Mechanoluminescent-Boosted NiS@g-C₃N₄/Sr₂MgSi₂O₇:Eu,Dy Heterostructure: An All-Weather Photocatalyst for Water Purification

Abstract

The vast majority of photocatalysts find it difficult to consistently and stably exhibit high performance due to the variability of sunlight intensity within a day, as well as the high energy consumption of artificial light sources. In this study, mechanoluminescent Sr₂MgSi₂O₇:Eu,Dy phosphors is combined with NiS@g-C₃N₄ composite to construct a ternary heterogeneous photocatalytic system, denoted as NCS. In addition to the enhanced separation efficiency of photogenerated charge carriers by the formation of a heterojunction, the introduction of Sr₂MgSi₂O₇:Eu,Dy provides an ultra-driving force for the photocatalytic reactions owing to its mechanoluminescence-induced excitation. Results show that the degradation rate of RhB increased significantly in comparison with pristine g-C₃N₄ and NiS@g-C₃N₄, indicating the obvious advantages of the ternary system for charge separation and migration. Moreover, the additional photocatalytic activity of NCS under ultrasound stimulation makes it a promising all-weather photocatalyst even in dark environments. This novel strategy opens up new horizons for the synergistic combination of light-driven and ultrasound-driven heterogeneous photocatalytic systems, and it also has important reference significance for the design and application of high-performance photocatalysts.

1. Introduction

The continuous water pollution by organic dyes poses a significant threat to both ecosystems and human health due to their high toxicity, potential carcinogenicity, and resistance to natural biodegradability [1]. To address this challenge, semiconductor-based photocatalysis has emerged as a promising strategy for degrading organic dyes. Among various photocatalysts, graphitic carbon nitride (g-C₃N₄) has attracted increasing attention due to its visible-light response, chemical and thermal stability, and low cost [2,3,4,5,6,7,8,9]. However, the rapid recombination of photogenerated electron–hole pairs in pure g-C₃N₄ severely limits its photocatalytic performance [10].

To overcome this limitation, constructing heterojunctions with suitable cocatalysts has been a widely adopted approach to facilitate charge separation and suppress recombination [11]. Nickel sulfide (NiS), a low-cost, earth-abundant transition metal sulfide, has shown great potential in enhancing photocatalytic activity. It has been reported that NiS cocatalyst can act as an efficient electron acceptor, thus promoting the migration of photogenerated electrons and reducing carrier recombination [12,13].

It is worth noting that although g-C₃N₄-based heterojunctions have shown potential in photocatalysis, traditional photocatalysts rely on external light sources, resulting in low efficiency in dark environments. Fortunately, mechanoluminescent materials (e.g., Sr₂MgSi₂O₇:Eu,Dy) have shown potential to enhance photocatalytic activity by converting external mechanical stress (e.g., ultrasound) into localized light emission, thus providing additional photon energy to photocatalysts [13,14,15]. This synergy between mechanoluminescence and catalysis not only promotes pollutant degradation but also opens a novel pathway for photocatalysis under dark conditions without the need for external light irradiation. Nevertheless, the integration of mechanoluminescent materials with semiconductor catalysts for photocatalytic degradation applications remains unexplored. Our hypothesis is that integrating mechanoluminescent Sr₂MgSi₂O₇:Eu,Dy with NiS@g-C₃N₄ can bridge this gap by enabling ultrasound-driven catalytic activity in the dark. The novelty lies in the first-ever integration of NiS@g-C₃N₄ heterojunction with Sr₂MgSi₂O₇:Eu,Dy mechanoluminescent phosphors, achieving dual-mode (light/ultrasound) catalytic activity for all-weather applications.

Herein, we rationally designed a novel heterojunction photocatalyst (NiS@g-C₃N₄/Sr₂MgSi₂O₇:Eu,Dy) by coupling NiS-decorated g-C₃N₄ with Sr₂MgSi₂O₇:Eu,Dy mechanoluminescent phosphors. The heterojunction demonstrates enhanced photocatalytic degradation of Rhodamine B (RhB) under visible light and, importantly, retains catalytic activity in dark environments under ultrasound stimulation via mechanoluminescence-induced photoexcitation. The degradation performance, underlying mechanism, and the role of mechanoluminescence were systematically investigated. This study not only provides a new strategy for developing high-efficiency photocatalysts but also introduces a novel paradigm for light-independent photocatalysis in environmental remediation.

