After a suitable ultrasonic treatment, such exfoliation can produce stable dispersions of very thin graphene oxide sheets in water

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The ultrasonic method and high shear method were used to exfoliate the as-prepared graphite oxide. In typical ultrasonication procedure, 25 ml as-prepared graphite oxide suspension (1 mg/ml) was loaded in a 30-ml glass beaker, and then sonicated by an ultrasonic bath cleaner (100 W, KX-1620, Beijing Kexi technology Co. Ltd., China) for 1, 2, 3 and 4 h (hereon denoted as u1, u2, u3, u4). In order to control the temperature of the water, the water was changed every half hour. In high shear exfoliation procedure, 350 ml as-prepared graphite oxide suspension (1 mg/ml) was loaded in a 500-ml double-walled glass beaker, which was outside cooled by ice water. The dispersion was then treated by AE300S-H high-speed shear mixer (Angni Instruments, China) at the speed of 10,000 rpm for 1, 2, 3 and 4 h (sampled as h1, h2, h3, h4), which are shown in Figure S1. In experiment, 20 ml GO samples were collected by straw every hour from the beaker. All samples were centrifuged at 4000 rpm for 1 h in L600-type centrifuge (Xiangyi Instruments Factory of Hunan, China). After centrifugation, the supernatant was carefully removed and retained for further characterisation.

Reference : Article Facile and size-controllable preparation of graphene oxide n...

You need to optimize the amplitude given the other operating parameters governing the ultrasonication process. You may be required to undertake more than one runs by varying various factors to get a desired since range of the particles of GO.