Breaking boundaries of soft photocatalysis: Overcoming limitations of carbon nitride as single-light absorber for overall water splitting

Abstract

Tapping solar energy for sustainable hydrogen production is expected to play a key role in the net-zero transition. While inorganic semiconductor materials have historically dominated the field, the search for a viable low-cost and environmentally benign photocatalyst has propelled carbon nitride as a flagship organic material, poised to challenge its inorganic counterpart. While the activity of carbon nitride photocatalysts has long been inherently hindered by tightly-bound excitons and restricted charge carrier mobility, typical of organic semiconductors, their ‘soft’ polymeric and molecularly precise backbone offer vast molecular-level design space to overcome these limitations. To date, innovations in carbon nitride have advanced from being primarily active in hydrogen half-reactions to achieving overall water splitting at competitive rates, even when acting as the sole-light absorber. As such, this review aims to unravel this paradigm shift by first scrutinizing some fundamental concepts behind the limited photocatalytic activity of carbon nitride in pure water from the context of excitonic effects associated to the photophysical and physicochemical properties. Several design principles and successful schemes in addressing this bottleneck will then be outlined. Lastly, the current state is reviewed and latest challenges and opportunities in this burgeoning field will be presented.