for decomposition Procion (a red-dye). The molecular interactions between the two were a simple electrostatic force that enabled the composite to undergo surface modification [138]. Fullerene FNM with silanes was proved for its effective photocatalytic activity on phenols for oxidative decomposition [139]. C70- TiO2 hybridized FNMs were proved stable for its photocatalytic behavior for five recycle runs while decomposing sulfathiazole (sulfonamides drug) a powerful antibiotic [140].
1.5.1.3 Graphene (G)/Graphene Oxide (GO)–Based FNMs
The supply of graphene/graphene oxide–based functionalization resolves the constraints delivered by metal oxide PCs. Of late, attentions are focused on FNMs of graphene/graphene oxide–based semi-conductor materials as functionalized PC due to their smaller size with larger specific surface area supported by high electron (e−) conductivity and high adsorption capacity [141]. Advanced research work has been augmented on MO-G/GO FMNs photocatalytic systems [142] for oxidativereduction of pollutants (BG) [143] and (MB) [144]. The unstable and aggregation tendency of the NMs are retarded by the advantages raised due to FNMs.
In one of their studies, researchers Rao, G. et al. synthesized TiO2-NW/Fe2O3-NP/GO FNM sheets by colloid-blending scheme, where the material was found to have 93% efficacy in getting rid of humic acid from water photocatalytically at a pH 6. TiO2 furnished h+ required for ·OH and GO the e− needed for ·O2− needed for the activity [145]. GO/MCU-C3N4/PVDF materials synthesized by vacuumized self-assembly and cross-linking process had exceptional self-cleaning property, which was proven fit for separating oil-in-water colloidal emulsions. Photocatalytic degrading capabilities were attributed to the e− transferences from CB (1.61 eV) to VB (1.18 eV), with π-π* transition giving h+ in VB. h+, ·O2−, and ·OH were controllers in the reaction for eradicating oil-foulants as observed by the researchers Shi, Y. et al. [146].
Scientific workers Gnanamoorthy, G. et al. synthesized AF-Bi2Sn2O7/rGO (AF-amine functionalized) FNMs for photocatalytic degradation of organic dye MB in the visible region was 75% (20 min). Bandgap between pure (2.6 eV) and FNMs (1.6 eV) decreased. VB with h+ and CB with e− that favored the reaction were supported by the formation of radicals ·O2− and ·OH. Stability and reusability of FNMs were persistent up to four cycles [147]. In one of their methods, the authors Liu, H. et al. used FNMs of Bi2Sn2O7/RGO to reduce and degrade Rh B and phenol photocatalytically in the bright visible region (420 nm) and noticed that the degrading efficiencies were 95.8% (125 min) and 81.1% (200 min) for Rh B and phenol, respectively. On embedding RGO on Bi2Sn2O7 (pure), they observed that there was a decrease in the bandgap from 2.48 eV (pure) to 1.85 eV FNM which served well for degrading the contaminant, where the active radicals involved for the reaction was h+ and ·OH [148].
1.5.1.4 Graphene-Carbon Nitride/Metal or Metalloid Oxide–Based FNMs
Recently, conjugation of C and N in a metal-free graphitic polymer is a hotspot that captivates the research workers to utilize the visible energy for the receptive photocatalytic zone in redemption of water pollutants [149]. Normally, hetero-junctions of g-C3N4–based PC are obtained by fusing g-C3N4 (semiconductor) PC and a co-catalyst (semiconductor). Significantly, type II hetero-junction and Z-scheme PC are predominantly employed by many co-workers for removing OPs. Z-scheme have been extensively utilized in BiOI/Pt/g-C3N4 [150], MoO3/g-C3N4 [151], g-C3N4/FeWO4 [152], g-C3N4/Ag/MoS2 [153], TiO2/g-C3N4 [154], and g-C3N4/Ag/Ag3VO4 [155]. While, straddling, staggered, and broken heterojunctions belonging to type 1, type 2, or type 3, with a small/large bandgap between CB and VB/or CB and VB with high potentials, are used in ZnO/g-C3N4 [156], Bi/Bi2WO6/g-C3N4 | Bi/Bi2MoO6/g-C3N4 [157], SmVO4/g-C3N4 [158], g-C3N4/CuWO4 [159], and BiVO4/g-C3N4 [160]. Thus, many FNMs have been used in fabrication, to name a few for the removal of organic toxics like MB, MO, Rh B, fuchsin, and X3B form water segments.
