Higher pressures load the ultrasound tool too much, and the ultrasonic generator begins its inevitable falling out of resonance and its power decreases. A liquid denser than water (ethylene glycol, glycerol, etc.) also leads to a higher output power, thanks to a higher cavitation threshold. When the liquid is exposed to intense ultrasound, the waves propagate
through the liquid causing an alternating of high-pressure and low-pressure cycles that is dependent on the frequency of the electric generator. During the low-pressure cycle, high-intensity RAD001 small vacuum bubbles are created, as the liquid vapor pressure is achieved. When the bubbles reach a certain size, they collapse strongly during a high-pressure cycle. During this implosion, very high pressures, high temperatures, and speed liquid jets are locally generated. This phenomenon is called
cavitation [23]. The resulting hydrodynamic GKT137831 forces are able to disintegrate agglomerates and to mill particles in solution. The ultrasonic vibrations are transferred into an elastic RO4929097 chemical structure environment by spreading the longitudinal or transverse waves. Transverse waves cannot propagate in a gas or a liquid because there is no mechanism for driving the motion perpendicular to the propagation of the wave; thus, they are transformed into standing (stationary) waves by the ultrasonic horn. Stationary waves are able to vibrate lamellar particles, using the vibration to overcome van der Waals forces. As a result, lamellar particles are gradually peeled off to reveal individual sheets. The particle milling effect is based
on intense ultrasonic cavitation, while delamination is caused by stationary waves. Increasing the density of the solvent or/and increasing the pressure of the solvent will also increase the cavitation threshold [24, 25]. Through the selection of suitable reaction conditions and factors (sonotrode shape, intensity of ultrasound, solvent density, pressure, etc.), it is then possible to favor the process of delamination over grinding and milling. Delamination of layered minerals [26] by ultrasound was successfully used for the preparation of exfoliated mica Niclosamide [27] and kaoline [28] under atmospheric pressure. Pressurized batch ultrasonic reactors were also used to exfoliate graphite to graphene [29], which then served as the precursor for the composite materials of graphene-anatase [30] and graphene oxide-anatase [31]. It can then be theorized that the exfoliation of IAGs using power ultrasound in an environment of strong polar aprotic solvents in a pressurized batch reactor could be achieved through this procedure. In this paper, we demonstrate simple and low-cost methods for the preparation of single- and few-layered nanosheets of inorganic analogues of graphene, MoS2, WS2, h-BN, h-BCN, and g-C3N4, using stationary ultrasound waves in a pressurized ultrasonic reactor.