Layer-by-Layer Assembly of MXene-Based Thin Films

Abstract

MXenes are a class of two-dimensional nanomaterials which have drawn interest in a wide variety of fields (e.g., energy storage, sensing, catalysis, separation, etc.). While there are hundreds of possible MXene compositions, most work to date has focused on Ti₃C₂Tz. In this work, we utilize new MXene types to make thin films using layer-by-layer (LbL) assembly. In order to do this, we first demonstrated that techniques for improving the oxidative of stability of Tiₙ₊₁CₙTz MXenes could be generalized to novel MXenes, specifically Nbₙ₊₁CₙTz. We utilized x-ray photoelectron spectroscopy (XPS) and spectrophotometry to quantify the degree of oxidation of MXene dispersions. By using an antioxidant and lowering storage temperature, we were able to noticeably improve the shelf life of the Nbₙ₊₁CₙTz MXenes. Further, we demonstrated that MXenes with higher “n” (e.g., Nb₄C₃Tz vs. Nb₂CTz) were more oxidatively stable.

After verifying methods to improve the oxidative stability of MXenes, we then used three different MXenes (e.g., Ti₃C₂Tz, Ti₂CTz, and Nb₂CTz) to fabricate polymer/MXene thin films using LbL assembly. By varying the MXene used for assembly, we could control both the optical and electrical properties of the films as well as their growth rates. Specifically, LbL films assembled with Ti₃C₂Tz were more electrically conductive and had lower optical band gaps than their Ti₂CTz containing counterparts. While polymer/Nb₂CTz LbL films were less electrically conductive than their Ti₂CTz counterparts, they demonstrated higher film densities and lower optical band gaps.

However, all polymer/MXene LbL films had lower electrical conductivities than their vacuum filtered MXene-only thin film counterparts. To address this issue, we utilized a silane functionalization process to obtain positively charged f-Ti₃C₂Tz which was then used to make Ti₃C₂Tz-only LbL films. Similar to polymer/MXene LbL films, the Ti₃C₂Tz-only LbL films demonstrated conformal coatings on a wide variety of substrates. Notably, they demonstrated considerably improved electrical conductivities (92,000 vs. 3,800 S/m) and areal capacitances (13 vs. 4.7 mF/cm²). Lastly, we fabricated mixed MXene heterostructures using LbL assembly which we expect will enable enhanced property control of MXene-based thin films in the future.