By using the density functional theory (DFT) method, we theoretically investigated the structural, electronic and optical properties of functionalized MXenes with three different geometries, namely M₂CT₂-I, M₂CT₂-II and M₂CT₂-III (M = Ti, Zr, Hf; T = F, O, OH). Except for the M₂CO₂-II and M₂CO₂-III, there is no negative frequency in the phonon spectra of M₂CT₂ materials, which suggests the kinetic stabilities of these functionalized MXenes. The M₂CT₂-I materials with functional groups located above the opposite-side metal atoms have the largest cohesive energies, and thus, are energetically most favorable. Significantly, we found that all the energetically favorable M₂CO₂-I materials have not only suitable band gaps (0.92–1.75 eV), but also visible-light absorption, high redox potential of photo-induced holes, efficient separation of e⁻–h⁺ pairs, and exceptional carrier mobilities, implying the great possibility of Ti₂CO₂, Zr₂CO₂ and Hf₂CO₂ materials to be utilized as visible-light driven photocatalysts. Additionally, the Hf₂CO₂ materials with appropriate band edges could be used for photocatalytic water splitting. Our theoretical studies are valuable for further exploring the potential applications of MXenes, which are worthy of experimental investigation and confirmations.

Via density functional theory computations, we investigated the photocatalytic activities of pure and alloyed bismuth oxyhalides (BiOXs). The dipole moments of the majority of pure and alloyed BiOXs are larger than 2.00 Debye, which can ensure their high photocatalytic efficiencies. Both the redox potentials of the photon-induced holes and the band gaps increase with an increasing content of lighter halogen atoms in the BiOXs, which competitively affects the photocatalytic efficiency. The hole mobility decreases a lot due to the hybridization of the halogennp states, while the electron mobility is not affected by such hybridization. Therefore, the alloying effect in BiOXs brings about a substantially lower electron–hole recombination rate and, accordingly, a much higher photocatalytic efficiency. Our investigation also suggests that O vacancies, which are energetically more favorable in alloyed BiOXs, could act as capture centers for excited electrons and, consequently, improve the separation of the electron–hole pairs. Our findings present a reasonable explanation for the recent experimental reports and provide some guidance for the design of alloyed BiOX photocatalysts.

The photocatalytic properties were compared for the {001}, {110} and {010} facets of bismuth oxyhalides (BiOXs) through density functional theory (DFT) computations. X-terminated bulk-like {001} facets with clear boundary of [Bi₂O₂] and halogen slabs result in high thermodynamic stability and efficient separation of photo-induced e⁻-h⁺ pairs. Moreover, surface O vacancies, which act as e⁻-h⁺ recombination centers, are energetically unfavorable within {001} facets. BiX-terminated {110} and other facets with surface O vacancies introduce deep defect levels to the band gap, which are detrimental to the separation of e⁻-h⁺ pairs. These findings can better understand the origin of facet-dependent photocatalytic activities in BiOXs, and provide guidance for the design of high-efficiency photocatalysts.