2. Materials and Methods

2.1. Materials

All the chemical reagents were of analytical purity and purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China.

2.2. Preparation of g-C₃N₄

g-C₃N₄ were synthesized by thermolysis of urea at 550 °C for 4 h. The as-prepared product was washed with diluted HNO₃ and then dried at 80 °C overnight.

2.3. Preparation of NiS@g-C₃N₄

A total of 100 mg of g-C₃N₄ (1.087 mmol) was mixed with 91 mg of NiCl₂∙6H₂O (0.383 mmol) and 21 mg of thioacetamide (0.279 mmol), then transferred to a 100 mL Teflon-lined autoclave and heated at 190 °C for 10 h. The resulting NiS@g-C₃N₄ composite was washed with water and ethanol before drying at 80 °C overnight.

2.4. Preparation of Sr₂MgSi₂O₇:Eu,Dy

Sr₂MgSi₂O₇:Eu,Dy mechanoluminescent phosphors were prepared via a solid-state reaction. Specifically, 1866.05 mg of SrCO₃ (12.640 mmol), 621.63 mg of (MgCO₃)₄∙Mg(OH)₂∙5H₂O (1.280 mmol), 769.02 mg of SiO₂ (12.800 mmol), 6.7 mg of Eu₂O₃ (0.019 mmol), and 23.87 mg of Dy₂O₃ (0.064 mmol) were thoroughly ground for 30 min and then annealed at 1050 °C for 2 h under a reducing atmosphere of 5% H₂ in Ar. After natural cooling to room temperature, the resulting product was ground again for 30 min to obtain the final Sr₂MgSi₂O₇:Eu,Dy mechanoluminescent phosphor.

2.5. Preparation of the NiS@g-C₃N₄/Sr₂MgSi₂O₇:Eu,Dy Composite

The NiS@g-C₃N₄/Sr₂MgSi₂O₇:Eu,Dy composite was prepared by thoroughly grinding 500 mg of Sr₂MgSi₂O₇:Eu,Dy with 100 mg of NiS@g-C₃N₄ (weight ratio 5:1), followed by annealing at 300 °C for 2 h under Ar.

2.6. Characterizations

The crystal structure of samples was characterized by X-ray diffractometer (XRD) (Bruker D8 Advance, Billerica, MA, USA) with Cu-kα (λ = 1.5406 Å) radiation at 40 kV and 40 mA. Scanning electron microscope (SEM) images were acquired using a Hitachi S-4800 SEM (Hitachi, Tokyo, Japan). The mechanoluminescent spectra were recorded by a fiber-coupled Ocean Optics QE Pro spectrometer (Ocean Optics, Dunedin, FL, USA). UV–Vis absorption spectra were collected using an Agilent Cary 5000 spectrometer (Agilent, Santa Clara, CA, USA) at room temperature.

2.7. Photocatalytic Activity Test

The photocatalytic performance of the NiS@g-C₃N₄/Sr₂MgSi₂O₇:Eu,Dy composite was systematically evaluated for the degradation of Rhodamine B (RhB) in aqueous solution. Experiments were conducted in a 100 mL quartz reactor containing 10 mg of the photocatalyst and 25 mL of RhB solution with a concentration of 0.01 mg/mL. Before light irradiation, the suspension was magnetically stirred in the dark for 30 min to reach adsorption–desorption equilibrium. Visible-light-driven photocatalysis was initiated using a 300 W xenon lamp equipped with a long-pass optical filter (λ > 420 nm). To explore the mechanoluminescence-induced photocatalysis, a Fisherbrand™ Model 120 Sonic Dismembrator (120 W) (Fisherbrand, Shanghai, China) was employed as an ultrasound stimulus. At predetermined time intervals, 0.5 mL aliquots were withdrawn and centrifuged to remove the photocatalyst particles. The concentration of residual RhB in the supernatant was then quantitatively analyzed using a UV–Vis spectrophotometer (Agilent Cary 5000, Agilent, Santa Clara, CA, USA), allowing the evaluation of photocatalytic degradation efficiency over time.

