Recent developments and challenges in practical application of visible-light-driven TiO₂-based heterojunctions for...

The ability of the TiO₂-based photocatalysis process to mineralize organic pollutants has attracted attention worldwide for the degradation of recalcitrant pharmaceuticals and personal care products (PPCPs). Nevertheless, (1) the limited exploitation of the solar spectrum, i.e., activation under UV light (only 2-3% of solar spectrum), and (2) the high recombination rate of photo-generated charge carriers, i.e., electrons and holes, have limited its application which can, however, be improved by developing a TiO₂-based heterojunction. The objective of this critical review paper is to discuss the recent developments (2009-2019) in visible-light-driven (VLD) TiO₂-based heterojunctions for PPCP degradation and their degradation mechanisms. Compared to the conventional heterojunctions, Schottky and Z-scheme heterojunctions, which are non-conventional heterojunctions, are found to be more effective for PPCP degradation due to their more efficient separation of charge carriers and the occurrence of redox reactions at a relatively higher redox potential. Furthermore, the enhancement strategies for the development of a VLD TiO₂-based heterojunction are also explored which can be achieved by selecting the (1) highly photocatalytically active {001} facet of anatase TiO₂, (2) synthesis methods governing the structural changes at the junction interface, and (3) heterojunction components which can efficiently generate the powerful •OH radicals. The challenges in practical applications are also discussed which include factors, viz., cost reduction, recycling, stability, byproducts analysis, evaluation of the environmental effectiveness, and reactor design and scale-up of the VLD TiO₂-based heterojunctions. Accordingly, the prospects of VLD TiO₂-based heterojunctions for PPCP degradation in real environmental applications are discussed.