The role of surfactant (Pluronic (R) F 127) concentration on the yield and morphological characteristics of graphene nanoplatelets (GNPs) produced from the sonication of aqueous graphene suspensions is investigated in this work. By employing a wide surfactant concentration range (0.1-15 wt%) and sonication power densities up to 420 W L-1 we identify two graphene exfoliation regimes: the first occurs at low sonication power densities (<340 W L-1) and produces GNPs with sizes 200-300 nm, aspect ratios between 70 and 100, and concentrations up 1 mg mL(-1). In that regime, the surfactant concentration has no effect on the exfoliation results. In the second exfoliation regime (>340 W L-1), surfactant concentrations greater than 10 wt% produce dramatic increases in GNP yields, namely up to 3.0 mg mL(-1), and overall larger GNPs (350-500 nm) with smaller aspect ratios (5-60). We attribute these changes to the onset of a more energy intensive mechanism, termed cleavage. Cleavage involves the separation of graphite clusters in sub-bulk multi-layered graphene entities, as opposed to exfoliation, which involves the separation of individual or few-layer GNPs. Choosing an exfoliation regime by tuning simple process parameters enables control over the yield, size and morphology of the produced GNPs. (C) 2018 Elsevier Inc. All rights reserved.