Conformal Layer-by-Layer Assembly of Ti3C2Tz MXene-Only Thin Films for Optoelectronics and Energy Storage

MXenes are a class of two-dimensional (2D) nanomaterials known for their diverse chemical structures and tunable properties. MXene-based films are typically produced by spray assembly, drop-casting, or vacuum filtration. Unfortunately, these methods typically result in coatings with poor surface adhesion or are limited to certain substrate types and geometries. Layer-by-layer (LbL) assembly of polycation/MXene thin films has demonstrated excellent conformability and adhesion, but the introduction of the polycation results in a decrease in the functional properties of the film due to the insulating nature of the polymer. The challenge, then, is to replace the polycation with a positively charged MXene, but as-prepared MXenes are negatively charged. Here, we report the layer-by-layer (LbL) assembly of Ti3C2Tz-only thin films enabled by the functionalization of Ti3C2Tz nanosheets to render them positively charged. This results in LbL-assembled thin films with electrical conductivities 24 times higher and capacitances 3 times higher than those of analogous polymer/MXene LbL films. As compared to typical assembly methods for MXene-only films, LbL assembly enables adherent coatings on a variety of surfaces (e.g., glass, Si, indium tin oxide (ITO)-coated glass, poly(ethylene terephthalate), silicone, nylon thread, etc.) and geometries (rough, macroporous, etc.). As a demonstration, the all-MXene LbL films were used as transparent conductive films and electrodes with high capacitance (13 mF/cm2, 2080 F/cm3, 570 F/g). We anticipate that this method can be extended to create MXene-only structures beyond just Ti3C2Tz to create increased control over coating properties to open the way for new applications such as optoelectronics, energy-storage devices, catalysis, and sterilization.