Versatile Preparation of Monodisperse Carbon Microspheres In a Simple Microfluidic Device

^{Versatile Preparation of Monodisperse Carbon Microspheres in a Simple Microfluidic Device}

Yichang Pan, Minhua Ju, Ya Liu, Chongqing Wang, Lixiong Zhang,* and Nanping Xu

State Key Laboratory of Materials-Oriented Chemical Engineering and College of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing 210009, P. R. China. Fax: 86 25 8317 2263; Tel: 86 25 8317 2265; E-mail: lixiongzhang@yahoo.com

Microspheres composed of polymeric or inorganic materials are of great interests because of their potential application as catalyst supports, carriers for cosmetics, drugs, protein and DNA delivery, electrodes, sensors and so on. They are prepared by various methods such as emulsion/micelles/vesicle polymerization, spray drying, template methods, and self¨Ctemplate method. Recently, a new technique was developed to synthesize uniform-sized hollow microspheres such as nylon, biopolymer, organosilicon and TiO₂ using microfluidic emulsion technique via interfacial polymerization at the surface of the microdroplet.

In this report, we fabricated hollow and solid poly(furfuryl alcohol) (PFA) microspheres with diameters in the range from tens to several hundreds of micrometers by the phase transfer of FA and polymerization in the aqueous droplets using microfluidic devices. We fed an oil solution containing FA, fatty acid methyl ester, and a surfactant (span 80) and an sulfuric acid aqueous solution separately into the microfluidic devices to form an aqueous solution microdroplets dispersed in the oil phase. FA in the oil phase would diffuse into the droplets, and consequently start to polymerize at the catalysis of sulfuric acid. Thus, a layer of PFA would be formed at the surface of aqueous droplet, resulting in formation of PFA microcapsules containing aqueous solution. The PFA microcapsules in-turn could be transformed to microporous carbon hollow microspheres by carbonization. Solid PFA microspheres could also be prepared by prolonging the reaction time.

We first prepared PFA microspheres with diameters of several hundreds of micrometers in a microfluidic device assembled by inserting a syringe needle perpendicularly into a polyvinyl chloride (PVC) tube. By changing the flow rates of oil and aqueous solutions, residence time of aqueous microdroplets in the fluidic device, FA concentration in oil solution and curing temperature, the particle size and wall thickness of PFA hollow microspheres can be easily adjusted. In addition, a magnetic precursor could be simply incorporated in PFA microspheres, leading to formation of magnetic carbon hollow microspheres. By introducing silica sols in the aqueous phase, PFA-SiO₂ composite microspheres were obtained, which were then converted into carbon-silica composite microspheres after carbonization and finally the carbon microspheres after HF etching. The resultant carbon microspheres were mesoporous because in-situ polymerized silica gel networks served as hard template, and their mesoporosity depended on the flow rate of the oil phase and the residence time of the microdroplets in the device during the preparation of PFA-SiO₂ microspheres. The highest mesoporous volume and mesoporosity of carbon microspheres reached 1.06 cm³.g^-1 and 82 %, respectively, and the mesopores were in the range of 2~20 nm. Mesoporous silica microspheres embeded with submicron-sized cavities could also be prepared by burning-off the carbon in the PFA- SiO₂ microspheres.