In 2004, Zhang et al. constructed a multi-layer composite long afterglow catalyst by coating Sr₄Al₁₄O₂₅:Eu²⁺,Nd³⁺ on the surface of TiO₂ catalyst, achieving the first-ever photodegradation of Rhodamine B under dark conditions. This study marks the formal commencement of applied research on long afterglow materials in the field of photo-/electrocatalysis. This work not only pioneered a new paradigm of “light storage-sustained release catalysis”, but also laid the foundation for applying long afterglow-semiconductor composite catalysts in pollutant degradation^[39]. In order to systematically sort out the development of this field, the following will be classified according to the general matrix types of long afterglow materials, and related research will be introduced in chronological order to help readers grasp the research history and current status of such catalysts in the field of photocatalysis /electrocatalysis.

The synthesis strategies for composite long persistent luminescent materials (LPLs) can be broadly categorized into physical mixing and chemical coupling. Early studies on the photocatalytic/electrocatalytic applications of composite LPLs predominantly adopted physical mixing. Li et al.^[40] prepared a composite long-afterglow catalyst by mechanically blending TiO_2−xN_y with CaAl₂O₄:(Eu⁺, Nd⁺) via planetary ball milling. CaAl₂O₄:(Eu⁺, Nd⁺) emits blue light at 440 nm, upon 325 nm UV excitation, while the nitrogen-doped TiO₂ extends its visible absorption band to 700 nm, resulting in effective spectral overlap between the two components. This enables the LPLs to act as a persistent light source for exciting the semiconductor. Under mercury lamp irradiation, the composite catalyst exhibited superior NO removal efficiency compared to pure TiO_2−xN_y. Moreover, it retained the ability to decompose NO for up to 3 h after light exposure ceases, demonstrating “light storage-controlled release catalysis” properties. This clearly demonstrates that “spectral matching” is the primary principle of in composite systems design, that is, the emission wavelength of the LPLMs must effectively overlap with the absorption band of the coupled semiconductor. However, simple mechanical mixing may result in poor interfacial contact, limiting the charge transfer efficiency. Li et al.^[21] coupled a series of aluminate-based LPLMs, including CaAl₂O₄:(Eu⁺, Nd⁺), Sr₂MgSi₂O₇:Eu⁺, Sr₄Al₁₄O₂₅:(Eu⁺, Dy⁺), SrAl₂O₄:(Eu⁺, Dy⁺) and SrAl₂O₄:Eu⁺, with TiO_2−xN_y to systematically investigate the photocatalytic degradation performance of gaseous acetaldehyde pollutants in a closed system. The amount of CO₂ produced by the different composite catalysts varied significantly. The overall CO₂ yield followed the order: CaAl₂O₄:(Eu⁺, Nd⁺) > Sr₂MgSi₂O₇:Eu⁺ > Sr₄Al₁₄O₂₅:(Eu⁺, Dy⁺) > SrAl₂O₄:(Eu⁺, Dy⁺) > SrAl₂O₄:Eu⁺. This trend correlates with the afterglow wavelength of the respective LPLs. Among them, the most optimal CaAl₂O₄:(Eu⁺, Nd⁺) exhibits the shortest emission peak at 440 nm, attributed to the 4f ⁶5d → 4f ⁷ electronic transition of the Eu⁺ ions. After 180 min of visible light irradiation, this system can completely degrad 1.161×10⁻³ of acetaldehyde, generating 1.642×10⁻³ of CO₂, corresponding to a mineralization rate of 70.7%. Although the initial luminescence intensity of CaAl₂O₄:(Eu⁺, Nd⁺) was relatively low, its extended afterglow lifetime enabled it to sustain photo-assisted catalytic activity for more than 360 min after the light source was turned off, producing approximately 5.0×10⁻⁵ of CO₂. This study preliminarily revealed the crucial influence of the afterglow wavelength on photocatalytic efficiency. Even within the same aluminate-based family, differences in the local crystal field environment of the Eu⁺ emission centers and the distribution of trap energy levels can lead to variations in afterglow properties, thereby affecting their effectiveness as “built-in light sources” for catalytic assistance. Compared to TiO_2−xN_y combined with LPLs materials, Ag₃PO₄-based photocatalysts demonstrate superior activity in degrading organic dyes. However, their application is limited by poor photostability due to the susceptibility of Ag⁺ to photocorrosion^[41]. To address this issue, Zhang et al.^[42] deposited Pt nanoparticles onto the Ag₃PO₄ surface to promote the separation of photogenerated charges, while simultaneously blending the long persistent luminescent material CaAl₂O₄:Eu⁺, Nd⁺ with a silicone-acrylic emulsion to form a corrosion-resistant coating. This coating emits blue-violet long afterglow at 440 nm, overlapping with the absorption band of Pt/Ag₃PO₄. In a Rhodamine B (RhB) degradation experiment, the composite catalyst achieved an 81.7% degradation rate within 5 h, outperforming the 54.1% achieved by pure Ag₃PO₄ (54.1%) alone. In the dark, the RhB solution gradually faded in the presence of the composite catalyst, while a blank coating without LPLs showed no such degradation. During the daytime, CaAl₂O₄:Eu⁺, Nd⁺ absorbs sunlight, exciting Eu⁺ and causing electron-hole separation. The electrons migrate through the conduction band and are trapped by VO⁺ and NdCa⁺ defects, later escaping due to thermal disturbance at room temperature and returning to the conduction band. Finally, they undergo radiative transitions back to the 4f ⁷ ground state of Eu⁺, simultaneously emitting 440 nm afterglow. This persistent luminescence further excites Pt/Ag₃PO₄ to generate electron-hole pairs and reactive species such as h⁺ and ·OH⁻, which oxidize and degrade dye molecules. Corrosion resistance tests confirmed that the diffraction peaks and 440 nm emission intensity of the CaAl₂O₄:Eu⁺, Nd⁺ coating remained stable after immersion, demonstrating that the silicone-acrylic emulsion strategy enhances chemical stability and shows promise in mitigating corrosion and degradation issues.

