Fabrication of Ti₃C₂T_x MXene Transparent Thin Films with Tunable Optoelectronic Properties

MXenes, a new class of 2D transition metal carbides and carbonitrides, show great promise in supercapacitors, Li-ion batteries, fuel cells, and sensor applications. A unique combination of their metallic conductivity, hydrophilic surface, and excellent mechanical properties renders them attractive for transparent conductive electrode application. Here, a simple, scalable method is proposed to fabricate transparent conductive thin films using delaminated Ti₃C₂ MXene flakes by spray coating. Homogenous films, 5–70 nm thick, are produced at ambient conditions over a large area as shown by scanning electron microscopy and atomic force microscopy. The sheet resistances (R_s) range from 0.5 to 8 kΩ sq⁻¹ at 40% to 90% transmittance, respectively, which corresponds to figures of merit (the ratio of electronic to optical conductivities, σ_DC/σ_opt) around 0.5–0.7. Flexible, transparent, and conductive films are also produced and exhibit stable R_s values at up to 5 mm bend radii. Furthermore, the films' optoelectronic properties are tuned by chemical or electrochemical intercalation of cations. The films show reversible changes of transmittance in the UV–visible region during electrochemical intercalation/deintercalation of tetramethylammonium hydroxide. This work shows the potential of MXenes to be used as transparent conductors in electronic, electrochromic, and sensor applications.