Graphene Sponges Prepared from Clusters of Soap Bubbles with Hierarchical Pores
Authors: Zhang Rujing 
Published: February 2020

Graphene Sponges Prepared from Clusters of Soap Bubbles with Hierarchical Pores
Rujing Zhang, Yachang Cao, Hongwei Zhu
School of Materials Science and Engineering, Key Laboratory of Materials Processing Technology, Tsinghua University, Beijing 100084, P. R. China
Inspired by the light weight and porosity of the foams of soap bubbles, three-dimensional graphene sponges were fabricated using clusters of soap bubbles as templates. The structure was prepared by simply adding the dispersion of graphene oxide (GO) into a cluster of soap bubbles and stirring the mixture evenly, with subsequent vacuum freeze-drying and thermal annealing. Mass production can be realized since the synthesis process was simple and the raw material was easy to get.
Typical hierarchical structure of the thermal-annealled GO (trGO) sponge was obtained with the stirring rate of 250 r min-1 and frozen by liquid nitrogen. SEM images showed the structure was ordered quasi-round pores with diameters as large as several hundred microns, having smaller slot pores on the walls. TEM characterization of the materials showed small pores in nanometer scale, with the diameters ranging from several nanometers to dozens of nanometers. Since the stirring rate in soap bubbles’ production could influence the volume of bubbles, and the freezing rate of bubbles after adding GO dispersion could affect the assembly of GO sheets, the finally obtained structure could be tuned by changing the stirring rate or freezing rate.With high sirring rates of 5000 r min-1 and 20000 r min-1, the obtained structure had no visible large ordered pores due to the collapse of thinner walls in bubbles with smaller volume. Spongy materials prepared with freezing medium as refrigerator were also fabricated, showing disordered pores with smooth walls, resulting from the rearrangement of GO sheets in the process of slow freezing. The thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS) were also conducted to character the residual impurities from the soap after thermal annealing.
In the compression tests, trGO sponge prepared with the stirring rate of 250 r min-1 and frozen by refrigerator could recover completely under 30%-70% compression strain, revealing the ultra-flexibility and softness of the materials. Under cyclic compression, the stress-strain curves didn’t change obviously in 100 cycles. For materials frozen rapidly by liquid nitrogen, the compressive strain was bigger compared with trGO sponges prepared with refrigerator, since the collapse of relatively ordered assembly of graphene sheets had made the structure more compact. The spongy materials could absorb both water and oil, partly because of the visible large pores, and partly because of the residual oxygen-containing groups. The specific capacitance of the trGO sponge was measured as 15.8 F g-1, relatively lower than those three-dimensional graphene-based structures that have been reported, mainly due to the large amount of visible pores with the diameters as large as several hundred microns increasing the benefit surface area much less than pores in nanoscale. The ultra-light, super-flexible three-dimensional graphene sponge, with hierarchical ordered pores with the diameters ranging from several nanometers to hundreds of microns, showed potential applications in stroage media, absorbents, thermal insulation, sound insulation, etc.
Figure (a) Optical image of laundry detergent bubbles.(b) Graphene sponge prepared with stirring rate of 250 r min-1 and frozen with liquid nitrogen