Havasi et al.^[43] combined the long persistent luminescent material Sr₄Al₁₄O₂₅:Eu⁺, Dy⁺ (SRAL) with a ZnO:Co+Ag catalyst to construct a composite system. Using the degradation of methyl orange (MO) in aqueous solution as a model reaction, they systematically investigated the performance of various ZnO:Co+Ag/SRAL composites for energy-saving wastewater purification. ZnO:Co+Ag can utilize violet and blue light, while SRAL (emitting at 495 nm, blue-green) can provide photoluminescence assistance. The study employed two experimental setups to optimize the photocatalytic performance of the composite long-afterglow catalyst. One is the “SRAL-mixed” scheme, where the composite catalyst was placed directly in the reaction vessel. The other is the “PDMS/SRAL-assisted” scheme, where the catalyst was immobilized in a PDMS layer positioned beneath the transparent bottom of the reaction vessel. A catalyst without SRAL served as the control group, placed directly within the reaction vessel. The composite catalysts were tested under three distinct illumination modes: continuous light (210 min illumination, 0 energy saving), 1–1 s pulsed light (105 min illumination, 50% energy saving), and 1–10 s pulsed light (21 min illumination, 90% energy saving), as detailed in Figure 5(a)^[43]. The complete experimental procedure consisted of (1) different pre-illumination treatments of the LPLs; (2) photocatalytic reaction; (3) solution sampling; (4) determination of MO concentration by UV-vis spectrophotometry. The results (Figure 5(b)) showed that the photocatalytic activity followed the order: no LPL assistance < “PDMS/SRAL-assisted” scheme < “SRAL-mixed” scheme; and 1–10 s pulses < 1–1 s pulses < continuous illumination. Notably, the 1–1 s pulsed mode in the “SRAL-mixed” scheme achieved a degradation efficiency of 81% within 210 min, approaching the 83.6% degradation efficiency of pure ZnO:Co+Ag under continuous illumination. It is worth mentioning that although the 1–10 s pulsed mode has the shortest illumination time, it can still degrade over 60% of MO within 210 min. This indicated that the presence of SRAL can maintain relatively high catalytic efficiency while significantly reducing the required lighting power. Moreover, photocatalytic activity was also observed in darkness solely via the afterglow emission from pre-illuminated LPLs. As shown in Figure 5(c), increasing the SRAL content in the PDMS/SRAL plate (black squares) significantly enhanced catalyst performance, confirming that the visible light emitted by SRAL can effectively drive the photocatalytic reaction of ZnO:Co+Ag nanoparticles. Compared to the “PDMS/SRAL-assisted” scheme (black squares), the “SRAL-mixed” scheme (green pentagons) exhibits superior catalytic activity, achieving 77.9% MO degradation. Indeed, both assistance modes effectively promoted the photocatalytic dye degradation under continuous and pulsed illumination conditions. Nevertheless, the researchers in this paper recommended the “PDMS/SRAL-assisted” scheme for practical applications, as it avoids potential environmental and economic risks associated with the dissolution and separation of LPLs, as well as issues related to polarization or shielding effects.

