Structural, electronic and thermodynamic properties of triatomic borate-terminated MXene surfaces

Abstract

Abstract MXenes, a rapidly growing family of two-dimensional carbides and nitrides, have attracted attention for their high electrical conductivity and highly tunable surface chemistry. The recent synthesis of MXenes featuring triatomic borate (BO $$_{2}$$ ) terminations via a molten route further expanded the range of achievable surface functionalities. Here, we employ density functional theory calculations to systematically investigate a selection of BO $$_{2}$$ -terminated MXenes, including Ti $$_{2}$$ N, Ti $$_{2}$$ C, V $$_{2}$$ C, Nb $$_{2}$$ C, Ta $$_{2}$$ C, Ti $$_{3}$$ C $$_{2}$$ , Ti $$_{4}$$ N $$_{3}$$ , Ti $$_{4}$$ C $$_{3}$$ , V $$_{4}$$ C $$_{3}$$ , Nb $$_{4}$$ C $$_{3}$$ , and Ta $$_{4}$$ C $$_{3}$$ . Our calculations reveal that such BO $$_{2}$$ polyanionic terminations significantly distort the MXene lattice, increasing the thickness of each M $$_{n+1}$$ X $$_{n}$$ layer compared to the corresponding parent MAX phases. These structural changes are accompanied by pronounced near-surface charge transfer, indicative of strong bonding interactions between the MXene and BO $$_{2}$$ functional groups. Electronic structure analysis further demonstrates that surface BO $$_{2}$$ units introduce additional electronic states near the Fermi level, potentially enhancing transport properties relative to Cl-terminated MXenes. Thermodynamic modeling confirms that triatomic borate terminations are energetically favorable under realistic experimental conditions, explaining why these groups can dominate over chlorine terminations during the reported synthesis route. Collectively, our results elucidate how borate functionalization reshapes the structural and electronic properties of MXenes, offering valuable insights into the strategic engineering of advanced two-dimensional materials tailored for multifunctional applications.