Ti3C2 MXene has emerged as a promising material for chemiresistive ethanol sensing due to its high surface area, tunable surface chemistry, and excellent electrical conductivity. The sensing performance is strongly influenced by surface terminations such as oxygen (-O) and fluorine (-F), which modify the interaction between the MXene and ethanol molecules. These terminations alter the charge transfer dynamics and adsorption behavior, directly impacting sensitivity, selectivity, and stability.

The chemiresistive response of Ti3C2 MXene arises from changes in electrical resistance upon ethanol adsorption. Ethanol molecules interact with the MXene surface, donating electrons and increasing the material's conductivity. The presence of -O terminations enhances this effect due to their electron-withdrawing nature, which facilitates stronger ethanol adsorption. In contrast, -F terminations reduce sensitivity because fluorine atoms passivate active sites, limiting ethanol interactions. Studies have shown that Ti3C2Tx (where Tx represents mixed -O and -F terminations) with a higher -O/-F ratio exhibits superior ethanol response, with resistance changes exceeding 20% at concentrations as low as 50 ppm.

Selectivity against other alcohols, such as methanol and isopropanol, is a critical factor for practical applications. Ti3C2 MXene demonstrates preferential ethanol detection due to differences in molecular polarity and steric effects. Ethanol's intermediate polarity allows optimal interaction with the MXene surface, whereas methanol, being more polar, competes with ambient moisture, reducing its adsorption. Isopropanol, with its bulkier structure, experiences weaker van der Waals interactions, leading to lower sensitivity. Experimental results indicate that the response to ethanol is at least 1.5 times higher than methanol and 2 times higher than isopropanol under identical conditions.

Humidity stability remains a challenge for MXene-based sensors. Ti3C2 is prone to oxidation and structural degradation in humid environments, which can deteriorate sensor performance over time. However, controlled surface terminations mitigate this issue. -O-rich surfaces form a more stable interface with water molecules, reducing unwanted side reactions. In contrast, excessive -F terminations lead to hydrophobicity but also reduce active sites for ethanol detection. Optimized termination ratios achieve a balance, with sensors retaining over 80% of their initial response after prolonged exposure to 60% relative humidity.

Long-term stability is further improved by encapsulation strategies and operating temperature modulation. Mild heating (50–100°C) minimizes water adsorption while preserving ethanol sensitivity. Additionally, annealing treatments partially convert -F to -O terminations, enhancing both stability and responsiveness. These modifications extend operational lifetimes beyond six months without significant performance degradation.

The sensing mechanism involves a combination of physisorption and charge transfer. Ethanol molecules adsorb onto the MXene surface, where hydroxyl groups interact with surface terminations. This interaction induces electron donation into the MXene, increasing carrier concentration and reducing resistance. The process is reversible, with desorption occurring rapidly upon ethanol removal, enabling real-time monitoring. The response time typically ranges between 10–30 seconds, while recovery completes within 20–50 seconds, depending on termination composition and ambient conditions.

Interference from common volatile organic compounds (VOCs) such as acetone and formaldehyde is minimal due to differing adsorption energies. Acetone, for instance, interacts weakly with Ti3C2, resulting in negligible resistance changes below 100 ppm concentrations. Formaldehyde’s higher reactivity does not significantly overlap with ethanol’s detection window, allowing selective identification in mixed environments. Cross-sensitivity tests confirm that ethanol signals remain distinguishable even with interfering species present at equal concentrations.

Future developments may focus on termination engineering via post-synthesis treatments to fine-tune selectivity and stability. Plasma processing and chemical functionalization offer pathways to tailor surface groups precisely. Hybrid structures incorporating polymers or metal oxides could further enhance ethanol specificity while mitigating humidity effects.

In summary, Ti3C2 MXene ethanol sensors leverage surface terminations to achieve high sensitivity and selectivity. Oxygen-rich surfaces optimize performance, while fluorine terminations trade some sensitivity for stability. The material’s inherent conductivity and tunable chemistry make it a strong candidate for reliable ethanol detection in complex environments, provided humidity and long-term degradation are managed through careful material design.