Poly Aniline Nanofibers Prepared By Free Flowing Polymerization And Their Gas Sensing Properties

Owing to its high surface area,high carrier mobility as well as reversible oxidation/reduction chemistry,polyaniline nanofiber has great potential applications in the sensing material.Among synthetic methods nowadays,compared with template method,free-template method could avoid complex post-treatment for removing the template and high risk of damages in polyaniline nanostructure,however,relative dilute concentration of aniline monomer was generally used,and moreover,it is not easy to control the growth of polyaniline nanofibers.In this work,a free flowing polymerization was proposed,in which the oxidation of aniline monomers was confined in tubular space,and the orientation of nanofibers along the flowing direction would be improved.Our main results and conclusions are listed as follows:(1)Optimization of free flowing polymerization:Aniline solution and initiator solution were introduced into a polytetrafluoroethene microtube,and then chemical oxidation of aniline monomers would take place along free flowing direction,leading to the formation of polyaniline nanofibers.The effects of reaction parameters such as concentration of aniline monomers,nature of organic solvents in collecting container,reaction time,reaction tube length and diameter,altitude intercept and ratio of aniline to oxidant on the oxidation polymerization of aniline monomers were discussed.Monitoring the process of the reaction through ultraviolet and visible spectroscopy and observing the morphology of the product by field emission scanning electron microscopy.Furthermore,reaction parameters for free flowing polymerization were optimized to obtain large yield,high conductivity and narrow band gap of as-prepared polyaniline nanofibers.It is found that the product are most nanofibers,the tube length,diameter and altitude intercept have larger influences on the formation of polyaniline nanofibers.With the increasing of the concentration of aniline monomers,the average length of nanofibers,conductivity and yield increased.The polymerization of aniline monomers could be restrained to some extent due to the presence of organic solvent such as acetone in collecting container.Relative longer reaction time was benefited to obtain long polyaniline nanofibers with high conductivity.(2)Structure and properties of polyaniline nanofibers:Chemical structure,crystallization property and thermal property of as-prepared polyaniline were investigated by Fourier Transform infrared spectroscopy,laser micro-Raman spectroscopy,X-ray diffraction and thermo-gravimetric analysis,respectively.It is found that in most cases,polyaniline nanofibers possessed bipolarons structure,and very few in the fully oxidized form.Moreover,with the increasing of the concentration of aniline monomers,the conjugation length of polyaniline nanofibers with more complete structure increased,whereas high thermal stability at low concentration.When the diameter of reaction tube reduced,that is to say,the space for polyaniline polymerization was restricted,the orientation of polyaniline nanofibers along free flowing direction could be improved,but the doping process of polyaniline was hindered.Longer tube length and appropriately increasing the reaction time were good to obtain polyaniline nanofibers with more complete structure and higher regularity.In addition,under different altitude intercepts,the reaction processes were different.It is demonstrated that more complete structure would form for relative low altitude intercept,while the flowing was interupted.By contrast,at high altitude intercept,the low crystalline polyaniline formed.(3)Gas sensing behaviors of polyaniline nanofibers:Polyaniline nanofibers were deposited on Au interdigital electrode,and their gas sensing behaviors for different organic vapors.It is found that polyaniline nanofibers can response for a varity of vapors suctah as formaldehyde,chloroform,acetone,methanol,benzene and ammonia,the chemical resistance would change,and the response ratio to ammonia was far larger than the other vapors.More attention was paid to the sensing properties for ammonia.The response time,recover time,cycle stability,and repeatability were evaluated.Moreover,the response mechanism of the polyaniline nanofibers to ammonia was presented.In summary,a simple and effective method for preparing polyaniline nanofibers was proposed,in which polyaniline polymerization occurred in aqueous solution without any templates and destructive post-processing.The structure and properties can be controlled by adjusting the reaction tube length,diameter and reaction time.More importantly,these polyaniline nanofibers exhibited great potential for application as sensing materials for volatile organic compounds.