Dual-Functional Cu₂O/g-C₃N₄ Heterojunctions: A High-Performance SERS Sensor and Photocatalytic Self-Cleaning System for Water Pollution Detection and Remediation

Academic Background

With the rapid expansion of industrialization and agricultural activities, water pollution has become a significant global environmental issue. Large quantities of harmful substances, such as dyes, antibiotics, and pesticides, are continuously discharged into aquatic ecosystems, directly or indirectly disrupting aquatic habitats and posing serious threats to human health. Traditional water treatment technologies struggle to completely eliminate or degrade these persistent, concealed, and complex pollutants. Therefore, the development of multifunctional devices capable of efficient detection and remediation has become essential.

Surface-Enhanced Raman Scattering (SERS) technology has emerged as a highly effective method for trace pollutant detection due to its high sensitivity and broad-spectrum detection capabilities. However, traditional SERS substrates rely on noble metals (such as gold or silver), which are costly and prone to corrosion, limiting their large-scale application. In contrast, semiconductor-based SERS substrates offer excellent chemical stability, biocompatibility, and cost-effectiveness, significantly reducing detection expenses. Moreover, certain semiconductor SERS substrates, when integrated with photocatalytic technology, can exhibit substantial photocatalytic degradation potential.

Cuprous oxide (Cu₂O), a p-type narrow-bandgap semiconductor, has a broad visible light response spectrum and high solar energy utilization, making it a commonly used photocatalytic material. However, Cu₂O faces challenges in practical applications, such as high recombination rates of photogenerated electron-hole pairs and susceptibility to photocorrosion. Graphitic carbon nitride (g-C₃N₄), an emerging n-type two-dimensional semiconductor, is considered an ideal material for modifying Cu₂O due to its large surface area, excellent chemical stability, low cost, and suitable bandgap structure. The band alignment between Cu₂O and g-C₃N₄ facilitates efficient electron transfer, reducing electron-hole recombination and thereby enhancing SERS sensitivity and photocatalytic efficiency.

Paper Source

This paper was co-authored by Shuo Yang, Kaiyue Li, Ping Huang, Keyan Liu, Wenhui Li, Yuquan Zhuo, Ziwen Yang, and Donglai Han, affiliated with the School of Materials Science and Engineering at Changchun University, the Laboratory of Materials Design and Quantum Simulation at Changchun University, and the School of Materials Science and Engineering at Changchun University of Science and Technology. The paper was published in Microsystems & Nanoengineering in 2024.

Research Process and Results

1. Material Preparation and Characterization

The study first synthesized Cu₂O microcubes (Cu₂O MCs), rounded-edge microcubes (Cu₂O RMCs), and truncated microcubes (Cu₂O TMCs) via a water bath method, and prepared lamellar flocculent g-C₃N₄ nanosheets (g-C₃N₄ NSs) through high-temperature calcination. Subsequently, Cu₂O MCs and g-C₃N₄ NSs were physically ground together in varying mass ratios (10% to 50%) to prepare Cu₂O/g-C₃N₄ heterojunctions (MPHs).

The structure, chemical composition, surface properties, and chemical states of the synthesized materials were analyzed using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), nitrogen adsorption-desorption isotherms (BET), and X-ray photoelectron spectroscopy (XPS). The results confirmed the successful formation of Cu₂O/g-C₃N₄ heterojunctions, and the introduction of g-C₃N₄ significantly increased the surface area of Cu₂O, providing more active sites for photocatalytic reactions.

2. Photoelectrical Performance Testing

The separation, recombination, and transport efficiency of photogenerated charge carriers were investigated using UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS), photocurrent intensity (I-t) testing, photoluminescence (PL) spectroscopy, and electrochemical impedance spectroscopy (EIS). The results showed that the Cu₂O/g-C₃N₄-0.2 heterojunction exhibited superior photogenerated charge carrier separation efficiency and lower charge transfer resistance, significantly enhancing photocatalytic performance.

