Visible light driven redox-mediator-free dual semiconductor photocatalytic systems for pollutant degradation and the ambiguity in applying Z-scheme concept

Semiconductor photocatalysis using single or modified semiconductors and heterojunction-type composites is limited due to lower visible light response, rapid recombination of photogenerated charge carriers and low redox potentials of photoactive materials. Recently the coordinated applications of two or more semiconductors are becoming popular, an idea borrowed from the thermodynamically uphill driven tandem Z-scheme concept used for photoelectrochemical splitting of water involving two different semiconductors with improved overall visible light response. Consequently, in thermodynamically downhill photocatalytic reactions for pollutant degradation involving dual semiconductors as well the word "Z-scheme" is being used to explain the observed efficient electron-hole separation mechanism. Whereas the Z-scheme classically represents only thermodynamically uphill photosynthetic reactions (Delta G > 0), the photocatalytic degradation of pollutants, on the other hand, are thermodynamically downhill reactions (Delta G < 0), and this usage is creating a misconception and ambiguity in the field of photocatalysis. Therefore, herein we discuss the principles of the often called 'Z-scheme photo systems' involving (i) shuttle redox-mediators, (ii) dual semiconductors photocatalysts with solid state electron mediators and (iii) redox-mediator-free direct systems. The review relevantly summarizes and discusses various representative dual semiconductor photocatalytic systems with solid state mediators as well as non-mediated direct dual semiconductor photocatalysts studied for the degradation of pollutants under visible light irradiation. The future requirements for the development of an efficient visible light driven redox-mediator-free and durable direct dual semiconductor photocatalytic system for the degradation of pollutants is also presented.