Overview

Industrial wastewater originated from metallurgy, machining, petrochemical industry, and textile industry contains mineral and dye pollutants. The toxicity of industrial wastewater poses great threats to human beings as well as other living creatures. The loss of biodiversity and soil fertility are also not negligible. As a consequence, developing new photocatalysts that are able to decompose dye pollutants is of great interest. Additionally, to solve the problem that strong light attenuation in the water system reduces the efficiency of photocatalyst, our group devotes to the invention of next-generation piezo- and flexocatalysts. These novel techniques enable to decompose of waste dye pollutants without light illumination and are a promising way of wastewater disposal.

Besides, hydrogen gas owns the largest energy density, and after the combustion process, the final product is water, which is pollution-free. It can be seen that energy supply depends on water and vice versa. The interdependence between water and energy will increase in the next few years. Among various renewable alternative energy sources, hydrogen fuel has attracted great attention in the world's advanced industrial countries. Hydrogen production by direct water decomposition under illumination is a promising and pollution-free method. However, sunlight is unstable and varies according to different seasons and regions, so for most industrial applications of photocatalysts, the strong light attenuation in the water system leads to problems such as low quantum conversion efficiency, which greatly limits its commercial applications. Therefore, for the development of next-generation catalysts, issues such as cost-effectiveness, high efficiency, and long-term stability are extremely significant.

Our strategies:
1. Wastewater treatment

Clean water is very crucial in daily life. Therefore, many scientists tried to develop various catalysts to replace high-cost and highly toxic methods for sewage treatment. We are the first to discover the single- and few-layered MoS2 and MoSe2 nanoflowers (NFs), having a highly efficient piezo-catalyst effect, which can dramatically improve their degradation activity for destructing the organic dyes by imposing an ultrasonic wave in the dark. Interestingly, only after 60 s, the Rhodamine B dye degradation ratio reached 93%, which is the fastest degradation rate among the reported values. After the degradation time increases to 300 s, the degradation ratio of the RB dye reaches 100%. The ultrarapid degradation rate is thanks to the majority and minority carriers of the electron and hole, which are efficiently separated in the presence of the electric field, as the mechanical stress is exerted on the MoS2 NFs.

Also, our study also discovered that MoS2 nanosheets (NSs) can catalyze the generation of reactive oxygen species (ROS) to inactivate bacteria either through a piezoelectric effect (mechanical vibration) or photocatalytic effect (light irradiation). After 60 min of mechanical vibration or visible-light irradiation, the MoS2 NSs can reduce Escherichia coli (E. Coli) by 99.999%. In addition, the ROS generation efficiency and bacterial disinfection performance of the catalyst can be enhanced by depositing Au nanoparticles (NPs) on MoS2 NSs. The period of mechanical vibration or visible-light irradiation that achieves the same 99.999% reduction in E. coli is shortened to 45 min. This MoS2 nanocatalyst is a promising candidate for next-generation water purification systems because of its ability to be triggered by diverse environmental stimuli.

Recent first/corresponding-author publications:

1. Advanced Materials 28 (19), 3718-3725

2. Nano Energy 31, 575-581

3. Nano Energy 40, 369-375

2. Hydrogen evolution reaction (HER)

The hierarchical heterostructure of quartz microrods@few‐layered MoS₂ (QMSH) - quartz microrods (MRs) abundantly decorated with active‐edge‐site MoS₂ nanosheets - is capable of conducting hydrogen evolution reaction and decomposing wastewater without light irradiation simultaneous. Through theoretical calculations, the quartz microrods act as a self-powered parallel-plate capacitor to provide an internal electric field to the few‐layered MoS₂ nanosheets. This piezoelectric potential facilitates redox reactions with the free carriers in MoS₂ effectively. Under the dark environment, the second‐order rate constant of the QMSH is ≈0.631 L mg⁻¹ min⁻¹, which is 650 times larger than that of quartz MRs. This indicates that the piezoelectric heterostructural catalysts display exceptionally high efficiency on piezoelectrocatalytic redox reactions rather than in the piezocatalytic process only. The H₂‐production rate of QMSH catalysts approaches ≈6456 µmo1 g⁻¹ h⁻¹ and peaks at ≈16.8 mmol g⁻¹ in 8 h. It is obvious that piezoelectrocatalytic process is a promising method for treating industrial wastewater and producing clean energy.

With the increase in energy consumption, finding clean and sustainable energy sources is in urgent need. Hydrogen energy, among chemical fuels, is a promising alternative energy source owing to its high energy density. It is promising in reducing the demand for fossil fuels and is thus environmentally friendly. Our group was the first to utilize hybrid 1T- and 2H-phase few-layered MoSe₂ nanosheets to facilitate the state‐of‐the‐art catalytic hydrogen evolution reaction. With the combination of two types of MoSe₂ nanosheets, active reaction sites which display strong piezoelectric responses and extraordinary flexoelectric potential (flexopotential) are demonstrated. The strain induces piezopotential is able to enhance charge separation. Moreover, the piezo- and flexopotential coupling effect observed not only on edge‐site MoSe₂ nanosheets but also on polarized surfaces of the MoSe₂ nanosheets across the top and bottom surfaces further promoted the separation of electron-hole pairs. With the existence of the coupling effect, the hydrogen gas generation rate can reach up to ≈5000 µmol g⁻¹ h⁻¹.

The ferroelectric R3c LiNbO₃‐type ZnSnO₃ nanowires, thermally annealed in a hydrogen environment, exhibit a highly efficient hydrogen evolution reaction (HER). With the increasing concentration of oxygen vacancies, the moderated 3H‐ZnSnO₃ NWs have the longest extended carrier lifetime of approximately 8.3 ns. The piezoelectricity‐induced HER reaches an optimal H₂‐production rate of approximately 3453.1 µmol g⁻¹ h⁻¹ through the piezocatalysis process (without light irradiation). Moreover, via the synergistic piezophototronic process, the HER reaches approximately 6000 µmol g⁻¹ in 7 h. The mechanical force-induced spontaneous polarization functions as a carrier separator, driving the electron and hole in opposite directions in ferroelectric ZnSnO₃ NWs; this separation reduces the recombination rate and thus enhancing the redox process. Theoretical analysis also indicates that the photocatalytic and piezocatalytic effects can enhance piezophototronic performance through capitalizing on well‐modulated oxygen vacancies in ferroelectric semiconductors synergistically.

Recent first/corresponding-author publications:

1. Advanced Energy Materials 10 (42), 2002082

2. Advanced Materials 32 (34), 2002875

3. Advanced Functional Materials 30 (5), 1907619