3. Results

Results of the Characterizations and Photocatalytic Activity Tests

The XRD patterns of the g-C₃N₄, NiS@g-C₃N₄, and NiS@g-C₃N₄/Sr₂MgSi₂O₇:Eu,Dy composite are shown in Figure 1a. The pristine g-C₃N₄ exhibits two characteristic diffraction peaks located at 12.8° and 27.5°, corresponding to the (100) and (002) crystal planes, respectively, which are attributed to in-plane structural packing and interlayer stacking of conjugated aromatic systems [4]. In the NiS@g-C₃N₄ sample, no distinct peaks corresponding to NiS are observed, likely due to the small amount of NiS loaded and its high dispersion on the g-C₃N₄ surface. Upon forming a composite with Sr₂MgSi₂O₇:Eu,Dy mechanoluminescent phosphors, the main diffraction peaks of the resulting NiS@g-C₃N₄/Sr₂MgSi₂O₇:Eu,Dy heterojunction match the standard PDF card #75-1736 (Sr₂MgSi₂O₇), with key peaks at 2θ = 28.3°, 32.7°, and 47.1° corresponding to the (112), (200), and (220) crystal planes, respectively. However, due to the relatively low content of NiS@g-C₃N₄ compared to Sr₂MgSi₂O₇:Eu,Dy, only the 27.5° peak from g-C₃N₄ is weakly visible. Using the Scherrer equation, the crystallite size of the composite is calculated as ~63 nm. SEM images (Figure 1b) reveal that the composite exhibits a monodispersed particle morphology with sizes ranging from 5 to 50 µm. A high magnification SEM image confirms the uniform distribution and successful surface integration of NiS@g-C₃N₄ onto the Sr₂MgSi₂O₇:Eu,Dy phosphor particles, forming a well-defined heterostructure. The micron-sized morphology of Sr₂MgSi₂O₇:Eu,Dy provides a stable substrate, while NiS@g-C₃N₄ loading creates a high-surface-area heterostructure.

The NiS@g-C₃N₄/Sr₂MgSi₂O₇:Eu,Dy heterojunction exhibits a mechanoluminescent emission peak centered around 470 nm (Figure 2a), which is consistent with the characteristic emission profile of Sr₂MgSi₂O₇:Eu,Dy phosphors [16]. This result indicates that the integration of g-C₃N₄ and NiS does not compromise the mechanoluminescent properties of the Sr₂MgSi₂O₇:Eu,Dy component. In terms of optical absorption, pure g-C₃N₄ displays a strong absorption band in the range of 200–500 nm (Figure 2b), primarily attributed to its π–π* transitions. Upon loading with NiS, the NiS@g-C₃N₄ composite shows a notable enhancement in this absorption band, likely due to the synergistic effect and improved charge separation. After further integration with mechanoluminescent Sr₂MgSi₂O₇:Eu,Dy phosphors, the resulting heterojunction demonstrates significantly stronger absorption across both the UV and visible regions compared to either g-C₃N₄ or NiS@g-C₃N₄ alone. We have also performed Tauc plot analysis on the UV–Vis diffuse reflectance spectra (see Figure S1) to calculate the bandgap energies. The results (see Figure S1) show that the bandgap of pristine g-C₃N₄ is ~2.70 eV, NiS@g-C₃N₄ is ~2.65 eV, and the NiS@g-C₃N₄/Sr₂MgSi₂O₇:Eu,Dy composite is ~2.61 eV. This bandgap narrowing is attributed to the synergistic effect of heterojunction formation, which enhances light absorption in the visible region. The enhancement effectively broadens the light-harvesting range of g-C₃N₄, promoting better utilization of external visible light and thereby improving the overall photocatalytic performance of the heterojunction system.