3. SERS Detection Performance

The SERS performance of the Cu₂O/g-C₃N₄ heterojunctions was evaluated using 4-aminothiophenol (4-ATP) as the probe molecule. The results demonstrated that the Cu₂O/g-C₃N₄-0.2 heterojunction exhibited the highest SERS signal intensity at 1438 cm⁻¹, with an enhancement factor (EF) of 2.43 × 10⁶, indicating high sensitivity and consistency. Additionally, the sensor achieved a detection limit of 10⁻⁶ M for methyl orange (MO), and the relative standard deviation (RSD) of SERS signals at 25 random points was below 15%, demonstrating excellent uniformity.

4. Photocatalytic Degradation Performance

The photocatalytic degradation performance of the Cu₂O/g-C₃N₄ heterojunctions for MO was evaluated. The results showed that the Cu₂O/g-C₃N₄-0.2 heterojunction achieved a degradation efficiency of 98.3% for MO within 90 minutes under visible light, and maintained a degradation efficiency of 93.7% after 216 days, indicating excellent long-term stability. Furthermore, after four cycles, the degradation efficiency remained at 84.0%, demonstrating good cyclic stability.

5. Photocatalytic Mechanism

The band structure of the Cu₂O/g-C₃N₄ heterojunction was analyzed using Mott-Schottky (M-S) curves and XPS valence band (VB) spectra. The results indicated that the Cu₂O/g-C₃N₄ heterojunction followed a Z-scheme charge transfer mechanism, effectively promoting the separation of photogenerated electron-hole pairs and generating active species such as h⁺, ·OH, and ·O₂⁻, which drove the self-cleaning and photocatalytic degradation processes.

6. SERS Self-Cleaning Performance

The self-cleaning performance of the Cu₂O/g-C₃N₄-0.2 SERS sensor was evaluated. The results demonstrated that the sensor could effectively degrade organic pollutants (such as MO, 2,4-D, TC, and MB) adsorbed on its surface and successfully regenerate after 180 seconds of irradiation, showcasing excellent self-cleaning functionality and reusability.

Conclusion

This study successfully developed a dual-functional Cu₂O/g-C₃N₄-0.2 heterojunction system that integrates SERS detection and photocatalytic degradation capabilities, demonstrating its potential as an efficient device for water pollution monitoring and remediation. The sensor exhibited high sensitivity, excellent uniformity, and reproducibility, capable of detecting various pollutants and showcasing superior photocatalytic degradation performance and long-term stability. The Z-scheme heterojunction structure played a critical role in promoting efficient charge separation and preventing recombination, providing a promising foundation for the development of multifunctional, sustainable, and efficient water quality monitoring devices.

Research Highlights

1. Multifunctional Integration: The Cu₂O/g-C₃N₄ heterojunction integrates SERS detection and photocatalytic degradation, enabling efficient pollutant detection and degradation.
2. High Sensitivity and Consistency: The Cu₂O/g-C₃N₄-0.2 SERS sensor achieved an enhancement factor of 2.43 × 10⁶, with a relative standard deviation below 15%, demonstrating high sensitivity and consistency.
3. Superior Photocatalytic Performance: The heterojunction achieved a degradation efficiency of 98.3% for MO under visible light within 90 minutes and maintained a degradation efficiency of 93.7% after 216 days, indicating excellent long-term stability.
4. Self-Cleaning Functionality: The sensor effectively degraded organic pollutants adsorbed on its surface and successfully regenerated within a short period, showcasing its self-cleaning and reusability.

Research Significance

This study provides new insights into the development of multifunctional, sustainable, and efficient water quality monitoring devices, with significant scientific value and application prospects. The Cu₂O/g-C₃N₄ heterojunction system not only enables efficient detection and degradation of water pollutants but also demonstrates excellent long-term stability and self-cleaning functionality, laying the foundation for future advancements in environmental monitoring and remediation technologies.