Loes Michael J, Bagheri Saman, Sinitskii Alexander
Department of Chemistry and Nebraska Center for Materials and Nanoscience, University of Nebraska-Lincoln, Lincoln, Nebraska 68588, United States.
ACS Nano. 2024 Sep 13. doi: 10.1021/acsnano.4c08225.
Monolayers of TiCT MXene and bilayer structures formed by partially overlapping monolayer flakes exhibit opposite sensing responses to a large scope of molecular analytes. When exposed to reducing analytes, monolayer MXene flakes show increased electrical conductivity, i.e., an n-type behavior, while bilayer structures become less conductive, exhibiting a p-type behavior. On the contrary, both monolayers and bilayers show unidirectional sensing responses with increased resistivity when exposed to oxidizing analytes. The sensing responses of TiCT monolayers and bilayers are dominated by entirely different mechanisms. The sensing behavior of MXene monolayers is dictated by the charge transfer from adsorbed molecules and the response direction is consistent with the donor/acceptor properties of the analyte and the intrinsic n-type character of TiCT. In contrast, the bilayer MXene structures always show the same response regardless of the donor/acceptor character of the analyte, and the resistivity always increases because of the intercalation of molecules between the TiCT layers. This study explains the sensing behavior of bulk MXene sensors based on multiflake assemblies, in which this intercalation mechanism results in universal increase in resistance that for many analytes is seemingly inconsistent with the n-type character of the material. By scaling MXene sensors down from multiflake to single-flake level, we disentangled the charge transfer and intercalation effects and unraveled their contributions. In particular, we show that the charge transfer has a much faster kinetics than the intercalation process. Finally, we demonstrate that the layer-dependent gas sensing properties of MXenes can be employed for the design of sensor devices with enhanced molecular recognition.
TiCT MXene的单层以及由部分重叠的单层薄片形成的双层结构，对多种分子分析物表现出相反的传感响应。当暴露于还原性分析物时，单层MXene薄片显示出电导率增加，即n型行为，而双层结构的导电性降低，表现出p型行为。相反，当暴露于氧化性分析物时，单层和双层都显示出电阻率增加的单向传感响应。TiCT单层和双层的传感响应由完全不同的机制主导。MXene单层的传感行为由吸附分子的电荷转移决定，响应方向与分析物的供体/受体性质以及TiCT的固有n型特性一致。相比之下，双层MXene结构无论分析物的供体/受体特性如何，总是显示相同的响应，并且由于分子插入TiCT层之间，电阻率总是增加。这项研究解释了基于多薄片组件的块状MXene传感器的传感行为，其中这种插入机制导致电阻普遍增加，对于许多分析物来说，这似乎与材料的n型特性不一致。通过将MXene传感器从多薄片级别缩小到单薄片级别，我们解开了电荷转移和插入效应，并揭示了它们的贡献。特别是，我们表明电荷转移的动力学比插入过程快得多。最后，我们证明MXenes的层依赖性气敏特性可用于设计具有增强分子识别能力的传感器器件。
