Recent advances and strategies to tailor the energy levels, active sites and electron mobility in titania and its doped/composite analogues for hydrogen evolution in sunlight

Hydrogen energy is realized to be promising as a potential replacement for fossil fuels; it is expected to become a prominent source of energy in the foreseeable future. Therefore, straightforward and cost-effective methods of hydrogen production are urgently required and are considered the current state-of-the-art in the field. Accordingly, the strategy of hydrogen production is very important; it can be achieved through photocatalytic water splitting under irradiation of suitable light sources, where materials are key factors to efficiently conduct this process. In this direction, titanium dioxide/titania (TiO2) has been demonstrated to be an excellent photocatalytic semiconductor towards hydrogen production through water splitting. However, the wide band gap structure, faster carrier recombination and inadequate charge transfer characteristics of TiO2 photocatalysts greatly limit their efficiency and restrain their practical applications in the field. In this context, the size, morphology and chemical properties of TiO2 have been tailored through a range of modification strategies. Closer insights into these processes reveal that the key factors of these modifications essentially involve tuning the crystal structures and surface characteristics and, thereby, the energy structures of titania. Therefore, in this review, we focus on energy structure modifications of TiO2 through various strategies, such as (i) tuning of size and morphology, (ii) creating defect structures, (iii) elemental doping, (iv) composites and (v) surface decoration with metal nanoparticles and co-catalysts. The content of this review provides insight into understanding the energy structure of TiO2 that supports photocatalytic hydrogen generation and therefore endows it with prospects for real-time application in the field to achieve sustainable hydrogen production.