Due to quantum confinement effect, quantum dots (QDs) would show totally different electronic and optical properties from two-dimensional (2D) nanosheets. For the first time, we have systemically investigated the optical, electronic and photocatalytic properties of Ti₃C₂O₂ QDs based on the first-principles calculations. Our results show that the most stable structures of Ti₃C₂O₂ QDs have features of hexagonal unit centered by O atom. The energy gaps of QDs with H passivation vary from 2.76 to 1.14 eV with the increase of lateral size. The lowest unoccupied molecular orbital (LUMO) is mainly contributed by Ti:3d, while the highest occupied molecular orbital (HOMO) is mainly contributed by C:2p and Ti:3d. The corresponding partial charge density at HOMO or LUMO is distributed around Ti and C atoms in the outside surface of QDs. The QDs show large absorption coefficient with the order of 10⁵ cm⁻¹ at visible light range, while the first absorption peaks vary from 2.8 to 1.4 eV (443–886 nm). Most importantly, the Ti₃C₂O₂ QDs present size-dependent photocatalytic selectivity for CO₂ reduction and photocatalytic activity for water splitting, caused by the shift of energy level positions of HOMO and LUMO. Our study reveal the potential of Ti₃C₂O₂ QD in electronic and photocatalytic applications and provide new ideals into the design of photocatalysis based on QD materials.

由于量子限制效应，量子点（QD）将显示出与二维（2D）纳米片完全不同的电子和光学特性。基于第一性原理计算，我们首次系统地研究了Ti ₃ C ₂ O ₂ QD的光学，电子和光催化性能。我们的结果表明，Ti ₃ C ₂ O ₂最稳定的结构QD具有以O原子为中心的六边形单元的特征。H钝化的QD的能隙随横向尺寸的增加而从2.76到1.14 eV不等。最低的未占据分子轨道（LUMO）主要由Ti：3d贡献，而最高的未占据分子轨道（HOMO）主要由C：2p和Ti：3d贡献。HOMO或LUMO处相应的部分电荷密度分布在QDs外表面的Ti和C原子周围。 在可见光范围内，量子点显示出较大的吸收系数，其吸收系数约为10 ⁵  cm ^-1，而第一个吸收峰在2.8至1.4 eV（443–886 nm）之间变化。最重要的是，Ti ₃ C ₂ O ₂ QD对CO呈现出尺寸依赖性的光催化选择性₂由HOMO和LUMO的能级位置的移动引起的水分解的还原和光催化活性。我们的研究揭示了Ti ₃ C ₂ O ₂ QD在电子和光催化应用中的潜力，并为基于QD材料的光催化设计提供了新的理想。