Maria Antoniadou

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.

Biomaterials are getting more recognition in drug delivery due to their unique intrinsic properties. Herein, graphitic carbon nitride (g-CN) is fabricated from urea precursors. g-CN is further incorporated with starch (Sta) and tragacanth gum (TG) (i.e., Sta/g-CN/TG) to form a hydrogel for use as a drug nanocarrier. Various analytical techniques, such as FTIR (Fourier transform infrared) spectroscopy, XRD (X-ray diffraction), ZP (zeta potential) analysis, BET (Brunauer–Emmett–Teller) analysis, DLS (dynamic light scattering), and SEM (scanning electron microscopy), were applied for the structural elucidation of the Sta/g-CN/TG hydrogel. ZP analysis reveals a +8.60 ± 0.01 mV charged surface, XRD shows amorphous structure, and SEM displays a mean particle diameter of 90 ± 0.01 nm. BET analysis reveals a Type II isotherm and a surface area of 0.540 ± 0.001 m² g⁻¹, and BJH analysis confirms a mean pore size of 19.08 nm and a mean pore volume of 0.0088 cm³ g⁻¹. DLS demonstrates a mean nanoparticle size of 180 ± 0.1 nm, and FTIR reveals OH and CO group stretching. To investigate hydrogel loading and release efficiencies (%LE and %RE), the lornoxicam (LXM) drug was loaded into the Sta/g-CN/TG hydrogel (i.e., LXM@Sta/g-CN/TG/LXM). The time interval, drug concentration, pH, and temperature impact LXM loading, while pH and temperature factors influence LXM release. The results showed that the Sta/g-CN/TG hydrogel made with a 1 : 0.5 : 0.2 mass ratio of Sta, g-CN, and TG had outstanding loading, release, and swelling qualities via hydrogen bonding. The results demonstrate a 66.94% greater %RE at acidic pH = 7.4 and 67.98% highest %RE at 50 °C respectively. However, the Peppas–Sahlin model emerged as the best-fitted model with linear regression (R²). The collected outcomes also confirmed Fickian diffusion in the LXM release. Finally, Sta/g-CN/TG is a promising pH-sensitive nanovehicle with increased LXM loading and sustained release properties.

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.

Hydrogel biomaterials are increasingly utilized as drug delivery systems due to their unique properties.

In this study, 3-dimensional molybdenum disulfide (MoS₂) structures, integrated with hematite (α-Fe₂O₃) nanoparticles, were fabricated under a convenient two-step hydrothermal route. The fabricated photocatalytic nanocomposites consist of well-arranged MoS₂ flakes, resembling spherical flower-like morphology, and the nanoparticulate α-Fe₂O₃ structures decorate the 3D network. By raising the α-Fe₂O₃ weight ratio, the composites’ specific surface area and morphology were not affected, regardless of the partial cover of the cavities for higher hematite content. Moreover, the crystallinity examination with XRD, Raman, and FTIR techniques revealed that the precursor reagents were fully transformed to well-crystalized MoS₂ and Fe₂O₃ composites of high purity, as no organic or inorganic residues could be detected. The photocatalytic oxidation and reduction performance of these composites was evaluated against the tetracycline pharmaceutical and the industrial pollutant hexavalent chromium, respectively. The improvement in the removal efficiencies demonstrates that the superior photoactivity originates from the high crystallinity and homogeneity of the composite, in combination with the enhanced charge carriers’ separation in the semiconductors’ interface.

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.

Photonic crystal structuring has emerged as an advanced method to enhance solar light harvesting by metal oxide photocatalysts along with rational compositional modifications of the materials’ properties. In this work, surface functionalization of TiO₂ photonic crystals by blue luminescent graphene quantum dots (GQDs), n–π* band at ca. 350 nm, is demonstrated as a facile, environmental benign method to promote photocatalytic activity by the combination of slow photon-assisted light trapping with GQD-TiO₂ interfacial electron transfer. TiO₂ inverse opal films fabricated by the co-assembly of polymer colloidal spheres with a hydrolyzed titania precursor were post-modified by impregnation in aqueous GQDs suspension without any structural distortion. Photonic band gap engineering by varying the inverse opal macropore size resulted in selective performance enhancement for both salicylic acid photocatalytic degradation and photocurrent generation under UV–VIS and visible light, when red-edge slow photons overlapped with the composite’s absorption edge, whereas stop band reflection was attenuated by the strong UVA absorbance of the GQD-TiO₂ photonic films. Photoelectrochemical and photoluminescence measurements indicated that the observed improvement, which surpassed similarly modified benchmark mesoporous P25 TiO₂ films, was further assisted by GQDs electron acceptor action and visible light activation to a lesser extent, leading to highly efficient photocatalytic films.

A bifunctional thin film photocatalyst consisting of graphitic carbon nitride on tungsten trioxide (g-C₃N₄/WO₃) is introduced for the improvement of photocatalytic activity concerning hexavalent chromium reduction and methylene blue dye removal in water, compared to the bare, widely used WO₃ semiconductor. A bilayered structure was formed, which is important for the enhancement of the charge carriers’ separation. The characterization of morphological, structural, optoelectronic, and vibrational properties of the photocatalysts permitted a better understanding of their photocatalytic activity for both dye degradation and Cr⁺⁶ elimination in water and the analysis of the photocatalytic kinetics permitted the determination of the corresponding pseudo-first-order reaction constants (k). Trapping experiments performed under UV illumination revealed that the main active species for the photocatalytic reduction of Cr⁺⁶ ions are electrons, whereas in the case of methylene blue azo-dye (MB) oxidation, the activation of the corresponding photocatalytic degradation comes via both holes and superoxide radicals.