)

where ΔE_i represents the color difference in the i-th test sample between the test illuminant and the reference illuminant (blackbody radiator of the same CCT) in the CIE 1964 UCS color space. The general color rendering index (R_a) is given as the average of the first eight test samples:

$${\mathrm{R}}_{\mathrm{a}}={\displaystyle \frac{1}{8}}{\sum }_{i=1}^8\mathrm{R}\mathrm{i}$$

(5)

The calculated results reveal that pristine CW:7%Eu³⁺ exhibits a low R_a of merely 52.0, attributable to its emission spectrum being overly concentrated in the red region (616 nm) with severe deficiency in the blue–green band. In contrast, the CW-CN-0.5 composite demonstrates a significantly enhanced R_a of 89.6, approaching the high-quality white-light LED standard (R_a > 90) This substantial improvement stems from the complementary spectral combination of g-C₃N₄’s broad blue–green emission (~460 nm) and Eu³⁺ red emission (~616 nm), which effectively broadens the visible spectral coverage and validates the efficacy of the heterostructure strategy in overcoming the spectral deficiency of conventional CaWO₄:Eu³⁺ phosphors.

Crucially, the photographic inset in Figure 8b captures the operational near-UV-pumped LED device, fabricated using the optimized CW-CN-0.5 composite phosphor, exhibiting uniform, intense white luminescence without perceptible color segregation or intensity inhomogeneity. This visual demonstration validates the practical viability of the heterostructure strategy, confirming that the engineered energy-transfer pathways between CaWO₄:Eu³⁺ and g-C₃N₄ successfully circumvent the spectral limitations of individual constituents. The realization of single-phase white-light emission from this cost-effective tungstate/carbon nitride system not only establishes a paradigm for designing rare-earth-based luminescent composites with tunable chromaticity but also offers a scalable technological solution for next-generation energy-saving w-LEDs, eliminating the need for multi-phosphor blends and their associated color-mixing complexities.

4. Conclusions

In summary, we have successfully constructed a CaWO₄:Eu³⁺/g-C₃N₄ heterostructured composite wherein the incorporation of g-C₃N₄ modulates interfacial interactions and reduces the optical bandgap without inducing structural decomposition. Photoluminescence analysis reveals that under 365 nm excitation, the composite exhibits dual-band emission components: characteristic red emission from Eu³⁺ and broad blue–green luminescence (450–550 nm) originating from g-C₃N₄, thereby enabling tunable, cold-white-light emission from a single-phase phosphor.

Furthermore, XRD and thermal-quenching analyses demonstrate that g-C₃N₄ incorporation enhances lattice rigidity and stabilizes the crystal field surrounding Eu³⁺ ions, increasing the thermal-quenching activation energy from 0.22 eV to 0.41 eV and improving emission retention to 78% at 175 °C compared to 74% for the pristine sample. Notably, consistent with the enhancement mechanism previously observed in our group’s Lu₂MoO₆:Eu³⁺/g-C₃N₄ system [26], the present work confirms that this heterostructuring strategy is equally effective in tungstate-based hosts, validating the universality of g-C₃N₄ coupling for designing advanced rare-earth luminescent composites with high thermal stability and tunable chromaticity.