How Does Sodium Polyphosphate Used to Prevent Ti3C2Tx Degradation in Water Impact Electronic Properties of Thin Films?

Titanium carbide, Ti3C2Tx, where Tx stands for surface terminations -OH, -F, and -O, is the first discovered member of 2D MXene family^[1]. Its hydrophilic nature enables fabrication of high-quality thin films from Ti3C2Tx aqueous suspensions. High conductivity of resulting films makes them attractive for flexible electronics, electromagnetic shielding, and THz devices^[1-3]. However, degradation of Ti3C2Tx in aqueous environments due to oxidation and hydrolysis presents a challenge. Capping MXene flake edges and defects with sodium polyphosphate improves chemical stability in solution^[4,5]. Here, we investigate the effect of sodium polyphosphate on the electronic properties of thin films using a four-point probe measurement of DC conductivity and time-domain THz spectroscopy measurement of complex frequency-resolved conductivity^[2,6]. We find that polyphosphates can improve MXene conductivity, possibly acting as bridges that facilitate inter-flake transport and passivating defects that act as carrier traps.<br/>Approved for public release PR2024-1144.<br/><br/>[1] VahidMohammadi, A., J. Rosen, and Y. Gogotsi, <i>The world of two-dimensional carbides and nitrides (MXenes).</i> Science, 2021. <b>372</b>(6547): p. eabf1581.<br/>[2] Li, G., et al., <i>Dynamical control over terahertz electromagnetic interference shielding with 2D Ti₃C₂T_y MXene by ultrafast optical pulses.</i> Nano Letters, 2020. <b>20</b>(1): p. 636-643.<br/>[3] Li, G., et al., <i>Terahertz polarizers based on 2D Ti₃C₂T_z MXene: spin cast from aqueous suspensions.</i> Advanced Photonics Research, 2020. <b>1</b>(2): p. 2000084.<br/>[4] Natu, V., et al., <i>Edge capping of 2D-MXene sheets with polyanionic salts to mitigate oxidation in aqueous colloidal suspensions.</i> Angew Chem Int Ed Engl, 2019. <b>58</b>(36): p. 12655-12660.<br/>[5] Huang, S., et al., <i>Understanding the effect of sodium polyphosphate on improving the chemical stability of Ti₃C₂T_z MXene in water.</i> Journal of Materials Chemistry A, 2022. <b>10</b>(41): p. 22016-22024.<br/>[6] Li, G., et al., <i>Two-dimensional MXenes Mo₂Ti₂C₃T_z and Mo₂TiC₂T_z: microscopic conductivity and dynamics of photoexcited carriers.</i> ACS Applied Energy Materials, 2020. <b>3</b>(2): p. 1530-1539.