The immense progress of humanity on the technological, domestic, and industrial fronts comes at the cost of polluting the planet. Aquatic pollution is particularly dangerous since all life forms are directly linked to it. Each year tons of industrial and domestic pollutants make their way into aqueous systems. Efficient removal/degradation of these pollutants is of prime importance for the sustainable future. Among many technologies, photodegradation is an emerging and promising method for the successful removal of aqueous pollutants since it is powered by abundant solar light. The last decade had shown that carbon dots are among the most promising materials that can be utilized as an efficient tool to derive various solar‐driven chemical reactions. Carbon dots possess unique photophysical and chemical properties such as light‐harvesting over a broad‐spectrum region, upconversion photoluminescence, photosensitizers, chemical inertness, and bivalent redox character, etc. The ease of synthesis of carbon dots at low cost also contributes hugely to their utilizations as an efficient photocatalyst for the degradation of aqueous pollutants. This review summarizes the recent progress made in the field of photodegradation of aqueous pollutants with the aid of carbon dots and their hybrids, highlighting the critical role carbon dots can play in the field.

This review article summarizes the synthesis and the fluorescence mechanism together with the most important applications in thermometry, bio-imaging, LSCs and photocatalysis of red/near-infrared C-dots.

Composite metal oxide semiconductors are promising candidates for photoelectrochemical water splitting (PEC WS) toward environmentally friendly hydrogen production. Among them, ZnO and α‐Fe₂O₃ hold great potential thanks to a series of benefits, including fast charge transport in single‐crystalline structures, large surface area and tunable shapes (ZnO), and energy bandgap falling in the visible spectral range (α‐Fe₂O₃). However, both materials present significant drawbacks, which hinder their successful application in high‐efficiency PEC WS: the wide bandgap of ZnO limits its absorption in the UV range, while the low charge carrier mobility results in heavy recombination losses in α‐Fe₂O₃ during charge collection. The synthesis of ZnO/hematite composites has recently proven to be an effective approach to improve the overall WS performances. In this review, the recent developments on the application of different morphologies (0D, 1D, 2D, and 3D structures) for PEC WS are illustrated, analyzing the role of the shape and morphology in boosting the functional properties, both in single systems and in composite nanostructures. Complex networks show higher photocatalytic efficiency than the single building blocks and, consequently, composite materials exhibit higher performances. Possible paths for the development of an effective lab‐to‐fab transition based on application of ZnO/α‐Fe₂O₃ composite structures are also suggested.

TiO₂–SiO₂ composites containing 10 wt%, 20 wt%, 30% and 40 wt% of TiO₂, obtained by using preformed mesoporous silica nanoparticles (MSNs) and titanium isopropoxide as titanium source, have been investigated in detail using a variety of techniques. All the samples were characterized by N₂-physisorption, X-ray powder diffraction (XRPD), diffusive reflective UV-vis spectroscopy (DRUV-vis), X-ray photoelectron spectroscopy (XPS) and imaged using transmission electron microscopy (TEM). The TiO₂–MSN composites, that exhibited a spherical morphology, high specific surface areas and titania in the anatase phase, owing to their specific chemical–physical properties were studied as catalysts in the photocatalytic degradation of methylene blue, methyl orange and paracetamol, as examples of polluted wastewaters. The well-defined porous structures of MSNs may offer a special environment for titania nanoparticles, increasing the specific surface area and the thermal stability of the composite, thus modifying the photocatalytic behavior of the materials. The TiO₂ loading, the particle size and the surface characteristics were related to the degree of UV absorption and the measured energy band gap of the nanocomposites. A cooperative effect between the two components (TiO₂ and SiO₂) could be the key factor at the basis of the good photocatalytic performances: nanostructured TiO₂ in intimate contact with MSN provides the sites for generation of OH˙ radicals by oxidation of water and the SiO₂ skeleton is able to adsorb the molecules of cationic dyes and prevent poisoning of the TiO₂ surface.

Ceria-doped titania photocatalysts (ceria loading 0.25–5.0 wt%) were synthesized by hydrothermal methods for water remediation. Nanotubes (CeTNTx) and nanoparticles (CeTNPx) were obtained. Ceria doping was applied to tune the electronic properties of nanostructured titania, boosting its photocatalytic activity. CeTNT nanostructures contained anatase as the only titania phase, whereas the CeTNP series consisted of both anatase and rutile polymorphs. The Ce addition induced a decrease in the energy gap, allowing enhancement of visible light harvesting. The photodegradation of methylene blue, MB, in aqueous solution was chosen to study the influence of the morphology and the ceria loading on the photocatalytic response, under UV and solar light. Both CeO₂–TiO₂ nanoparticles and nanotubes were found to be very active under UV light. The highest MB degradation rates were obtained for the 0.25 wt% CeO₂ doping, for both nanotubes and nanoparticles (0.123 and 0.146 min⁻¹, respectively), able to photodegrade completely the dye after 120 min. The two samples are stable after a 3-cycle reusability test. The photo-response under simulated solar light confirmed that doping titania with ceria allows harvesting visible light absorption, enhancing its photoactivity. A maximum efficiency of 85% under simulated sunlight at a degradation rate of 0.054 min⁻¹ was obtained. Transient photoluminescence confirmed that MB acts as a charge scavenger for the composite system. These results pointed out ceria-doped titania nanostructures as a promising class of photocatalysts for the degradation of dyes and other hazardous organic compounds in wastewater.

In the framework of the circular economy, spent coffee grounds were converted into powdered activated carbon by means of pyrolysis, using potassium hydroxide as the activating agent. Its adsorption capacity on a panel of phenolic compounds was compared with those of two commercial powdered activated carbons, after preliminary studies on organic dyes with different ionic properties, to assess the affinity between adsorbates and adsorbents. Pseudo-first-order and pseudo-second-order kinetic models were carried out, together with Freundlich and Langmuir isotherms. They were useful to calculate the breakthrough at 5%, 10%, and 50% of adsorption and the partition coefficients for the comparison of performance between different sorbent systems in a less biased manner (e.g., reducing bias associated with operational settings like sorbate concentration and sorbents dosage). The results showed that the removal efficiency for SCGs-AC was comparable with that of the commercial activated carbons with the highest partition coefficients for methylene blue (12,455 mg/g/μM, adsorption capacity = 179 mg/g) and 3-chlorophenol (81.53 mg/g/μM, adsorption capacity = 3765 mg/g). The lower efficiency in bromothymol blue and bisphenol-A adsorption was due to its different morphology and surface properties.