Layer-Dependent\nGas Sensing Mechanism of 2D Titanium\nCarbide (Ti<sub>3</sub>C<sub>2</sub>T<i><sub><b>x</b></sub></i>) MXene

Monolayers of Ti<sub>3</sub>C<sub>2</sub>T<i><sub>x</sub></i> MXene and bilayer structures\nformed by partially overlapping monolayer\nflakes exhibit opposite sensing responses to a large scope of molecular\nanalytes. When exposed to reducing analytes, monolayer MXene flakes\nshow increased electrical conductivity, i.e., an n-type behavior,\nwhile bilayer structures become less conductive, exhibiting a p-type\nbehavior. On the contrary, both monolayers and bilayers show unidirectional\nsensing responses with increased resistivity when exposed to oxidizing\nanalytes. The sensing responses of Ti<sub>3</sub>C<sub>2</sub>T<i><sub>x</sub></i> monolayers and bilayers are dominated by entirely\ndifferent mechanisms. The sensing behavior of MXene monolayers is\ndictated by the charge transfer from adsorbed molecules and the response\ndirection is consistent with the donor/acceptor properties of the\nanalyte and the intrinsic n-type character of Ti<sub>3</sub>C<sub>2</sub>T<i><sub>x</sub></i>. In contrast, the bilayer MXene\nstructures always show the same response regardless of the donor/acceptor\ncharacter of the analyte, and the resistivity always increases because\nof the intercalation of molecules between the Ti<sub>3</sub>C<sub>2</sub>T<i><sub>x</sub></i> layers. This study explains\nthe sensing behavior of bulk MXene sensors based on multiflake assemblies,\nin which this intercalation mechanism results in universal increase\nin resistance that for many analytes is seemingly inconsistent with\nthe n-type character of the material. By scaling MXene sensors down\nfrom multiflake to single-flake level, we disentangled the charge\ntransfer and intercalation effects and unraveled their contributions.\nIn particular, we show that the charge transfer has a much faster\nkinetics than the intercalation process. Finally, we demonstrate that\nthe layer-dependent gas sensing properties of MXenes can be employed\nfor the design of sensor devices with enhanced molecular recognition.