SrAl₂O₄:Eu⁺,Dy⁺ (SAO) is one of the most classic aluminosilicate-based long persistent luminescent materials (LPLs). Lu et al.^[44] designed a SAO-based Cu₂O nanocrystal/TiO₂ microsphere rotating-disk reactor. Compared to conventional fixed-bed systems, the rotating catalyst disk generates a continuously renewed thin water film, which shortens the light-path length, reduces solution light absorption, accelerates the refreshment of surface pollutants and their intermediates, and thereby enhances photocatalytic activity. The built-in SAO component absorbs excess light energy and produces long-lasting afterglow, exciting Cu₂ONCs/M-TiO₂ to generate photogenerated charge carriers, thereby enabling continuous catalysis without an external light source. This rotating disk reactor achieved 24 h, round-the-clock degradation rates of 98% for rhodamine B and 93% for bisphenol A. However, this study merely employed the LPL as an external auxiliary light source and did not explore the potential intrinsic performance enhancement that could arise from its composite integration with other catalysts. Numerous studies have shown that SAO can be chemically integrated with various semiconductor materials to construct novel photocatalytic systems, particularly with g-C₃N₄. Aliabadi et al.^[45] synthesized SAO via the gel-combustion method and integrated it with g-C₃N₄/WO₃/Bi₂WO₆ to construct a dual Z-scheme heterojunction photocatalyst. This composite increased the dye removal rate under illumination from 88% to 98%. Moreover, under weak light conditions (simulating cloudy weather), the composite still degraded 87% of the pollutant within 20 min. In another study, Li et al.^[46] combined SAO with g-C₃N₄ through high-temperature calcination to fabricate a binary composite catalyst. This system achieved up to 80% degradation rate of basic fuchsin (BF) within 10 min, approaching complete degradation within 20 min. After the light source was removed, the composite maintained 90% BF degradation rate within 3 h, whereas pure g-C₃N₄ exhibited no catalytic activity. Interestingly, pure SAO exhibited higher degradation efficiency in the dark than the SAO/g-C₃N₄ composite, suggesting that the incorporation of g-C₃N₄ has a certain effect on the light absorption and energy storage of SAO. Mechanistic studies reveal that under visible-light irradiation, photogenerated electrons in the valence band of g-C₃N₄ are excited to its conduction band, leaving holes behind. Because the conduction band potential of SAO (+0.32 eV) is higher than that of g-C₃N₄ (−1.03 eV), electrons transfer from g-C₃N₄ to SAO. Simultaneously, holes reverse-migrate to g-C₃N₄, as the valence band potential of SAO (+2.47 eV) is higher than that of g-C₃N₄ (+1.65 eV). This bidirectional charge separation significantly enhances the separation efficiency of photogenerated carriers, boosting photocatalytic activity. In SAO, photoexcitation promotes Eu⁺ from ground state (4f ⁷) to an excited state (4f ⁶5d), oxidizing Eu⁺ to Eu⁺, while Dy⁺ traps electrons and is reduced to Dy⁺. Under illumination, conduction-band electrons react with O₂ to generate superoxide radicals, which synergize with holes to mineralize pollutants (e.g., BF) into H₂O and CO₂.

In darkness, stored energy in SAO is released: Dy⁺ releases electrons to the conduction band, which are transferred to Eu⁺, reducing it back to Eu⁺. Meanwhile, the afterglow excitation of SAO enables g-C₃N₄ to continuously generate active charge carriers, achieving long-term catalysis in dark environments. Another example is the work by Chen et al.^[47] who constructed a dual Z-scheme ternary composite photocatalyst by integrating SAO with g-C₃N₄@NH₂-UiO-66 via a solvothermal method. Under 30 min of visible-light irradiation, this composite achieved a 95% degradation rate for methylene blue (MB), which is 5.86, 4.04 and 10 times higher than that of pure SAO, g-C₃N₄, and NH₂-UiO-66, respectively. Moreover, it maintained a 50% MB degradation rate over 5 h in the dark. While chemical integration generally leads to closer interfacial contact between LPLs and catalysts, the coupling does not always result in straightforward positive gains. Overall, composite catalysts tend to exhibit a lower band gap (E_g) and enhanced visible-light absorption capabilities.

