Graphitic carbon nitride (gC₃N₄) has been deployed in various applications, including photocatalysis. Among photocatalytic reactions studied, H₂O splitting for the production for H₂ is the most common one, and carbon nitride can serve as pure catalyst, cocatalyst, catalyst support, or part of a heterojunction. Photocatalytic CO₂ reduction is another reaction of interest, combining utilisation of CO₂ emissions and production of sustainable fuels and chemicals. Research on the use of carbon nitride for this purpose is less extensive, and almost always includes the use of dopant materials or heterojunctions. For instance, the role of boron (B) as dopant for gC₃N₄ has started to be explored but mostly for application in zinc batteries, photodegradation of organics, and photocatalytic H₂O splitting/H₂ production. Only a couple of studies have investigated the role of B-doping on CO₂ photoreduction and B-gC₃N₄ seems superior to the pristine material, for all reactions studied. Yet, the relationship between the structure/chemistry of B-doped gC₃N₄ on its chemical, sorptive and optoelectronic properties, as well as CO₂ photoreducing activity remains largely unknown. If understood, a greater control of and more efficient B-doped gC₃N₄ could be reached.

In our study, we aim to bridge this knowledge gap in (photo)chemistry of B-gC₃N₄. We produced two sets of B-doped gC₃N₄ samples through calcination of melamine mixture with varying amount of either amorphous boron, or boric acid.  Once synthesised, we characterized our samples using: XPS, XRD, N₂ sorption (77 K), CO₂ sorption (288, 298, 308 K), DRS UV-Vis, steady-state PL, TAS and EPR. We confirmed the successful B-doping of gC₃N₄ using both B precursors (from 0.5 to 11 at% B). We could control better the amount of doping using boric acid, owing to its greater reactivity with melamine. Introducing B causes oxygen (O) to be also included in the structure in analogous amounts and B-O bonds. High B content results in increased BET area and enhanced CO₂ adsorption. B-doping lowers the band edges, without changing the bandgap of the material. All samples show similar tri-s-triazine structure and light absorbance, however different relaxation patterns and creation of mid-gap states. The samples share similar charge carrier lifetimes and kinetics, even though B-doping up to 5 at% increases the amount of excitons. We noticed differences in the amount of unpaired electrons, which are potentially connected to the chemical structure changes caused by B integration from different precursors. Most of the samples show change in EPR signal intensity before and after irradiation, an indication of excited electrons. Our study provides for the first time a comparison between B precursors for B-doping of C₃N₄, and thorough investigation of their effect on the material’s sorptive and optoelectronic properties. We will next study the B-gC₃N₄ materials’ photocatalytic properties.