Exploitation of Carbon Nitride Films for Photoelectrochemical Solar Fuel Production

Photoelectrochemical (PEC) solar fuel production is a promising way to alleviate the increasing energy demand of humankind. Polymeric carbon nitride (CN) has been attracting tremendous interests in energy-related fields since 2009. However, utilization of CN for photoelectrochemical water splitting is still at its infancy due to the poor electronic conductivity, severe electron-hole recombination, sluggish hole utilization, and the difficulty in CN film synthesis. The direct growth of crystalline CN film with good contact to conductive substrate can overcome these issues though it is challenging. In our group, we developed several techniques for successful CN film fabrications starting from monomer organization on substrate, including doctor-blade and two-step seeding-crystallization, followed by thermal condensation. The doctor blade technique is verified as a general pathway to grow highly porous and large-scale CN films with controllable chemical and photophysical properties on various substrates. The obtained CN films demonstrate high electrochemically active surface area, excellent dye adsorption and photodegradation properties, and promising PEC application. By incorporating this technique with reduced graphene oxide (rGO), the CN/rGO film exhibits a photocurrent up to 75 μA/cm² at 1.23 V vs RHE in 0.1 M KOH, while the one in presence of hole scavenger reaches 660 μA/cm². Another work is that porous hierarchical crystalline CN film on FTO is synthesized via a two-step seeding-crystallization monomer film assembly followed by a thermal condensation. Results show the obtained CN film exhibits improved electron transport, reduced structural defects, and advantageous photo-generated hole extraction at the CN/electrolyte interface. Accordingly, the CN electrode produces a high photocurrent of 116 μA/cm² upon one sun illumination at 1.23 V vs RHE in 0.1 M KOH aqueous solution with the IPCE value at 400 nm reaches 8.53%. Our works open doors to CN film on substrates, which holds great promise in photoelectrochemical devices, photovoltaics, and light emitting fields.