Critical parameters in exfoliating graphite into graphene

Dispersing graphite into few-layers graphene sheets (GS) in water is very appealing as an environmental-friendly, low-cost, low-energy method of obtaining graphene. Very high GS concentrations in water (0.7 mg mL(-1)) were obtained by optimizing the nature of dispersant and the type of ultra-sonic generator. We find that a multi-step sonication procedure involving both tip and bath sources considerably enhances the yield of exfoliated GS. Raman and transmission electron microscopy indicate few-layers graphene patches with typical size of similar to 0.65 mu m in one dimension and similar to 0.35 mu m in the other. These were further employed in combination with water-dispersed CNTs to fabricate conductive transparent electrodes for a molecularly-controlled solar-cell with an open-circuit voltage of 0.53 V.