Review of bio-inspired green synthesis of titanium dioxide for photocatalytic applications

Titanium dioxide (TiO2) is an important photocatalyst that is widely studied for environmental applications, especially for water treatment by degradation of pollutants. A range of methods have been developed to produce TiO2 in the form of nanoparticles and thin films. Solution-based synthesis methods offer the opportunity to tune the synthesis through a choice of reagents, additives and reaction media. In particular, the use of biomolecules, such as proteins and amino acids, as bio-inspired additives in TiO2 synthesis has grown over the last decade. This review provides a discussion of the key factors in the solution-based synthesis of titania, with a focus on bio-inspired additives and their interaction with Ti precursors. In particular, the role of bio-inspired molecular and biomolecular additives in promoting the low-temperature synthesis of titania and controlling the phase and morphology of the synthesised TiO2 is discussed, with a particular focus on the interaction of TiO2 with amino acids as model bio-inspired additives. Understanding these interactions will help address the key challenges of obtaining the crystalline TiO2 phase at low temperatures, with fast kinetics and under mild reaction conditions. We review examples of photocatalytic applications of TiO2 synthesised using bio-inspired methods and discuss the ways in which bio-inspired additives enhance photocatalytic activity of TiO2 nanomaterials. Finally, we give a perspective of the current challenges in green synthesis of TiO2, and possible solutions based on multi-criteria discovery, design and manufacturing framework.