The photocatalytic performance of metal-free graphitic carbon nitride (g-C₃N₄) was examined using visible light-emitting diodes (LEDs). A comparative and parametric study was conducted using the photocatalytic degradation of phenol as a model reaction. The g-C₃N₄ photocatalyst was synthesized from melamine using thermal condensation, followed by a thermal exfoliation that increases the catalyst surface area from 11 to 170 m²/g. Different characterization techniques, namely X-ray powder diffraction, X-ray photoelectron spectroscopy, nitrogen adsorption using the Brunauer–Emmett–Teller method, ultraviolet-visible (UV–vis) spectroscopy, transmission electron microscopy, photoluminescence spectroscopy (PL), and zeta potential analysis, were used to characterize the photocatalyst. A comparison of the photodegradation experiments conducted with a full-spectrum xenon lamp and a custom-made single-wavelength LED immersion lamp showed that the photocatalyst performance was better with the LED immersion lamp. Furthermore, a comparison of the performance of exfoliated and bulk g-C₃N₄ revealed that exfoliated g-C₃N₄ completely degraded the pollutant in 90 min, whereas only 25% was degraded with bulk g-C₃N₄ in 180 min because the exfoliated g-C₃N₄ enhances the availability of active sites, which promotes the degradation of phenol. Experiments conducted at different pH have shown that acidic pH favors the degradation process. The exfoliated g-C₃N₄ has shown high photocatalytic performance in the photodegradation of other phenolic compounds, such as catechol, m-cresol, and xylenol, as well.

Four commercial titanium dioxide (TiO₂) photocatalysts, namely P25, P90, PC105, and PC500, were immobilized onto steel plates using a sol-gel binder and investigated for phenol degradation under 365 nm UV-LED irradiation. High-performance liquid chromatography (HPLC) and total organic carbon (TOC) analyses were performed to study the impact of three types of oxygen sources (air, dispersed synthetic air, and hydrogen peroxide) on the photocatalytic performance. The photocatalyst films were stable and there were significant differences in their performance. The best result was obtained with the P90/UV/H₂O₂ system with 100% degradation and about 70% mineralization within 3 h of irradiation. The operating conditions varied, showing that water quality is crucial for the performance. A wastewater treatment plant was developed based on the lab-scale results and water treatment costs were estimated for two cases of irradiation: UV-LED (about 600 EUR/m³) and sunlight (about 60 EUR/m³). The data show the high potential of immobilized photocatalysts for pollutant degradation under advanced oxidation process (AOP) conditions, but there is still a need for optimization to further reduce treatment costs.

Phosphomolybdic acid (PMA) was used to achieve simultaneously P-doping and heterojunction construction of graphitic carbon nitride (gCN). P-gCN/PMA_x composites were obtained via post-thermal annealing of bulk gCN/PMA_x and characterized in detail using different techniques including XRD, XPS, SEM, and DRS. As an in situ hard templating agent and doping source, the loaded PMA helps to provide porous structured materials and reinforced electronic properties whereby many transitions are enhanced resulting in the improvement of light-harvesting. The electronic properties of gCN are improved strongly with the increased amount of PMA loaded, but the crystallinity becomes worse and the material becomes much more amorphous and disordered. The materials were investigated for the hydrogen evolution reaction (HER) showing the highest H₂ evolution performance of 625 μmol g⁻¹ h⁻¹ for the P-gCN/PMA_1.5 sample, which is almost 4 times higher than that of gCN with 167 μmol g⁻¹ h⁻¹. The P-gCN/PMA_1.5 sample was investigated under long-term irradiation and in recycling tests indicating the good photocatalytic stability of this material. The apparent quantum efficiency (AQE) exhibited by P-gCN/PMA_1.5 (0.7%) is 7 times higher than the AQE measured exhibited by gCN. This is evidence of the boosting effect exhibited by the loading of PMA onto the surface of gCN (0.1%). Thus, this study aims to offer a novel strategy to improve the activity of gCN by applying polyoxometalates as modifiers for better light harvesting.