Facet-Selective Electrostatic Assembling of 2D MXene onto Anisotropic Single-Crystal Metal Oxides for Enhanced Photocatalysis

Designing composite photocatalytic systems with nanoscale precision is crucial. While conventional facet-selective photo-deposition successfully utilizes spherical co-catalysts, the directed deposition of pre-synthesized two-dimensional (2D) materials onto specific facets remains extremely challenging. This work demonstrates an electrostatic assembly strategy for the precise deposition of 2D transition metal carbides (MXenes) onto anisotropic single-crystal semiconducting metal oxides. By precisely controlling the solution pH, we modulated the surface charge of the MXenes and the distinct crystallographic facets of the metal oxides, enabling selective deposition driven by electrostatic attraction. Negatively charged Mo_4/3C MXenes were selectively deposited on the electron-rich (101) surface of TiO₂ at pH 3, the (100) surface of Cu₂O exposed at pH 11, and the (010) surface of BiVO₄ at pH 1.5. The high facet selectivity was confirmed through a combination of advanced techniques, including electron microscopy, electron spectroscopy, and synchrotron-based spectromicroscopy. This selective interfacial engineering promotes spatially separated charge carrier migration toward distinct facets, while Schottky barriers form at the MXenes/oxides interfaces. The MXenes act as efficient reduction co-catalysts, facilitating the rapid consumption of electrons, thereby enhancing photocatalytic hydrogen evolution. This work establishes a generalizable, non-photolytic method for integrating challenging 2D co-catalysts with facet-engineered semiconductors for designing composite photocatalysts.