NOVELTY - Photocatalytic system comprises a heterodimer comprising: a first nanomaterial comprising titanium dioxide (TiO2) having a first bandgap (101) energy characterized by a first highest occupied molecular orbital (HOMO) (103) and a first lowest unoccupied molecular orbital (LUMO) (105); and a second nanomaterial comprising semiconducting metal oxide and/or metal sulfide having a second bandgap energy characterized by a second HOMO and a second LUMO, where the second bandgap energy is in the range of energies for visible light spectrum, and the second LUMO is higher than the first.    USE - The photocatalytic systems are useful: in a water filtration system and a water electrolysis system; for generating free radicals in water; for harvesting visible light for photocatalysis (all claimed); for producing hydrogen gas for hydrogen cars; and for treatment/filtration of contaminated water by oxidation of organic matter by the generated free radicals.    ADVANTAGE - The photocatalytic systems possess enhanced photocatalytic activity.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for:    (1) a method (A) of harvesting visible light for photocatalysis comprising: providing a heterodimer comprising a first nanomaterial comprising TiO2, and a second nanomaterial comprising semiconducting metal oxide and/or semiconducting metal sulfide; and exposing the heterodimer to electromagnetic (EM) radiation, where at least part of visible light spectrum of the EM radiation is absorbed by the second nanomaterial to excite an electron from a HOMO to LUMO of the second nanomaterial;    (2) a method (B1) of fabricating a heterodimeric photocatalytic (HDP) structure comprising impregnating a host matrix with a second nanomaterial comprising semiconducting metal oxide and/or metal sulfide whose bandgap energy is in the range of energies for visible light spectrum and coating a first nanomaterial comprising TiO2 onto at least part of the surface of an integrated structure comprising the second nanomaterial;    (3) a method (C1) of fabricating a HDP structure comprising: forming a heterodimer comprising a first nanomaterial comprising TiO2, and a second nanomaterial comprising semiconducting metal oxide or metal sulfide whose bandgap energy is in the range of energies for visible light spectrum; and impregnating the heterodimer into a host matrix;    (4) a photocatalytic system (F) comprising a photocatalytic heterodimer comprising: a UV light responsive nanomaterial; and a visible light responsive nanomaterial, where the UV light responsive material and the visible light responsive nanomaterial are attached to or proximally positioned with respect to each other such that a photogenerated electron from the visible light responsive nanomaterial can transfer to the UV light responsive nanomaterial to participate in a photocatalytic activity;    (5) a water filtration system comprising (F); and    (6) a water electrolysis system comprising (F).    DESCRIPTION OF DRAWING(S) - The figure shows an electronic energy diagram of TiO2.    Electronic energy diagram (100)    Bandgap (101)    Highest occupied molecular orbital energy level (103)    Lowest unoccupied molecular orbital energy level (105)    Conduction band (120)