Heterogeneous photocatalysis has emerged as a promising alternative for both micropollutant removal and bacterial inactivation under solar irradiation. Among a variety of photocatalysts explored in the literature, graphite carbon nitride (g-C₃N₄) is a metal-free semiconductor with acceptable chemical stability, low toxicity, and excellent cost-effectiveness. To minimize its high charge recombination rate and increase the photocatalyst adsorption capacity whilst keeping the metal-free photocatalyst system idea, we proposed the heterojunction formation of g-C₃N₄ with diamond nanocrystals (DNCs), also known as nanodiamonds. Samples containing different amounts of DNCs were assessed as photocatalysts for pollutant removal from water and as light-activated antibacterial agents against Staphylococcus sureus. The sample containing 28.3 wt.% of DNCs presented the best photocatalytic efficiency against methylene blue, removing 71% of the initial dye concentration after 120 min, with a pseudo-first-order kinetic and a constant rate of 0.0104 min⁻¹, which is nearly twice the value of pure g-C₃N₄ (0.0059 min⁻¹). The best metal-free photocatalyst was able to promote an enhanced reduction in bacterial growth under illumination, demonstrating its capability of photocatalytic inactivation of Staphylococcus aureus. The enhanced photocatalytic activity was discussed and attributed to (i) the increased adsorption capacity promoted by the presence of DNCs; (ii) the reduced charge recombination rate due to a type-II heterojunction formation; (iii) the enhanced light absorption effectiveness; and (iv) the better charge transfer resistance. These results show that g-C₃N₄/DNC are low-cost and metal-free photoactive catalysts for wastewater treatment and inactivation of bacteria.

Bismuth vanadate (BiVO₄ or BVO) is one of the most studied photocatalysts for water oxidation because of its excellent visible light absorption and appropriate band energy positions. However, BVO presents a low charge mobility and a high electron–hole recombination rate. To address these fundamental limitations, this study proposes the coating of previously synthesized phase-pure monoclinic scheelite BVO with different amounts of naked cobalt (further oxidized to cobalt hydroxide) nanoparticles (NPs) via a modified magnetron sputtering deposition. The resulting BVO/Co photocatalysts were investigated for methylene blue (MB) photodegradation, photocatalytic oxygen evolution, and photoelectrochemical (PEC) water oxidation. In the MB photodegradation tests, the BVO/Co sample prepared with a deposition time of 5 min (BVO/Co(5 min)) presented the highest photoactivity (k = 0.06 min⁻¹) compared with the other sputtering investigated times (k = 0.01–0.02 min⁻¹), as well as the pristine BVO sample (k = 0.04 min⁻¹). A similar trend was evidenced for the PEC water oxidation, where a photocurrent density of 23 µA.cm⁻² at 1.23 V (vs. RHE) was observed for the BVO/Co(5 min) sample, a value 4.6 times higher compared with pristine BVO. Finally, the BVO/Co(5 min) presented an O₂ evolution more than two times higher than that of the pristine BVO. The increased photocatalytic performance was ascribed to increased visible-light absorption, lesser electron–hole recombination, and enhanced charge transfer at the liquid/solid interface. The deposition of Co(OH)₂ NPs via magnetron sputtering can be considered an effective strategy to improve the photocatalytic performance of BVO for different target catalytic reactions, including oxygen evolution, water oxidation, and pollutant photodegradation.