Another commonly studied long persistent luminescent materials (LPLs) is the silicate-based Sr₂MgSi₂O₇:Eu⁺,Dy⁺ (SMSO). Bu et al.^[48] constructed an SMSO/g-C₃N₄ Z-scheme heterojunction via mechanical mixing followed by sintering. Under visible-light irradiation, the composite catalyst achieved a 95% degradation rate of methylene blue within 30 min, with a reaction rate constant 3.2 times that of pure g-C₃N₄. Density functional theory calculations revealed that the Fermi levels of g-C₃N₄ and SMSO are –0.70 and –0.80 V (vs. NHE), respectively. The proposed mechanism involves the following steps: upon illumination, electrons in g-C₃N₄ are excited from the valence band (VB) to the conduction band (CB), while electrons in SMSO remain stable in its VB. Through the heterojunction interface, the photoexcited electrons in g-C₃N₄ transfer to the VB of SMSO and recombine with its holes. This process suppresses the recombination of electron-hole pairs in g-C₃N₄ itself, allowing holes in the VB of g-C₃N₄ (with strong oxidizing ability) and electrons in the CB of SMSO (with strong reducing ability) to accumulate, thereby synergistically enhancing the redox capacity of the system. Electron spin resonance (ESR) tests confirmed significantly enhanced signals of $\cdot {\mathrm{O}}_2^- $ and ·OH radicals under light. In the dark, the material sustained catalytic activity for up to 5 h, achieving 50% degradation with a 32.6 times that pure g-C₃N₄. This is attributed to the spectral overlap between the photoluminescence peak of SMSO and the absorption spectrum of g-C₃N₄, enabling SMSO to persistently excite g-C₃N₄ in the absence of light, generate electron-hole pairs, and maintain charge separation through the Z-scheme interface to sustain the catalytic reactions.

Wu et al.^[49] prepared SMSO via a solid-state method and combined it with Ag₃PO₄ using a solution mixing process. The absorption band of Ag₃PO₄ matches the 470 nm photoluminescence peak of SMSO originating from the Eu⁺ (4f ⁶5d→4f ⁷) transition. The loading of Ag₃PO₄, through the inner-filter and light-reabsorption effects, caused a 71%–83% attenuation in the PL intensity of SMSO (the quantum yield decreased from 7%–12% to about 2%) and weakened its afterglow. Nevertheless, the composite catalyst exhibited a 2.5-fold and 1.9-fold increase in photocatalytic efficiency for methyl orange (MO) under sunlight and visible light, respectivel. Moreover, residual MO was further degraded in the dark for 20 min after the light exposure ceased (reaching 32% and 9% degradation following solar and visible light excitation, respectively), whereas pure Ag₃PO₄ completely lost its degradation capability once illumination ceased. Ma et al.^[50] fabricated a dual S-scheme heterojunction SMSO/ZnIn₂S₄/UiO-66-NH₂, via hydrothermal self-assembly method. The composite catalyst showed significantly higher degradation efficiencies for Rhodamine B (RhB) and methylene blue (MB) under visible light compared to its individual components, achieving degradation rates of 72.2% and 81.7% after 40 min (RhB) and 60 min (MB) irradiation, respectively. MB after 60 min of irradiation. After the light was turned off, the material continued to catalyze degradation in the dark via its afterglow for 4 h (RhB) and 6 h (MB), increasing the total degradation rates further to 86.7% and 96.8%, respectively.

In addition to aluminate- and silicate-based long persistent luminescent materials (LPLs), certain LPLs with other matrix compositions have also demonstrated excellent performance in photocatalytic degradation. Vaiano et al.^[51] prepared nitrogen-doped TiO₂ (N-TiO₂) via a sol-gel method and combined it with a commercial blue-emitting LPL, Sr_2.90Eu_0.03Dy_0.07Al₄SiO₁₁ to construct a photocatalytic system driven by ultraviolet LEDs, aiming to reduce the energy consumption in photocatalytic wastewater treatment. The emission spectrum of this LPL (peak at 440 nm) can excite N-TiO₂, achieving a 90% decolorization efficiency for crystal violet through alternating light-dark cycles (total duration 60 min, actual illumination 30 min). In contrast, continuous illumination requires 60 min to attain the same efficiency. This strategy reduced energy consumption and economic cost by 42%.

He et al.^[52] pioneered the covalent integration of ultralong-persistence phosphor (afterglow >16 d) with metal-organic framework, constructing, Zn₃Ga₂Ge₂O₁₀:0.5%Mn/NH₂-MIL-101(Fe) (LPLs/MOFs) photocatalyst, significantly extended the timescale of dark-state catalysis from “hours” to “days,” greatly expanding the material’s potential for long-term catalytic applications. The study systematically compared two composite strategies—physical mixing and chemical bonding—demonstrating the significant advantages of the chemical approach in enhancing interfacial interactions and catalytic performance. This study utilized an aminosilane coupling agent to covalently modify hydrothermally synthesized LPLs onto the surface of NH₂-MIL-101(Fe), forming a heterojunction. The afterglow emission spectrum of the composite (peak at 535 nm) overlapped strongly with the absorption spectrum of the MOF, enabling an efficient persistent luminescence resonance energy transfer (PLRET) mechanism. After 120 min of UV irradiation, LPLs/MOF (1∶1) composite completely degraded Rhodamine B (RhB), exhibiting significantly higher degradation efficiency than the physically mixed LPLs+MOF (1∶1) counterpart. This enhancement is attributed to the heterojunction formed via covalent bonding, which effectively suppresses the rapid recombination of photogenerated electron-hole pairs, while the high specific surface area of the MOF provides abundant active sites. After turning off the light following 30 min of irradiation, the composite continued to degrade RhB for up to 12 h, achieving a total degradation rate of 98% within 24 h. In contrast, the physically mixed system showed reduced PLRET efficiency due to excessive spacing between LPLs and MOFs.

