Modification of S-doped g-C₃N₄ with CDs and brown TiO_2-x: Impressive S-scheme photocatalysts with QDs sizes for degradation of pollutants and disinfection of bacteria

Background: It has been reported that defect engineering in TiO₂ via the generation of mid-band gap states and elemental doping in g-C₃N₄ can improve the characteristics of TiO₂ and g-C₃N₄ semiconductors, resulting in boosted photocatalytic ability for removal of contaminants. Methods: In this research, for the first time, S-doped g-C₃N₄ (denoted as S-GC) was modified with CDs and brown TiO_2-x (denoted as TO_X) by a facile scalable procedure, and the resultant QDs sized nanocomposites were utilized for detoxifying water contaminants, containing metronidazole (MNZ) and tetracycline (TC) (as pharmaceutical pollutants), malachite green (MG), fuchsine (FS), and methylene blue (MB) (as organic pollutants), and disinfection of two bacteria, containing Staphylococcus aureus and Escherichia coli upon visible light. Findings: Among the photocatalysts, the binary S-doped g-C₃N₄/TiO_2-x (1:2) and ternary S-doped g-C₃N₄/TiO_2-x/CDs (3 mL) photocatalysts presented superior activity. The photocatalytic activity of S-doped g-C₃N₄/TiO_2-x/CDs (3 mL) nanocomposite against TC degradation was 1335 × 10^‒4 min^‒1, which was respectively 134, 16.0, 88.1, 31.1, and 3.41 times faster than g-C₃N₄, S-doped g-C₃N₄, TiO₂, TiO_2-x, and S-doped g-C₃N₄/TiO_2-x (1:2) photocatalysts. The facile charges transfer among the S-doped g-C₃N₄ and TiO_2-x components via CDs mediator along with the provided active sites played a significant role in the promoted activity, which was confirmed by EIS, PL, and BET analyses. Besides the catalyst stability tests, the plant toxicity studies were conducted through the growth of wheat seeds in a polluted solution with TC after detoxification, and the results showed that the treated system has less toxicity for using in agricultural purposes.