Superior NO2 sensing via semiconductor–metal transition in undoped Hf2CO2 MXene: doping-mediated trade-offs between surface reactivity and electronic properties

Abstract

In this paper, we employ density functional theory (DFT) and non-equilibrium Green's function (NEGF) calculations to investigate the gas-sensing properties of monolayer Hf₂CO₂ MXene, focusing on intrinsic vacancy defects and carbon-site substitutional doping. Among tested gases, NO₂ exhibits exceptional sensitivity due to strong adsorption energy and significant charge transfer. Boron doping at carbon sites enhances charge transfer and adsorption strength through d-band center upshift (−0.349 eV → −0.076 eV) and induces metallic conductivity that diminishes current modulation sensitivity. By analyzing current–voltage (I–V) relationships for pristine and B-doped Hf₂CO₂ with/without NO₂ adsorption, we demonstrate that the pristine Hf₂CO₂ monolayer exhibits superior gas-sensing performance, achieving a two-order-of-magnitude current surge upon NO₂ adsorption compared to doped systems. This study reveals the critical trade-off between doping-induced adsorption enhancement and conductivity-driven sensitivity loss, providing a design framework for MXene-based sensors through strategic optimization of surface reactivity and electronic response.