p-type graphitic carbon nitride with boron bridge atoms designed for photoelectrochemical conversion of CO2

Here, we report the results of studies dedicated to a synthesis of a series of graphitic carbon nitride materials doped with different amounts of boron, designed for photoelectrochemical conversion of $CO₂$. The crystallographic structure of B-doped and pure heptazines was analyzed using XRD, while their chemical structure was analyzed using NMR, XAS, XPS, and FTIR spectroscopy. Based on of experimental spectra and DFT-based ones, we proved that boron in the polyheptazine structure is incorporated through the substitution of N atoms located only in bridge positions. The carbon nitride materials were also characterized in terms of their band structure based on the results of DRS spectroscopy, Mott-Schottky analysis, and DFT modelling. The experimental band gap energies and the decrease of band gap values resulting from the presence of boron bridge atoms were also confirmed by DFT calculations. The localization of valence and conduction bands indicates, along with p-type conductivity resulting from the presence of boron in these specific sites, that B-doped g-$C₃N₄$ may be utilized for light-supported PEC conversion of $CO₂$. Photoluminescence measurements demonstrate more effective charge separation and significantly longer charge carrier lifetime resulting from the incorporation of boron. Finally, photocurrent spectroscopy showed that the material containing 2.5 at. % of B (B1–CN) generates photocurrent of the highest intensity of − 0.99 μA in $CO₂$-saturated 0.5 M $KHCO₃$, making it the most promising one for photoelectrochemical conversion of $CO₂$. The GC-MS/TCD analysis of syngas produced during photoelectrochemical $CO₂$ reduction confirmed the formation of $H₂$ and CO.