Currently, numerous studies have explored photocatalytic systems based on long persistent luminescent materials (LPLs) to achieve persistent photocatalytic degradation of organic pollutants. However, most of these studies rely on combinations of two materials. Whether through physical blending or chemical coupling via heterojunction formation, the LPL component typically does not serve as the primary catalytic agent. Instead, it acts as a persistent light source for the coupled semiconductor catalyst, enabling continued activity in the dark. In contrast, single-phase LPLs can simultaneously exhibit both persistent luminescence and high catalytic efficiency. Consequently, the research focus is gradually shifting from composite systems to single-phase LPLs to explore their potential for direct photocatalytic degradation of organics. Relevant studies remain relatively limited. The following section introduces two distinct strategies for designing single-phase LPL catalysts: one focuses on “intrinsic modification” through bulk ion doping to tailor the material’s native defects and energy-level structure, while the other employs “extrinsic modification” via surface nano-loading to introduce new optical and catalytic active sites. The former emphasizes structural homogeneity and stability, whereas the latter offers greater flexibility for integrating diverse functional nanomaterials.

Zhou et al.^[53] developed a single-phase, “dual-functional” long persistent luminescent material (LPL) Zn₂SiO₄:Ga⁺, which significantly extends the lifetime of photogenerated charge carriers by introducing positive charge traps through Ga⁺ doping, solving the issue of insufficient light penetration depth in conventional photocatalysts under high-concentration conditions. The catalyst was synthesized via a sol-gel method with a Ga⁺ doping concentration of 1% (atomic percent). Upon 254 nm UV excitation, Zn₂SiO₄:Ga⁺ exhibits broad-band emission from 320 to 650 nm, featuring a main UV peak at 362 nm and a shoulder peak at 462 nm attributed to Ga⁺ doping (Figure 6(a)). Its long persistent luminescence covers 325–550 nm, with a dominant peak at 398 nm, and the afterglow intensity is sufficient for naked-eye observation in the dark for over 5 min (Figure 6(b)). The decay curve of Zn₂SiO₄:Ga⁺ follows a double-exponential model (fast decay constant τ₁ = 0.87 min, slow decay constant τ₂ = 4.30 min), with the afterglow signal remains detectable for over than 30 min, extending the charge-carrier separation lifetime to the minute scale (Figure 6(c)). In degradation tests using 10 mg/L permethrin, Zn₂SiO₄:Ga⁺ (2 g/L) achieved a total organic carbon (TOC) removal rate of 97.1% under 2 h of UV irradiation, significantly higher than the 59.7% removal by undoped Zn₂SiO₄ (Figure 6(d), (e)). A key advantage of this material is its adaptability to high catalyst concentrations: even at 10 g/L, Zn₂SiO₄:Ga³⁺; maintained a TOC removal efficiency of 97%, whereas conventional Zn₂SiO₄ dropped to 28%. This phenomenon is attributed to the positive charge traps formed by Ga³⁺ doping, which capture photogenerated electrons and suppress recombination. Simultaneously, the persistent luminescence enables the catalyst to transfer stored ultraviolet energy deep into the solution, overcoming light-scattering limitations in high-concentrated systems. Moreover, the catalyst showed stable performance across a pH value range of 5–9 and under various water conditions (pure water, tap water, saline water), with activity declining only from 92.1% to 88.6% after five cycles, demonstrating excellent reusability.

Hai et al.^[54] systematically investigated the performance of the classic silicate long persistent luminescent material Sr₂MgSi₂O₇:Eu⁺,Dy⁺ (SMSO) in the degradation of persistent organic dyes. To enhance its catalytic efficiency, they uniformly loaded silver nanoparticles onto the material surface via a photochemical reduction method. Optically, SMSO exhibits an excitation spectrum ranging from 250 to 450 nm, with an emission peak at 470 nm. Afterglow testing revealed that the initial afterglow brightness of pristine SMSO was 700 mcd/m², while Ag-loaded SMSO increased remarkably to 6800 mcd/m² (an 871% enhancement), with an afterglow duration exceeding 15 h. In photocatalytic experiments, SMSO exhibited a degradation rate of 45.1% for Rhodamine B (RhB) after 8 h of UV irradiation. After the light was turned off, the degradation rate increased to 76.5% over the subsequent 24 h in the dark. Under the same 24 h period, SMSO also degraded methylene blue (MB) and methyl orange (MO) by 39.5% and 20.4%, respectively. After Ag modification, the degradation performance improved significantly: 88.9% for RhB, 75.9% for MB and 46.5% for MO. The variation in degradation efficiency among the dyes stems from their structural properties: the N⁺ group in RhB is relatively susceptible to destruction, whereas the azo structure in MO is more stable. Furthermore, pH significantly influenced the degradation performance. The highest degradation rate for RhB (80.6%) occurred under acidic conditions, while optimal degradation for MO (40.2%) and MB (63.1%) was observed under alkaline conditions.

