Band-Edge Engineering at the Carbon Dot–TiO<sub>2</sub> Interface by Substitutional Boron Doping

We investigated the heterostructures of pristine and boron-doped carbon dots (CDs) with TiO<sub>2</sub> using a first principles approach. Heterojunctions of CDs and TiO<sub>2</sub> demonstrated type II band alignments that promote spatial separation of charge carriers and were well suited for sensitizer configuration. However, large CD band gaps severely restricted their optical efficiencies. Substitutional boron doping of the CDs introduced new electronic states and pulled the CDs’ conduction band minimum (CBM) down, resulting in dramatic reduction of CD band gaps (by 48–57%). The resulting band alignment of the boron-doped CD–TiO<sub>2</sub> heterostructures were found to be type I, in which CBM and valence band maximum of TiO<sub>2</sub> straddled those of CDs and photoexcited carriers could migrate to CD side. This indicated better suitability for device configurations where conducting channel lies through the CDs but elevated the chance of recombination loss. However, the internal electric fields at these heterojunctions were found to selectively promote electron migration and hinder hole migration, which we postulate could counter the recombination loss in these systems. The obtained results offer insights into designing a new generation of water-splitting photocatalysts that would be able to utilize a broader spectrum of solar radiation than the current solutions.