(Invited) Piezocatalytic Degradation of Dye Molecules, Water Splitting, and Formation of Hydroxyl Radical through Piezoelectricity of MoS2 Nanoflowers
Authors: Sun Yu-Guang , Lee Jyun-Ting , Wu Jyh-Ming 
Published: February 2020

Two-dimensional (2D) materials such as MoS2, MoSe2, WS2, WSe2, etc, have received a greater amount of attention in a variety of applications including field-effect transistors, hydrogen production, nanogenerator and optoelectronic devices, and degradation of organic pollutants. Recently, the environmental issues including water and air pollution, and industrial and hazardous waste, are considerable and becoming a serious problem for many countries. The catalyst materials have provided a promising solution to suppress the environmental pollution from deteriorating. The high-performance catalysts are the prerequisites to meet the following aspect points such as eco-friendly processing, low investment costs, and remarkable catalytic activity. Therefore, many researchers have been engaged in developing highly efficient catalysts materials (i.e. photocatalysis) through a variety of technologies, including hybrid nanostructures, surface/interface engineering, and novel catalytic behavior. This work, we have demonstrated that the hydroxyl radicals (OH•) and hydrogen gas can be generated by the application of mechanical force to the 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. Based on the fluorescence (FL) spectra, the solution contained with the high concentration of highly oxidized free radicals, which was strongly dependent on the concentration of the MoS2 nanoflowers in the aqueous solution. The formation of the M- and S-vacancies 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 hours. 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 single-layered MoS2 nanoflowers with the application of mechanical force in a dark environment.
References:
1. Jyh Ming Wu*, Wei En Chang,Yu Ting Chang , and Chih Kai Chang, "Piezo-Catalytic Effect on the Enhancement of the Ultra-High Degradation Activity in the Dark by Single- and Few-Layers MoS2 Nanoflowers" Advanced Materials 28, 11(2016).
2. Jyh-Ming Wu*, Yu-Guang Sun, Wei-En Chang, Jyun-Ting Lee,"Piezoelectricity induced water splitting and formation of hydroxyl radical from active edge sites of MoS2 nanoflowers "Nano Energy 46 (2018) 372–382

Figure 1