To evaluate the photocatalytic performance of the NiS@g-C₃N₄/Sr₂MgSi₂O₇:Eu,Dy heterojunction, RhB was selected as the model pollutant (Figure 3). Prior to the photocatalytic degradation experiments, the photocatalysts were dispersed in an aqueous RhB solution and stirred in the dark for 30 min to establish adsorption–desorption equilibrium. As shown in Figure 3a, the RhB solution without any photocatalyst exhibits negligible self-degradation under the experimental conditions. Pure g-C₃N₄ shows a moderate photocatalytic activity, effectively degrading RhB only under light irradiation, but it remains inactive in the dark even with ultrasound. The NiS@g-C₃N₄ composite demonstrates similar photocatalytic behavior, but with a slightly higher degradation rate due to improved charge separation facilitated by NiS. Remarkably, upon integration with mechanoluminescent Sr₂MgSi₂O₇:Eu,Dy phosphors, the resulting NiS@g-C₃N₄/Sr₂MgSi₂O₇:Eu,Dy heterojunction not only exhibits enhanced photocatalytic activity under light irradiation but also enables photocatalytic degradation of RhB in the dark under ultrasound stimulation. This is attributed to the ultrasound-induced in situ mechanoluminescent emission that activates the photocatalyst even without external light. Notably, after 300 min of reaction, the NiS@g-C₃N₄/Sr₂MgSi₂O₇:Eu,Dy heterojunction achieves an RhB degradation efficiency of 86.9%, which is significantly higher than that of pure g-C₃N₄ (37.8%) and NiS@g-C₃N₄ (48.5%) (Figure 3b). Furthermore, post-recyclability characterization (XRD and SEM; see Figure S2a and S2b, respectively) confirms no significant changes in crystal structure or morphology. The composite retains its heterostructure, with NiS@g-C₃N₄ uniformly dispersed on Sr₂MgSi₂O₇:Eu,Dy surfaces, revealing the structural stability of the catalyst. Moreover, the NiS@g-C₃N₄/Sr₂MgSi₂O₇:Eu,Dy heterojunction exhibits excellent photostability, maintaining consistent performance over four consecutive degradation cycles (Figure 3c). These results highlight the potential of the NiS@g-C₃N₄/Sr₂MgSi₂O₇:Eu,Dy heterojunction as an efficient and recyclable photocatalyst for environmental remediation applications.

4. Discussion

Based on the above results, a proposed mechanism for the photocatalytic degradation of RhB under visible light and ultrasound has been illustrated in Figure 4. Upon visible light irradiation (λ > 420 nm), electrons in the valence band of g-C₃N₄ are excited to its conduction band, generating electron–hole pairs. The photogenerated electrons are subsequently transferred to NiS, which acts as an efficient electron acceptor, thereby enhancing charge separation and suppressing recombination. These electrons can then reduce dissolved O₂ molecules to generate ∙O₂⁻ radicals, which actively degrade RhB molecules in the solution. When the heterojunction is subjected to ultrasound stimulation, the Sr₂MgSi₂O₇:Eu,Dy phosphors emit strong mechanoluminescence centered around 470 nm. This in situ light emission can excite g-C₃N₄ even in the absence of external illumination, enabling the heterojunction to maintain photocatalytic activity in dark conditions. Therefore, the synergistic integration of mechanoluminescence and heterojunction design significantly facilitates the photocatalytic degradation of RhB.

5. Conclusions

In summary, a mechanoluminescent NiS@g-C₃N₄/S_r2MgSi₂O₇:Eu,Dy heterojunction was rationally designed and synthesized for the first time. By benefiting from the enhanced and broadened spectral response, the heterojunction exhibited superior photocatalytic performance for the degradation of RhB under visible light irradiation. More importantly, owing to the unique mechanoluminescent properties of Sr₂MgSi₂O₇:Eu,Dy phosphors, the heterojunction also demonstrated effective photocatalytic activity in the dark under ultrasound stimulation, achieving light-independent organic dyes degradation. This dual-mode catalytic capability expands the practical applicability of photocatalysts beyond conventional light-driven systems. Therefore, this study not only provides a promising strategy for developing efficient, multifunctional photocatalysts but also introduces the new concept of incorporating mechanoluminescence to enable photocatalytic degradation of pollutants in water treatment, particularly under limited-light or deep-environment conditions.