To solve the problem of water pollution, researchers have proposed a photocatalytic degradation technology, in which the key factor is the development of efficient photocatalytic materials. Graphitic carbon nitride (g-C3N4), an n-type semiconductor, has been widely studied due to its suitable band gap (2.7 eV), low cost, easy preparation, non-toxicity, and high photostability. However, the pure-phase g-C3N4 still has defects such as low specific surface area, insufficient visible light absorption, low charge mobility, few active sites for interfacial reaction, and easy recombination of photogenerated electron–hole pairs, which leads to the lower photocatalytic activity of g-C3N4. Aiming at the problems mentioned above, this paper focus on the synthesis of g-C3N4-based composites with high photocatalytic activity via lemon juice induction method. Thiourea and lemon juice were selected as precursors, and carbon quantum dots (CQDs) as electron mediators were introduced anchoring on the surface of g-C3N4 to build g-C3N4/CQDs with compact interface. The results showed that small-sized CQDs are uniformly distributed on the surface of g-C3N4, and the g-C3N4/CQDs composite has a 2D0D structure, which reduces the recombination of photogenerated electron–hole pairs. The photocatalytic degradation efficiency of 4% g-C3N4/CQDs for RhB reaches the highest data of 90.9%, and the photocatalytic degradation rate is 0.016 min− 1, which is about 2.3 times that of g-C3N4. After four cycles of photocatalytic reaction, the photocatalytic degradation efficiency of the material remained at 81.7%. Therefore, the g-C3N4/CQDs synthesized via lemon juice induction has a more stable microstructure, and the charge separation efficiency is greatly improved, which is suitable for practical photocatalytic environmental protection.

    Abstract 
    Graphical abstract
    1 Introduction
    2 Experimental section
        2.1 Chemicals and reagents
        2.2 Preparation of g-C3N4
        2.3 Preparation of CQDs precursors
        2.4 Preparation of g-C3N4CQDs
        2.5 Characterization
        2.6 Photocatalytic degradation
    3 Results and discussion
        3.1 Characterization of photocatalysts
        3.2 Photocatalytic activities
    4 Conclusion
    Acknowledgements 
    References

• Tao Jin(Jiangsu Key Laboratory for Environment Functional Materials, Suzhou University of Science and Technology, School of Materials Science and Engineering, Suzhou University of Science and Technology)
• Chengbao Liu(Jiangsu Key Laboratory for Environment Functional Materials, Suzhou University of Science and Technology, School of Materials Science and Engineering, Suzhou University of Science and Technology, Jiangsu Collaborative Innovation Center of Technology and Material for Water Treatment, Suzhou University of Science and Technology)
• Feng Chen(Jiangsu Key Laboratory for Environment Functional Materials, Suzhou University of Science and Technology, School of Materials Science and Engineering, Suzhou University of Science and Technology, Jiangsu Collaborative Innovation Center of Technology and Material for Water Treatment, Suzhou University of Science and Technology)
• Junchao Qian(Jiangsu Key Laboratory for Environment Functional Materials, Suzhou University of Science and Technology, School of Materials Science and Engineering, Suzhou University of Science and Technology, Jiangsu Collaborative Innovation Center of Technology and Material for Water Treatment, Suzhou University of Science and Technology)
• Yongbin Qiu(Jiangsu Province Ceramics Research Institute Co., Ltd)
• Xianrong Meng(Suzhou Institute of Environmental Science)
• Zhigang Chen(Jiangsu Key Laboratory for Environment Functional Materials, Suzhou University of Science and Technology, School of Materials Science and Engineering, Suzhou University of Science and Technology, Jiangsu Collaborative Innovation Center of Technology and Material for Water Treatment, Suzhou University of Science and Technology)