This behavior is correlated with the generation efficiency of radicals (·OH) and the dye adsorption capacity. Mechanistic studies indicated that under illumination, Eu⁺ excitation generates electron-hole pairs, with some electrons being captured by Dy⁺. In the dark, the gradual release of electrons from these trap levels sustains the catalytic reaction. Ag nanoparticles enhanced light absorption capacity through surface plasmon resonance effect while promoting interfacial electron migration, effectively inhibiting electron-hole pair recombination.

Currently, composite long-afterglow materials applied to photocatalytic water splitting for hydrogen evolution primarily rely on heterojunctions constructed through chemical coupling strategies. Liu et al.^[59] successfully fabricated a g-C₃N₄@Au@SrAl₂O₄:Eu⁺,Dy⁺ composite catalyst using a Pechini-type sol-gel method combined with mechanical mixing and high-temperature calcination.In this system, the afterglow emission spectrum of SrAl₂O₄:Eu⁺,Dy⁺ (SAO, 450–600 nm) shows strong overlap with the absorption spectrum of g-C₃N₄@Au, while Au nanoparticles enhance visible-light absorption through surface plasmon resonance.

Under visible-light irradiation, the order of hydrogen evolution activity was: g-C₃N₄@Au@SAO > g-C₃N₄@SAO > g-C₃N₄@Au > g-C₃N₄. After the light was turned off, only g-C₃N₄@Au@SAO maintained hydrogen evolution activity. This work first demonstrated photocatalytic hydrogen evolution in an aqueous phase using a composite LPL material. Tu et al.^[60] coupled SAO with Cu|CuO via an incomplete solid-state reaction to fabricate a Z-scheme heterojunction Cu|CuO/SAO composite film photocatalyst. This material utilizes the green afterglow emitted by SAO in the dark to excite CuO and generate photogenerated charge carriers. XRD and SEM characterization revealed that a tight Z-scheme heterojunction formed between fibrous CuO (~200–300 nm) and SAO particles (~1000–1500 nm), which helps suppress electron-hole recombination and thereby enhances the redox capability of the material. The total hydrogen production of the composite film reached 672.29 μmol/g over 180 min of simulated sunlight irradiation followed by 240 min in darkness.

Sr₂MgSi₂O₇:Eu⁺,Dy⁺ (SMSO), a classic silicate-based long persistent luminescent material (LPL), has also garnered significant attention for photocatalytic hydrogen evolution. Tang et al.^[61] constructed a novel Z-scheme composite SMSO/Ag₃PO₄ via a solvent-based method for hydrogen production. Experimental results showed that the composite photocatalyst with a mass ratio of 15∶1 between SMSO and Ag₃PO₄ exhibits, producing 491 μmol/g of H₂ after 1 h of simulated sunlight irradiation. The hydrogen yield further increased by 100 μmol/g over 3 h in the dark, reaching a cumulative total of 591 μmol/g. In a separate study, Tang et al.^[62] built a Z-scheme photocatalytic system by integrating SMSO with BiFeO₃ and further enhancing its performance with silver nanoparticles (Ag NPs). Herein, BiFeO₃ was synthesized via the hydrothermal method, while Ag NPs were deposited on SMSO via photoreduction, followed by the construction of a BiFeO₃/Ag/SMSO/Ag composite through sequential ionic layer adsorption, isoelectric precipitation, and high-temperature calcination. The loading of Ag NPs significantly boosted the luminescence intensity of SMSO and improved the sustained catalytic activity of the entire system. This catalyst achieved a hydrogen production of 444.64 μmol/g after 240 min of light irradiation—3.63 times that of bare BiFeO₃. Moreover, it produced an additional 57.07 μmol/g of H₂ during a subsequent 240 min dark period, representing a 3.15-fold improvement over the single-component catalyst. Wang et al.^[63] employed a solvothermal method to fabricate a Type-II heterojunction composite photocatalyst by coupling SMSO with CdS (SMSO@CdS). Under UV-vis irradiation for 0.5 h, this material exhibited a remarkably high hydrogen evolution rate of 18.07 mmol/g, which is 326 times and 1.9 times that of pure SMSO and CdS, respectively (Figure 7(a)). In the dark, the composite continued to produce hydrogen for 4 h, with a cumulative yield of 6.31 mmol/g (Figure 7(b)), significantly outperforming both individual components (SMSO and CdS) and most reported counterparts. The superior performance of this system can be attributed to three key factors: (1) the heterojunction structure effectively suppresses the recombination of photo-generated electron-hole pairs; (2) the afterglow emission of SMSO (peaking at 470 nm) overlaps well with the absorption band of CdS, enabling hydrogen production even without light; (3) the intrinsic electric field (IEF) interface synergistic effect and electron redistribution guides the directional migration of charges. Under dark conditions, electrons released thermally from Dy⁺ traps are continuously transferred to the surface of CdS via the IEF to participate in hydrogen evolution (Figure 7(c)).