Li, H. et al., fabricated WO3/Cu/g-C3N4 nanohybrids to degrade 4-nonylphenol [161]. While, Yang, Y. et al. used Ag@AgBr/g-C3N4 FNMs as nano-composites to degrade MO [162]. Similarly, the authors Fu, J., et al., in their recent publication of CdS/g-C3N4, demonstrated a comparable output in enhancement-factor as 20.5 and 3.1 for dye-degradation of MO while using the composites of two active semiconductors g-C3N4 and CdS individually [163]. Later, in another experiment, the authors Yang, Y. et al. investigated SPR results of Ag NMs while studying the performance of Ag-coated-g-C3N4 over MO dye-degradation [164]. In another situation, researchers Ma, D. et al. revealed that g-C3N4/RGO/Bi2WO6 FNMs that fit the Z-scheme had RGO as a bridge to transfer the e− electrons between the two bands g-C3N4 and Bi2WO6. The photoelectrons formed in the CB of the later Bi2WO6 moves rapidly into the VB of the former g-C3N4 (holes) to accumulate sufficient (e−) electrons in the CB of the former and holes of VB in the later. FNMs were found effective to photocatalytically degrade and remove TCP from water [165].
In a separate work, Jiang, Z. et al. engineered TiO2/g-C3N4 by solvothermal method and proved its photocatalytic degrading properties over Rh B, MB, and CIP. H+ and superoxide ·O2− had significant role over ·OH radical in this reaction. Excitonic PL signals indicated that n-π* electronic shifts were involved by lone pairs e− present in N atoms of g-C3N4. Hetero yolk-shell structure formed significantly promoted the charge transference efficacy [166]. In a new protocol, facile magnetic g-C3N4/Fe3O4/p-Ru NP FNMs photo-nano catalyst got by deposition-precipitation process showed excellent degradation capacity and reusability with only 5% efficacy lost detected after five cycles. Photocatalysts degraded organic matters—aromatic amines and coloring pigments—azo dyes (CR, CB, EB, and RR-120) efficiently from industrial aqueous water. Formation of photo-electron creates h+ (holes), where the reactive ·OH formed induces a responsible oxidative photo-degradation and h+ (holes)/·O2− radicals have insignificant roles [167].
1.5.1.5 Graphene-Carbon Nitride/QD-Based FNMs
Hydrothermally synthesized BWO fixed as ultrathin Bi2WO6 NSs embedded on g-C3N4 QDs as (CNQDs/BWO), belonging to Z-scheme, efficiently degraded organic contaminants of antibiotic TC and dye Rh B, with % efficacy of 92.51 and 87 in NIR and visible regions, in ~1 h. Langmuir-Hinshelwood model adopted by the authors Zhang, M. et al. later showed that the bandgap energy of 2.70 eV (BWO) and 2.60 eV (CNQDs) was sufficient to bring the change [168]. The authors Zhou, L. et al., proved that GCNQD-treasured on modified g-C3N4 had a worthy photocatalytic degrading activity against organic Rh B [169]. The experimentalists Lin, X. et al. observed that hydrothermally synthesized nano-heterostructures of CNQDs/InVO4/BiVO4 on a leaf-like material of InVO4/BiVO4 had ·O2− radical as the main force behind the efficient oxidative-degradation of Rh B organic dye [170].
Similarly, heterostructure GCNQDs/Ag/Bi2MoO6
