Structural Engineering of Al-Enriched Ti₃C₂Tₓ MXene for Enhanced NO₂ Gas Sensing

Abstract

Nitrogen dioxide (NO₂) is a hazardous air pollutant generated primarily from industrial processes and vehicle emissions. The development of highly sensitive gas sensors operating at room temperature is essential for environmental monitoring and public health protection. Ti₃C₂Tₓ MXene has emerged as a promising sensing material due to its high electrical conductivity, large specific surface area, and tunable surface chemistry. However, the influence of precursor stoichiometry on the structural and sensing properties of Ti₃C₂Tₓ remains largely unexplored [1,2]. Ti₃C₂Tₓ MXenes were synthesized from Ti₃C₂Tₓ MAX-phase precursors prepared with different aluminum contents: stoichiometric composition (MX-EST) and compositions containing 10 at.% (MX-10) and 20 at.% (MX-20) excess Al. Structural and chemical characterization was performed using X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). NO₂ gas-sensing performance was evaluated under ambient conditions by measuring sensitivity, response/recovery times, and detection limits [2,3]. Structural analyses revealed that Al enrichment modifies the MXene morphology and surface chemistry. XRD and Raman results indicated increased interlayer spacing and enhanced surface functionalization in Al-enriched samples. SEM observations showed the formation of larger and thinner flakes, particularly for MX-20. XPS confirmed the presence of –O, –OH, and –F surface terminations and demonstrated that MX-10 possesses an optimal balance between the carbide core and surface functional groups. Gas-sensing measurements showed that MX-10 exhibited the best performance, achieving a tenfold increase in sensitivity at 50 ppm NO₂ compared with MX-EST, with response and recovery times of 55 and 72 s, respectively, and a detection limit of approximately 1.8 ppm [3–5]. These results demonstrate that precursor stoichiometry provides an effective strategy for tuning the structural and surface properties of Ti₃C₂Tₓ MXenes. Moderate Al enrichment significantly enhances NO₂ sensing performance, offering a simple and scalable route toward high-performance, low-power gas sensors for environmental monitoring.

Keywords: Ti₃C₂Tₓ MXene; NO₂ gas sensing; Structural engineering