Through a solvothermal-calcination process, Abulimiti et al.^[64] combined another silicate-based long-afterglow material, ZnGa₅Si₅O₁₈ (ZGSO), with sulfur-doped carbon nitride (SCN). Under xenon lamp irradiation, the ZGSO/SCN (1∶1) composite catalyst achieved a photocatalytic hydrogen evolution rate of 3963.80 μmol/(g·h), which is about 15 times higher than that of pure ZGSO (307.1 μmol/(g·h)) and 1.4 times higher than that of pure SCN (2800.80 μmol/(g·h)). Li et al.^[65] employed a strategy of in situ growing covalent organic framework (COF) on the surface of Zn₂GeO₄:Mn⁺, Li⁺ (ZGO) LPLs to construct a core-shell heterojunction composite catalyst (ZGO@COF).

ZGO exhibits an emission peak at 540 nm, originating from the ⁴T₁(⁴G) → ⁶A₁(⁶S) electronic transition of Mn⁺ ions. Its emission spectrum highly matches the absorption spectrum of the COF (Figure 8(a))^[65], indicating that the afterglow luminescence from ZGSO can be effectively absorbed by the COF, thereby sustaining photocatalytic reactions even after light cessation. Photocatalytic performance testing revealed that ZGO@COF (1∶6) exhibited higher hydrogen evolution activity (Figure 8(b)) than pure COF (26.97 mmol/(h·g)) and pure LPLs (0.38 mmol/(h·g)). However, ZGO itself showed very weak hydrogen evolution performance, primarily due to its narrow light absorption range.

Unlike the research approach of composite long persistent luminescent materials (LPLs) in photocatalysis, which follows a “degradation-then-hydrogen evolution” pathway, studies on single-phase LPL catalysts for hydrogen evolution actually predate their exploration for organic degradation. Cui et al.^[66] were the first to propose using a single-phase LPL directly as a photocatalyst to enhance light energy utilization efficiency. They synthesized Sr₂MgSi₂O₇:Eu⁺,Dy⁺ via a sol-gel method and systematically investigated its structure and properties. The results indicated that rare-earth ion doping induced oxygen vacancies and numerous lattice defects on the material surface, thereby enhancing photocatalytic activity. Eu⁺ and Dy⁺ introduced dopant energy levels within the forbidden band of Sr₂MgSi₂O₇, reducing its absorption bandgap from the intrinsic value of 7.9 to 2.34 eV. The valence band top and conduction band bottom of Sr₂MgSi₂O₇:Eu⁺,Dy⁺ were located at −1.44 eV, while the intermediate state formed by Eu⁺ was positioned at 1.58 eV, with the intrinsic valence band of the host at 6.46 eV. This band structure meets the potential requirements for the hydrogen evolution reaction. Furthermore, the material exhibited long persistent luminescence with a maximum emission peak at 465 nm following UV excitation, persisting for over 20 h. Photocurrent measurements further confirmed its favorable photoelectric response characteristics. Evaluated its hydrogen evolution performance via methanol–water reforming under UV light, visible light, and dark conditions. When irradiated with a 500 W high-pressure mercury lamp (peak emission at 365 nm), Sr₂MgSi₂O₇:Eu⁺,Dy⁺ achieved a maximum hydrogen production rate of 14 mmol/(h·g), with a solar-to-hydrogen (STH) conversion efficiency of 5.18%—one of the highest light-energy conversion efficiencies reported in the field of photocatalytic hydrogen evolution at the time. After 15 min of UV exposure, hydrogen production continued to increase over time in the dark, reaching a cumulative 264 μmol/g after 8 h, with an STH conversion efficiency of 0.39% (Figure 9(a)). The possible mechanism reveals (Figure 9(b)) that under illumination, electrons in the ground state 4f of Eu⁺ are excited to the 5d level, then thermally excited into the conduction band of Sr₂MgSi₂O₇, while Eu⁺ is oxidized to Eu⁺ by losing electrons. Most of the excited electrons originating from Eu⁺ in the conduction band can directly interact with the reaction substrate, improving catalytic efficiency during illumination. Unconsumed electrons can be temporarily stored by Dy⁺ ions, which are reduced to Dy⁺, thereby prolonging the carrier lifetime (pathway I, indicated by red arrows in Figure 9(b)). In the dark, influenced by thermal disturbances, electrons are released from Dy⁺ and continue to participate in the reaction, enabling sustained hydrogen production.

