This study presents a method to precisely control the thickness and functionalization of Ti3C2Tx MXene flakes to enhance their electrical properties, environmental stability, and gas-sensing performance. A hybrid method involving high-pressure processing, stirring, and immiscible solutions was used to achieve sub-100 nm MXene flake thickness on a Si wafer. Functionalization was controlled by defunctionalizing MXene at 650°C under vacuum and H2 gas in a CVD furnace, followed by refunctionalization with iodine and bromine vaporization from a bubbler attached to the CVD. The introduction of iodine, with a larger atomic size, lower electronegativity, and reduced shielding effect, significantly improved the surface area, oxidation stability, and film conductivity of MXene. It also enhanced the gas-sensing performance, including sensitivity, response, and response/recovery times. The hydrophobicity, larger atomic size, lower electronegativity, and reduced shielding of -I contributed to the excellent sensing enhancement of I-MXene. The study highlights the importance of terminal functionalization in enhancing the gas-sensing performance of MXenes, providing valuable insights for the development of highly sensitive and faster-responsive gas sensors. The results demonstrate that I-MXene exhibits outstanding stability and performance in both aqueous and ambient environments, making it a promising material for future gas-sensing applications. The study also addresses challenges in MXene synthesis, including streamlining the etching process using a Teflon-based high-pressure reactor instead of stainless steel. Future studies aim to investigate various surface terminations on different carbide-based MXenes to gain insights into the influence of different functional groups and MXene materials on their electrical, environmental, and gas sensing properties. The proposed methods hold promise for application to other MXene structures and compositions, offering versatility and the potential for further advancements in MXene research across various fields.This study presents a method to precisely control the thickness and functionalization of Ti3C2Tx MXene flakes to enhance their electrical properties, environmental stability, and gas-sensing performance. A hybrid method involving high-pressure processing, stirring, and immiscible solutions was used to achieve sub-100 nm MXene flake thickness on a Si wafer. Functionalization was controlled by defunctionalizing MXene at 650°C under vacuum and H2 gas in a CVD furnace, followed by refunctionalization with iodine and bromine vaporization from a bubbler attached to the CVD. The introduction of iodine, with a larger atomic size, lower electronegativity, and reduced shielding effect, significantly improved the surface area, oxidation stability, and film conductivity of MXene. It also enhanced the gas-sensing performance, including sensitivity, response, and response/recovery times. The hydrophobicity, larger atomic size, lower electronegativity, and reduced shielding of -I contributed to the excellent sensing enhancement of I-MXene. The study highlights the importance of terminal functionalization in enhancing the gas-sensing performance of MXenes, providing valuable insights for the development of highly sensitive and faster-responsive gas sensors. The results demonstrate that I-MXene exhibits outstanding stability and performance in both aqueous and ambient environments, making it a promising material for future gas-sensing applications. The study also addresses challenges in MXene synthesis, including streamlining the etching process using a Teflon-based high-pressure reactor instead of stainless steel. Future studies aim to investigate various surface terminations on different carbide-based MXenes to gain insights into the influence of different functional groups and MXene materials on their electrical, environmental, and gas sensing properties. The proposed methods hold promise for application to other MXene structures and compositions, offering versatility and the potential for further advancements in MXene research across various fields.