Tuning pyrolyzing temperature to switch electrorheological effect from negative to positive in MIL-125(Ti)-derived carbon/TiO2 composite particles: Crucial role of interfacial polarization

In this study, carbon/TiO2 composite particles were prepared by pyrolyzing metal-organic framework (MOF) particles. The structure and conductivity of composite particles was characterized and the electrorheological (ER) effect of composite particles dispersed in silicone oil was measured under electric fields. It is interesting that the composite particle suspensions exhibit a tunable electro-response from negative to positive ER effect depending on pyrolysis temperatures. The suspension containing particles produced at 400 °C shows abnormal negative ER effect, which can be explained by the conduction model because 400 °C-pyrolysis destroys the bonds between organic linkers and Ti—O clusters to yield composite with lower conductivity than silicone oil. However, the suspension containing particles produced at 440 °C still shows negative ER effect, which cannot be explained by the conduction model because the conductivity of composite exceeds that of silicone oil. Through microscopic observation and dielectric spectra analysis, we clarified that the absence of available interfacial polarization in the suspensions of particles produced at lower pyrolysis temperature than 440 °C is the real reason of negative ER effect. Increasing pyrolysis temperatures to 480 and 520 °C improves the carbonization level and conductivity of composites, leading to large interfacial polarization and positive ER effect of corresponding suspensions. The suspension containing particles produced at 520 °C exhibits the optimal positive ER effect. This work demonstrates the crucial role of interfacial polarization in determining positive or negative ER effect through using MOF-derived carbonaceous composites with tunable structure and electric properties.