Carbon nanodots (C-dots) have generated enormous excitement because of their superiority in water solubility, chemical inertness, low toxicity, ease of functionalization and resistance to photobleaching. In this review, by introducing the synthesis and photo- and electron-properties of C-dots, we hope to provide further insight into their controversial emission origin (particularly the upconverted photoluminescence) and to stimulate further research into their potential applications, especially in photocatalysis, energy conversion, optoelectronics, and sensing.

High quality carbon nanodots (C-dots) with high purity were synthesized through a mild, one-step electrochemical approach, without the assistance of any chemicals but only pure water. This high productivity method makes the synthetic process of C-dots synthesis both economical as well as environment-friendly. The as prepared C-dots are predominantly multi-layer graphene oxide, with luminescence and high up-conversion photoluminescence (emission of light at shorter wavelengths than the excitation wavelength). Meanwhile, C-dots showed peroxidise mimetic function and visible-light-sensitive photocatalytic activity for methyl orange degradation. In addition, a novel photocatalyst (TiO₂/C-dots) was obtained by combining C-dots with TiO₂ through an easy hydrothermal method. Remarkably, TiO₂/C-dots exhibited an excellent visible-light photocatalytic activity.

In this study, we report the facile fabrication of Ag₃PO₄, CQDs/Ag₃PO₄, Ag/Ag₃PO₄ and CQDs/Ag/Ag₃PO₄ photocatalysts (CQDs, carbon quantum dots). For CQDs/Ag₃PO₄ and CQDs/Ag/Ag₃PO₄, because of the insoluble, photoinduced electron transfer, upconversion luminescence and electron reservoir properties of CQDs, the complex photocatalysts exhibit enhanced photocatalytic activity and structural stability over the photodecomposition of organic compounds (methyl orange) under the irradiation of visible light. Furthermore, the synergistic effect of CQDs and the intense surface plasmon resonance of Ag lead to the highest photocatalytic activity of CQDs/Ag/Ag₃PO₄ among Ag₃PO₄ and the corresponding complex photocatalysts. Our results provide an invaluable methodology for designing high-performance photocatalysts based on CQDs and related functional materials, which is promising for catalytic and new energy applications.

Water-soluble fluorescent N-doped carbon dots (NCDs) were synthesized by a facile one-pot ultrasonic reaction between glucose and ammonium hydroxide. Besides exhibiting strong luminescence in the visible-to-near infrared range, the obtained NCDs displayed clear upconversion photoluminescence properties. Moreover, the NCDs showed excellent photocatalytic property in the photodegradation of methyl orange under visible light.

Artificial photosynthesis of H₂O₂ from H₂O and O₂, as a spotless method, has aroused widespread interest. Up to date, most photocatalysts still suffer from serious salt-deactivated effects with huge consumption of photogenerated charges, which severely limit their wide application. Herein, by using a phenolic condensation approach, carbon dots, organic dye molecule procyanidins and 4-methoxybenzaldehyde are composed into a metal-free photocatalyst for the photosynthetic production of H₂O₂ in seawater. This catalyst exhibits high photocatalytic ability to produce H₂O₂ with the yield of 1776 μmol g⁻¹h⁻¹ (λ ≥ 420 nm; 34.8 mW cm⁻²) in real seawater, about 4.8 times higher than the pure polymer. Combining with in-situ photoelectrochemical and transient photovoltage analysis, the active site and the catalytic mechanism of this composite catalyst in seawater are also clearly clarified. This work opens up an avenue for a highly efficient and practical, available catalyst for H₂O₂ photoproduction in real seawater.