Mesoporous Silica/conducting Polymers Composite Materials For Humidity Sensing

Sensitive material is the core of humidity sensor.Over the past decades,metal oxides,polyelectrolytes,graphene and its derivatives and various composite materials have been developed and appeared as a promising class for humidity sensing applications.Organic/inorganic composites possess the excellent properties of both monomers,overcoming the limited uses of the pure counterparts.Via the regulation of the chemical composition and the composites proportion,various humidity sensitive materials and humidity sensors have been prepared with high-performance properties.In this paper,two series of mesoporous silica/conductive polymer based humidity-sensitive materials were fabricated by one-step in-situ polymerization,leading to different composites with adjustable contents of conductive polymers.The humidity characteristics and the humidity sensing mechanism were investigated and discussed,of which big change of dielectric constant or super high porosity accommodates more water molecules and tailors the performance of these capacitive humidity sensors.The main results of this paper are shown as follows:1.The monomer pyrrole was prepared on the MCM-41 framework coated on the ceramic substrate under the condition of 0°C in the closed system.MCM-41/PPy(X,X=30,60,90,120)humidity sensors were prepared by in-situ growth and adjusting the polymerization time of pyrrole.The successful compositions of the two materials were verified by relevant characterization,such as FT-IR,XRD,SEM and TGA.The changes of PPy content and surface morphology of the composites at different polymerization time were analyzed.It was concluded that the content of PPy increased with the addition of polymerization time.The humidity sensitivities of MCM-41/PPy materials were tested and the performances were analyzed with the obtained humidity sensitivity curves,response-recovery characteristic curves and frequency characteristic curves.Compared with the monomer MCM-41 and PPy counterparts,the composites combine the advantages of the two monomer materials,which avoids the defects of low humidity insensitivity of MCM-41 and humidity saturation of PPy material,while maintain the high sensitivity and excellent linearity.However,the most obvious disadvantage of the material system is that the response-recovery time is relatively long(Response time: 915 s,recovery time: 100 s,11%-75%-11% RH).2.In order to improve the response-recovery characteristics of the above materials,we continued to construct a new material system,by in-situ polymerization of EDOT materials into MCM-41 estimation,and using the same means to control its content accordingly.Finally,MCM-41/PEDOT(X,X=30,60,90,120)humidity sensors were obtained,and their humidity characteristics were tested and characterized as well.The system has a much shorter response-recovery time without weakening the high sensitivity.Compared with the above system,the MCM-41/PEDOT humidity sensors exhibit significant enhancement,with the 165 s of response time and the 115 s of recovery time.(11%-95%-11% RH).To further analyze the humidity sensing mechanism of mesoporous silica/conductive polymer materials,we tested their dielectric loss spectra,complex impedance plots and the phase angel-frequency relationship of the composite humidity sensing materials.Conductive polymers contain a large number of ? bonds,which can improve the dielectric properties of silicon monomer and make it enough active sites at low humidity.The high specific surface area and porosity of mesoporous silica provides stable frameworks for conductive polymer materials,thus preventing the loss of polymers in high humidity.The composites contain the advantages of both organic and inorganic materials,able to serve as high performance capacitive humidity sensors in the whole humidity detection range.In this paper,through the above two aspects of work,a novel method for the development of capacitive humidity sensor is designed and successfully applied in humidity sensing field.Through the construction of composite materials,the sensor with full humidity range sensitivity can be fabricated,with high sensitivity and rapid response-recovery at the same time.