Synthesis Of Bis-azobenzene/Graphene Hybrid Materials For Solar Thermal Fuels

Azobenzene molecules could undergo trans?cis isomerization,which store the solar energy in the chamical bonds of metastable cis-isomer.O nce triggered by an external power,the metastable cis-isomer azobenzene will reverts to stable trans-isomerization,which release the energy as heat accomplicating the cycle progress.The photoresponsibility,excellent stablity and structural diversity of azobenzene are important fundmental for synthesizing solar thermal storage materials.However,the conditional zobenzene derivatives have some defacts such as short half-life time(?1/2)and low isomerization enthalpy(?H)limiting its application for solar energy strorage.In this paper,bis-Azo was prepared based on molecular design.Moreover,bis-Azo was grafted to reduced graphene oxide(RGO)surface which possessed both certain conjugated structure and good dispersion via nucleophilic substitution reaction obtaining high energy desity,long half-life time and controlling RGO-bis-Azo composite.Subsequently,the influences of intra-and/or intermolecular steric hindrance of bis-Azo covalently bound to RGO on improving the solar thermal storage stability and energy density were studied.TEM images revealed the bis-Azo conformed dense organic layer on the surface of RGO indicating high high-density grafting.The results of thermogravimetric analysis(TGA)and X-ray photoelectron spectroscopy(XPS)displayed that approximately every 50-52 carbon atoms on the RGO covalently bound with one bis-AZO molecule in average.High high-density grafting and packing interaction strengthen intra-and/or intermolecular interaction and steric hindrance.The UV-Vis spectra showed the steric hindrance improved the thermal stability of cis-isomer of bis-Azo covalently bound to RGO with half-life time of 1320 h which increased by one order of magnitude.The reversion of bis-Azo could be accelerated under the irradiation of blue light(450 nm)with a excellent cycling perfomance.The energy density of RGO-bis-Azo reaches 80 Wh/kg with an increasing of 2.5 times which was in same order of magnitude with DFT calculation.At the same time,RGO-bis-Azo displayed good heat release rate character which the power density reached 2259 W/kg.The results showed that multi-branch azobenzene covalently grafted to graphene composites are ideal materials for high energy,long-term and controlling solar heat storage materials by optimizing the interactions and steric hindrance between molecules.