Previous studies have shown that when the sum of 4f electron counts of two different trivalent rare-earth ions is an integer multiple of 7 (e.g., Ce⁺ (4f) and Eu⁺ (4f ⁶)), a conjugated electronic structure can form. This enhanced electron transfer and light absorption in long persistent luminescent materials (LPLs)^[67]. Fan et al.^[68] first synthesized an LPL Sr₂MgSi₂O₇:Eu,Ce, with such a conjugated electronic structure via a sol-gel method and applied it to photocatalytic hydrogen evolution (HER) and oxygen evolution reactions (OER). The material exhibited a main emission peak in the blue region at 467 nm, with afterglow lasting over 8 h (Figure 10(a)). The introduction of Eu⁺ and Ce⁺ ions created intermediate energy levels, resulting in a band gap of 2.67 eV—significantly narrower than that of the undoped host (estimated from the absorption edge in Figure 10(b))^[68]. Ultraviolet photoelectron spectroscopy (Figure 10(c)) revealed an intermediate state (acting as a valence band) at 2.82 eV, compared to the intrinsic valence band of the Sr₂MgSi₂O₇ host at 6.46 eV.

Considering the critical role of the host’s intrinsic conduction band in transferring photogenerated electrons for afterglow, the energy level at −1.44 eV was assigned as the conduction band minimum of Sr₂MgSi₂O₇:Eu,Ce. Under irradiation from a 500 W mercury lamp, the hydrogen and oxygen evolution rates reached 13.0 and 0.67 mmol/(g·h), respectively (Figure 10(d), (e)). More notably, after 2 h of UV pre-irradiation and subsequent placement in the dark, the material continued to produce hydrogen and oxygen, accumulating 0.75 mmol/g of H₂ and 2.34 mmol/g of O₂ over 8 h (Figure 10(f), (g)). Compared to Sr₂MgSi₂O₇:Eu and Sr₂MgSi₂O₇:Ce, the co-doped Sr₂MgSi₂O₇:Eu,Ce demonstrated superior photocatalytic efficiency under both light and dark conditions, indicating that the coexistence of Eu and Ce is crucial for achieving efficient water splitting, particularly for the OER half-reaction. In Sr₂MgSi₂O₇:Eu,Ce, oxygen vacancy defects serve as the primary electron traps, Eu⁺ acts as the luminescent center, and Ce⁺ functions as a photosensitizer that enhances Eu⁺ emission via energy transfer. As illustrated in Figure 10(h), under illumination, 4 f electrons of Eu⁺ are excited to the conduction band of the host and participate directly in reactions. The remaining unexhausted excited electrons are captured by electron traps and released slowly to interact with reactants (Path I). Simultaneously, excited electrons generated from Ce⁺ absorption follow two competitive transfer paths (Path II), both contributing to enhanced catalytic efficiency: (1) they can be captured by electron traps or directly transferred to Eu⁺, to form Ce⁴⁺, oxidizing surface OH⁻ to produce oxygen. (This is enabled by the facile electron transfer between Eu⁺ and Ce⁺ within the conjugated structure, leading to the anomalous valence states Ce⁴⁺ and Eu⁺). (2) Excited electrons can relax to the ground state, with the released energy being absorbed by Eu⁺, thereby increasing the number of photogenerated carriers from Eu⁺ and reducing non-radiative losses (Path II). XPS analysis (Figure 10(i), (j)) confirmed the coexistence of Ce⁺, Ce⁴⁺, Eu⁺ and Eu⁺, indicating a dynamic valence equilibrium and charge transfer within the system. This conjugated structure promotes charge transfer between active sites and improves the efficiency of the catalytic cycle. Although the material’s band structure satisfies the thermodynamic requirements for overall water splitting, the strong association of Ce⁺ with lattice-oxygen migration significantly enhances oxygen evolution capability. This leads to a mismatch between HER and OER rates, making it difficult to achieve simultaneous, efficient overall water splitting. This study provides a novel perspective for designing high-performance long-persistent photocatalysts based on conjugated electronic structures.