[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 NSs had a very good 100% degrading capability of Rh B in visible region, where h+ of VB (Bi2MoO6) and e− of CB (CNQDs) worked effectively for oxidation/reduction to cause degrading reaction to give H2O and CO2 [171]. Si NWs (silicon nano-wires) on g-C3N4 QDs as Si NWs @ g-CNQDs, photoelectrocatalytically could decompose 85.1% of 4-CP in ~ 2 h from aqueous solution, had a notable charge separation and good stability [172]. π-conjugated GCNQDs implanted on metalloid sulfide Sb2S3/supported by ultrathin-g-C3N4, with a bandgap of 2.7 eV, were proved to be good candidate for photocatalytic disposals of MO from unwanted water and had a very good electron (e−) transference [173]. In a novel approach, the co-workers Patel, J. et al. synthesized Mn:ZnS/QDs, for photo-degrading fluoroquinolone: Norfloxacin in an ambient condition of solar-light/UV-light, where Mn and ·OH fortified the reaction to 4 reapplied cycles [174].
1.5.2 Polymer Composite–Based FNMs as Photocatalysts
Polymer TiO2/CS/glass FNMs were powerful in decomposing RR4 organic dye in visible region. h+ and ·OH generated from TiO2 layer circulate to TiO2/CS boundary to cause oxidation of RR4. A total of 100% efficiency was noticed with the stability up to seven reusable cycles [175]. CdS/TiO2-PAN FNM degraded MB (66.29%) in 210 min [176]. Researchers studied the photocatalytic action and inferred a repeated utility to protect the water system. Chitosan-AgCl/Ag/TiO2 synthesized by the team Jbeli, A. et al. was reported to be cost-effective photocatalytic degrader of organic components ABA, O-TD, and SA under visible radiations [177]. Similarly, surface modified FNM TiO2/ZnO/chitosan had a powerful photo-degradability of MO (97%) when excited by solar radiations [178]. An organic/inorganic FNM as composites P3HT/PNP-Au NP got by re-precipitation method showed positive spectral line in UV region (~427 nm), had an enhanced photocatalytic decomposition of MB (90.6%), and inferred that it may be due to a strong π-π* shift [179]. A 3D honey-comb like ordered macro-porous NM-3DOM Ag/ZrO2 had significant photocatalytic degradability over CR when stimulated by multi-modules of microwave-assisted, simulated-solar, UV, and visible radiation [180].
1.5.3 Metal/Metal Oxide-Based FNMs as Photocatalysts
Metal/Metal oxide when entrapped as FNMs, on photo-irradiation leads to photo-excitation of particles that undergoes transference between the CB and VB, where the CB transfers the e− to degrade the pollutant [181]. Components like (1) semiconductor, (2) metal, and (3) metal-supporters assist interfacial-interaction for photocatalytic degradative actions. In one of their reports, Park, S.J. et al., proved that T-ZnO-CNO FNMs with nano-onions prepared by green routes removed the challenging pollutant of water 2,4-DNP photocatalytically with an efficacy of ~92%. Conversion of O2 to ·O2− and formation of ·OH favored the degradation using 3D hybrid structures [182]. Degradation of phenol (63%) and MB (52%) at 420 nm (visible) by FNMs of Au/(WO3/TiO2) and WO3/TiO2 were noticed by Rhaman, Md. et al. [183]. Electron transference is retarded but hole movement is favored due to Au embedded on the surface of WO3/TiO2. Sufficient bandgap energies cause photocatalytic excitations. Flower-like functionalized Au-ZnO NMs hydrothermally were responsible contributors for photo-decomposition of Rh B into CO2 and H2O, with h+(holes) and ·OH formed by light radiations were functional for the activity. A total of 99% reduction in 10 min was noticed by authors Hussain, M. et al. [184].
1.6 Nanocatalyst Antimicrobials as FNMs
FNMs as nanocatalyst have been authenticated with a promising note for cleansing and sanitization treatment for a mixture of waste and normal water from different sources. With an excellent potentiality to inactivate the active disease-causing dreadful pathogenic micro-organisms like fungi, bacteria, and viruses, FNMs behold their role to safeguard the water bodies. Increase in the potential momentum for anti-microbial activity is efficaciously observed by surface