Liquid phase laser ablation as a general green physical etching method to prepare MXene dots with arbitrary surface functionalization. Implications in overall water splitting photocatalysis

A novel green, fluoride-free etching method allowing installation of surface groups at will for the preparation 0D MXene dots with notable photocatalytic activity is reported. The method consists in the liquid-phase laser ablation using a pulsed Nd-YAG laser (532 nm, 7 ns fwhp, 30 mJ pulse−1, 10 Hz) that produces the physical Al etching of the Ti3AlC2 precursor forming nanometric Ti3C2 particles. Simultaneously with etching, the surface of the MXene dots become functionalized with organic moieties inherited from the liquid phase as confirmed by XPS and in several cases by FT-IR spectroscopy. The lateral particle size of the Ti3C2 dots depends on the viscosity of the liquid medium and varies from a few nm to above 50 nm. Crystallinity and structure of the Ti3C2 dot is confirmed by high resolution TEM and by selected area electron diffraction of the samples. Ti3C2 dots promote photocatalytic overall water splitting into H2 and O2, the activity depending on the nature of the surface terminal groups. However, upon reuse the Ti3C2 dots become significantly degraded into amorphous material, decreasing gradually in photocatalytic activity. This degradation is proposed to be caused by self-oxidation of Ti3C2 dots by the photogenerated holes, as evidenced by the fact that the use of methanol as hole quencher stops deactivation during H2 production. In sharp contrast with the decay in photocatalytic activity the presence of hole quenchers, the Ti3C2 dots even increase substantially their photocatalytic activity upon reuse, a fact that has been attributed to the optimization of the nature of the surface terminal groups during the photocatalytic process.