Multi-functional flexible sensor based on composite of polyimide aerogel and TiO2 nanoparticles with applications in humidity sensing, water sensing in liquid and photocatalysis

Fast development of flexible electronics has stimulated great interests in researches on high performance multi-functional flexible sensors. Herein, we report a flexible sensor based on the composite of polyimide (PI) aerogel and TiO2 nanoparticles with multiplex applications in humidity sensing, detection of water content in liquids and photocatalytic degradation. Specifically, PI aerogel with 3,3 ',4,4 '-biphenyltetracarboxylic dianhydride and 4,4 '-diaminodiphenyl ether as the monomers was deposited with TiO2 nanoparticles, and then covered by reduced graphene oxide as top and bottom electrodes to fabricate a self-standing flexible capacitive humidity sensor (FCHS). The flexible sensor exhibited linear change in capacitance on a semi-logarithmic scale from similar to 10 pF to similar to 10000 pF between relative humidity of 11-98 %, showing ultrahigh sensitivity. Moreover, it showed excellent deformation stability (humidity response changed little under bending to similar to 180 degrees for 200 times), and successfully measured the respiration process and monitored tiny movement of fingertip in non-contact way. Further, the flexible sensor could detect water content (ppm level) in polar ethanol and non-polar diesel with good sensitivity. Particularly, the FCHS could photodegrade dyes (methyl orange, orange II, Sulforhodamine B) and diesel under ultraviolet illumination, demonstrating desirable feature of self-cleaning from contaminations. The work offers valuable references for designing multi-functional flexible opto/electronic devices.