Piezoelectric Spontaneous Polarization Research Articles

Molecular-statistical approach to the description of tilted smectic phases

ABSTRACT We have investigated the origin of spontaneous polar ordering in smectic liquid crystals in the framework of molecular-statistical approach. The polar order parameter is introduced as , where P is the polarization, μ is the molecular transverse electric dipole and ρ is the concentration of molecules. We have shown that piezoelectric spontaneous polarization should generally be related to the bent shape of molecules (even for the molecules which are not normally considered as bent-shaped), while both chirality and polarization appear due to the presence of the electric dipole in the direction perpendicular to the plane of molecular bend. The molecular origin of the flexoelectric effect in tilted smectic phases and the influence of the dipole–dipole interaction on the polar order parameter is also studied. The principal result of the paper is elaboration of a self-consistent mean theory for the description of polarization in Sm- and Sm- phases.

(Invited) Piezocatalytic Degradation of Dye Molecules, Water Splitting, and Formation of Hydroxyl Radical through Piezoelectricity of MoS2 Nanoflowers

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.

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.