Percolated carbon nanotube networks for high-speed humidity sensors.

Improving the response time for polyimide (PI)-based capacitive humidity sensors is critical for real-time sensing. Multi-walled carbon nanotube (MWCNT) films were used to form the upper electrode of humidity sensor to realize an extremely short response times. MWCNT films were spray-deposited on a moisture-sensitive PI layer and subsequently patterned by oxygen plasma. Random-network MWCNT electrodes have a well-entangled and open porous structure that is almost impossible to obtain with conventional metal electrodes. Compared with porous metal electrode-based sensors as an upper electrode, the fabricated capacitive humidity sensors with MWCNT electrodes showed an exceptionally short response time of less than 2.5 s and a good linearity of 0.998. An analysis of the long-term (100 days) stability data revealed that the MWCNT electrode humidity sensors showed little drift even after 100 days aging, indicating that they are suitable for practical and reliable humidity measurements. These improvements in performance may stem from their interconnected microscopic porous structure, which is more accessible to water molecules through the conductive electrode.