Band Positioning in Boron Nitride and Metal-Free Photocatalysts via Photoelectron- and UV-Vis Diffuse Reflectance Spectroscopy

New photocatalysts, particularly porous ones such as porous boron nitride, have emerged that exhibit complex structures and for which, there is limited knowledge of the electronic structure. Gaining insight into their complete band structure on the absolute energy scale will help assessing their suitability for a given photocatalytic reaction. To address this, we rationalise key concepts of band positioning alignment for both porous and non-porous semiconductors on the absolute energy scale. The approach employs a range of techniques generally accessible to many research groups. It involves a combination of spectroscopic techniques, namely X-ray photoelectron spectroscopy to determine the work function and valence band offset, and UV-Vis diffuse reflectance spectroscopy to measure the band gap. We apply this to present the complete band structure of boron nitride, in both porous and non-porous forms. We validate our methodology by comparing the experimentally obtained band structure for graphitic carbon nitride and amorphous boron, both amorphous semiconductors with a known band structure. We show how this can help predict possible photocatalytic reactions and demonstrate this in the context of CO2 photoreduction. With porous materials, such as porous BN, garnering increasing interest for photocatalytic applications, shedding light on their band structures could pave the way towards a methodical tuning and optimization of the photochemistry of these materials.