Photocatalytic Reduction of Cr(VI) by Titanium Dioxide Coupled to Functionalized CNTs: An Example of Counterproductive Charge Separation

Polymeric nanofibers containing both carbon nanotubes and titanium dioxide were produced by electrospinning and used to photocatalytically reduce Cr(VI) ions. A comparison was made between multiwall carbon nanotubes (CNTs) and single-wall CNTs, as well as between OH-functionalized and nonfunctionalized CNTs. The reduction rate was found to decrease upon using OH-functionalized CNTs, whereas the opposite phenomenon was observed with nonfunctionalized CNTs. The systems under study demonstrate the counterintuitive possibility that charge separation for specific target species can be quite deleterious for photocatalysis. Whether enhanced charge separation is benevolent or deleterious depends on the type of reaction (oxidation or reduction) as well as the location of the reaction (the photocatalyst or the inert electron sink) and the adsorpticity of both the photocatalyst and the inert partner.