Graphite oxide (GO) is a precursor for large-scale synthesis of graphene-based materials and an intermediate for monolayer graphene oxide [1, 2]. Graphene and graphene oxide, with the theoretical surface area as high as 2600 m~2g~(-1), have great potential in the fields of catalysis, separation and gas storage if effectively converted into bulk materials. Graphene oxide processing such as drying often leads to layer stacking and, therefore, to reduced surface area in bulk materials [3]. In simple restacking processes, the surface area of the bulk material drops quickly as the number of individual sheets per stack increases. Therefore, most of the so-called graphene-based materials labeled as 'graphene' are actually reduced graphite oxide (rGO) platelets, with layer numbers of 10 and above. In the scientific community, it is well known that GO can be thermally unstable and should be regarded as an energetic material [4-7]. Due to GOs unique chemical nature (low C/O ratio, abundant epoxide functionalities) it may undergo self-heating and follow explosive mode thermal exfoliation. GO thermal exfoliation is highly exothermic, which for bulk samples above a critical size, can lead to significant heat accumulation and a thermal runaway reaction [7, 8]. Thermal decomposition of GO yields large volumes of gaseous products, typically from 40 up to 60 wt.-% of the original sample depending on the initial C:0 ratio [7]. If the self-heating event occurs spontaneously in an unexpected processing or storage step, it can lead to gas release, vessel overpressure or ignition of the volatile gases, which potentially can lead to a large-scale fire hazard [5, 7]. Most GO films or GO bulk powders have low surface area and available porosity. However, if properly engineered to increase the area and porosity, these materials could still offer excellent opportunities in gas storage, catalysis and selective membrane applications [3]. Non-explosive mode of thermal exfoliation of GO films seems to be desired to obtain high surface area product, since it offers time for the exfoliation product gases (mainly CO2 at temperatures below 300°C) to generate the fine porosity necessary for large surface area development prior to rGO complete disintegration [3]. In addition, thermal exfoliation of GO at low-to-moderate external heating rates (up to 50 Kmin~(-1)), is reported to increase the surface area and porosity [9]. In line with these results, it is reasonable to believe that very large external heating rates could theoretically help to attain very high rGO surface areas and large micro-, mesopore volumes for various applications. Herein, we systematically explore the influence of moderate and high external heating rates on rGO surface area and porosity during GO explosive mode thermal exfoliation.
展开▼
