(328i) Porous Phosphorus-Doped Boron Nitride Materials for Photocatalytic CO2 Reduction

CO₂ photoreduction is a potential avenue for the production of sustainable fuels, which will be required to help achieve our climate goals in hard-to-abate sectors. Currently, the state-of-the-art route is the photothermal one using ceria (CeO₂) which is expensive due to the high temperatures required (~1,500 ^oC). Alternatively, photocatalysis could be a cost-effective and safer route, operating under mild conditions. However, there are numerous challenges which need to be overcome; a major one being the design, production, and deployment of an efficient photocatalyst. To date, multiple oxides and sulphides have been studied at the lab-scale, with titania (TiO₂) as the benchmark photocatalyst. TiO₂ offers a combination of efficiency, cost-effectiveness, and robustness, while on the other hand, presents only UV light absorption due to its wide band gap. It is essential to design photocatalysts which also absorb in the visible light portion of the solar spectrum, as it occupies the largest percentage.

Porous amorphous boron nitride (BN), as opposed to crystalline BN, has proved to be a promising material for CO₂ photoreduction with photocatalytic activity competitive with the benchmark TiO₂ P25 and the ability to harvest visible light¹. The introduction of other atoms/functional groups to BN-based materials can further alter their bandgap, and electron transfer mechanisms. Therefore, it is expected that photocatalytic activity can be influenced through chemical doping. Studies have shown that non-metal heteroatoms (S or P) can alter the optoelectronic properties of graphitic carbon nitride^2–4, while for BN materials only C, O and Si have been studied for this purpose^5–7. The electronic properties of P could make it a suitable dopant for porous BN.

In our study, we explore P doping of porous BN materials and the impact on the material’s optoelectronic and photocatalytic properties. To do this, we have synthesized amorphous pure and P-doped BN, and have used XPS and NEXAFS to quantify the P content and their chemical environment, depending on the synthesis route and precursors. The materials exhibit micro- and mesoporosity with high surface area, and good CO₂ adsorption capacity as determined by N₂ adsorption at 77 K, and CO₂ adsorption at 298 K, respectively. DR-UV/Vis combined with XPS analysis show that our P-doped samples have a reduced bandgap, and as a result can harvest larger portion of the visible light spectrum during CO₂ photoreduction_, as compared to the pure BN sample.

References:

[1] Shankar, R. et al. Porous boron nitride for combined CO2 capture and photoreduction. J. Mater. Chem. A 7, 23931–23940 (2019).

[2] Chegeni, M. & Dehghan, N. Preparation of phosphorus doped graphitic carbon nitride using a simple method and its application for removing methylene blue. Phys. Chem. Res. 8, 31–44 (2020).

[3] Li, H., Zhang, N., Zhao, F., Liu, T. & Wang, Y. Facile fabrication of a novel au/phosphorus-doped g-c3 n4 photocatalyst with excellent visible light photocatalytic activity. Catalysts 10, (2020).

[4] Qin, H. et al. Sulfur-doped porous graphitic carbon nitride heterojunction hybrids for enhanced photocatalytic H2 evolution. J. Mater. Sci. 54, 4811–4820 (2019).

[5] Weng, Q. et al. Tuning of the Optical, Electronic, and Magnetic Properties of Boron Nitride Nanosheets with Oxygen Doping and Functionalization. Adv. Mater. 29, 1–8 (2017).

[6] Huang, C. et al. Carbon-doped BN nanosheets for metal-free photoredox catalysis. Nat. Commun. 6, (2015).

[7] Cho, Y. J. et al. Electronic structure of Si-doped BN nanotubes using X-ray photoelectron spectroscopy and first-principles calculation. Chem. Mater. 21, 136–143 (2009).