A comparative review of dopant strategies for efficient photo-electrocatalytic water splitting: 3d transition metal oxides vs. carbon-based materials.

Photocatalytic water splitting is an essential technology in the production of hydrogen on a sustainable basis, to combat the energy limits the world is facing by conserving solar energy in chemical bonds. Nevertheless, an inefficient visible light absorption and recombination of charges usually restrict the efficiency of semiconductor photocatalysts. This review is comparative analysis of two prominent dopant-based methods to overcome these limitations, namely, Transition Metal Oxides (TMOs) and Carbon-Based Materials (CBMs). We debate on the latest advancements in TMO dopants including Sc, V, Cr, Co, Ni and Cu, which is a TMO dopant but acts by decreasing bandgaps and altering electronic band structures in order to reach visible light. On the other hand, CBMs as graphene, carbon nanotubes (CNTs), and carbon quantum dots (CQDs), are electron sinks, as well as, conductive scaffolds, which enable separation of charge, and charge movement. This review emphasizes the trade-off between TMO-based bandgap engineering and CBM-based charge-mobility optimization. Additionally, we are referring to the combination of these strategies into hybrid systems that combine the optical benefits of TMOs and kinetic benefits of CBMs. The current review gives a design rulebook to the next-generation of high-efficiency photocatalysts in that it outlines some of the special conditions under which each type of dopant can work effectively.