Carbon dots (CDs), a rapidly emerging class of zero-dimensional (0-D) nanomaterials with small particle sizes (<10 nm), have garnered significant scientific interest owing to their exceptional physicochemical properties, non-toxicity, low-cost synthesis, and versatile applications. In recent years, the combination of various inorganic photocatalysts (e.g., metal oxides, metal chalcogenides, metal oxyhalides, MXenes, non-metallic semiconductors) with CDs has gained momentum as a promising strategy to enhance their photocatalytic efficiency. By incorporating CDs, researchers have addressed fundamental challenges in photocatalytic systems, including limited light absorption range, rapid electron–hole recombination rate, low quantum efficiency, etc. The present review is focused on the most recent developments in CDs-based heterostructures for advanced photocatalytic applications, particularly in the field of environmental remediation, providing a comprehensive overview of emerging strategies, synthesis approaches, and the resulting enhancements in photocatalytic water treatment applications.

CDs/TiO₂ nanohybrids were synthesized and tested for photocatalytic H₂ production from aqueous media through simulated solar light-driven photocatalytic reactions. Firstly, three different types of CDs were prepared through green methods, specifically hydrothermal treatment and microwave irradiation, using citric acid and urea as precursors in varying molar ratios. After a multi-step purification procedure, impurity-free CDs were obtained. The as-synthesized CDs were thoroughly characterized using UV-Vis, FT-IR, and PL spectroscopy, along with HR-TEM. The results revealed that the size and optical and physicochemical properties of CDs can be tailored by selecting the precursors’ ratio and the synthetic approach. The heterostructured CDs/TiO₂ photocatalysts were formed solvothermally and were analyzed using UV-Vis/DRS, FT-IR, and XPS techniques, which confirmed the effective incorporation of CDs and the improved properties of TiO₂. The use of sacrificial reagents is among the most common strategies for enhancing H₂ production from water through photocatalytic processes; herein, ethanol was selected as a green liquid organic hydrogen carrier. A maximum H₂ production rate of 0.906 μmol H₂/min was achieved, while the recyclability study demonstrated that the photocatalyst maintained stable performance during multiple cycles of reuse. Thus, optimizing the synthesis conditions of CDs/TiO₂ nanohybrids resulted in the creation of environmentally friendly and reusable photocatalysts.

The present work focuses on TiO₂ modification with carbon dots (CDs) using a hydrothermal process, which results in the synthesis of CD/TiO₂ nanocomposite photocatalysts characterized by exceptional optoelectronic properties. The structural and physicochemical properties of the obtained nanocomposites, which contained varying amounts of CDs, were precisely assessed. HR-TEM analysis showed that the prepared nanocomposites consisted of rod-shaped TiO₂ nanoparticles and CDs well-dispersed on their surface. The optical properties of the nanocomposites were studied using UV–vis diffuse reflectance spectroscopy. All CD/TiO₂ samples presented decreased energy gap values compared with bare TiO₂ samples; the band gap was further decreased as the CD concentration rose. Electrochemical measurements revealed that the presence of CDs improved the photocurrent response of the TiO₂, presumably due to enhanced charge separation and decreased recombination. The synthesized nanomaterials were used as photocatalysts to produce hydrogen via the photoreforming of ethanol and glycerol green organic compounds, under 1-sun illumination. The photocatalytic experiments confirmed that the optimum loading of CDs corresponded to a percentage of 3% (w/w). Ethanol photoreforming led to a H₂ production rate of 1.7 μmol∙min⁻¹, while in the case of the glycerol sacrificial agent, the corresponding rate was determined to be 1.1 μmol∙min⁻¹. The recyclability study revealed that the photocatalyst exhibited consistent stability during its reuse for hydrogen production in the presence of both ethanol and glycerol.

Photodynamic therapy (PDT) is a promising cancer treatment employing photo-induced reactive oxygen species (ROS) generation by a photosensitizer (PS). Titanium dioxide (TiO₂) is a potential PS due to its superb photocatalytic features and biocompatibility. However, its clinical potential is restricted by its predominant ultraviolet (UV) absorption. To address this limitation, this work introduces TiO₂/carbon dots (CDs) nanohybrid materials for improving the photophysical properties of TiO₂ and its photodynamic performance. TiO₂ and CDs were synthesized through wet chemical and hydrothermal techniques, and subsequently combined via a facile ex situ solvothermal process to produce hybrid materials containing 1–50% w/w CDs. The materials were characterized using XRD, Raman, TEM, FT-IR, zeta potential, TGA, UV-Vis and PL. PDT studies on A431 skin cancer cells indicated improved photosensitizing ability of TiO₂/CDs, with TiO₂/CDs (10%) inducing 47% cell toxicity, versus 20% for TiO₂ after 10 min of red-light irradiation (661 nm, 18 mW/cm², 12.96 J/cm²). Intracellular localization studies revealed enhanced cellular uptake of TiO₂/CDs (10%), compared with TiO₂. In vitro studies on 3T3 healthy fibroblasts confirmed PSs’ safety both with and without light. Overall, this study elucidates the key role of CDs in the photophysical and photodynamic behavior of TiO₂-based systems, providing design guidelines for the next-generation inorganic PSs.