Piezoelectricity induced water splitting and formation of hydroxyl radical from active edge sites of MoS2 nanoflowers

We have demonstrated that the hydroxyl radicals (OH•) and hydrogen gas can be generated by the application of mechanical force to the active edge sites of the MoS2 nanoflowers in a dark environment. The non-centrosymmetric structure of the single- and few-layered MoS2 nanoflowers were uniformly dispensed in the aqueous solution. A mechanical force was applied to the aqueous solution, and the hydroxyl radicals were generated from the MoS2 nanoflowers because a piezoelectric spontaneous polarization was created around the single- and odd number of layers. To exclude the dye adsorption effect during the decomposition process, the MoS2 nanoflowers with 99.9 wt% were removed from the solution by the centrifuged process to obtain the solution without the nanoflowers. Interestingly, the solution can decompose the organic dye (i.e. Rhodamine B) instantly (see the video recording in the Supporting Information). Based on the fluorescence (FL) spectra, the solution contained with the highly oxidized free radicals, which was strongly dependent on the concentration of the MoS2 nanoflowers in the aqueous solution. The formation of the M- (VMo′′′′) and S-vacancies (VS••) sites on the single-layer MoS2 acted as the F-center defects, leading to the hydroxyl radicals being accommodated on the sites to decompose the organic dye molecules and prolong the radical's lifetime up to 6 h. More importantly, with applying the ultrasonic vibration to the MoS2 nanoflowers in the deionized water and ethanol solution, a remarkable hydrogen peak was observed by the gas chromatography mass spectrometry. Moreover, as increasing the concentration of the MoS2 nanoflowers in the solution, the intensity of the hydrogen generation was increased. This is the first work to demonstrate that the dye molecule can be instantly decomposed by the free radicals solution and produced the hydrogen gas using the piezoelectricity of the MoS2 nanoflowers with the application of mechanical force in a dark environment.