Conformal Layer-by-Layer Assembly of Ti<sub>3</sub>C<sub>2</sub>T<i><sub>z</sub></i> MXene-Only Thin Films for Optoelectronics and Energy Storage
Ian J. Echols, Junyeong Yun, Huaixuan Cao, Ratul Mitra Thakur, Anubhav Sarmah, Zeyi Tan, Richard Littleton, Miladin Radović, Micah J. Green, Jodie L. Lutkenhaus
Abstract
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.