[Retracted] Heavy Metal Removal from Aqueous Effluents by TiO₂ and ZnO Nanomaterials

Table 1

Modifications of TiO₂ for improving remediation capacity [52].

Technique	Examples of application materials	Effects of modification on TiO2 removal performance
Doping	Graphite, graphene oxide, semiconducting metal oxides (ZnO), metals (Cu, Zr).	Band gap between VB and CB is difficult to overcome by UV and visible light. Doping introduces midgap states and decreases the band gap energy.
Composites	Carbon nanotubes, metallic oxides, graphene oxides.	Combination of favourable properties for photocatalysis of all constituents.
Complexes	Porphyirin-TiO2	Absorption of UV and visible light.
Nanorods	Anatase/rutile/titanate rods	Increases specific area and mesoporosity.
Nanosheets	TiO2, graphene	High specific surface area, large number of active sites/facets. Increased separation and delayed recombination of holes.