Determination of the key role to affect the piezocatalytic activity of graphitic carbon nitride for tetracycline hydrochloride degradation in water.

Graphitic carbon nitride (g-CN) has been proved to possess intrinsic piezoelectricity and its two-dimensional (2D) nanosheets present piezocatalytic activity to produce hydrogen from water splitting and eliminate organic pollutants in wastewater. Specific surface area and piezoelectric polarization are of great significance to achieve high piezocatalytic activity, but it is difficult to simultaneously improve both of them. Herein, to reveal the dominant role in the piezocatalysis of g-CN, we investigated the effect of exfoliation level on the piezocatalytic activity for degrading tetracycline hydrochloride (TC). Characterization results indicated that the specific surface area of the bulk g-CN was much lower than those of exfoliated g-CN samples due to the decrease of size and thickness. However, piezoresponse force microscopy (PFM) and kelvin probe force microscopy (KPFM) examinations suggested the bulk g-CN possessed the biggest piezoelectric polarization that gradually declined as increasing the exfoliation temperature. Through testing the piezocatalytic abatement of TC, the activity decline following the order of decrease in polarization was confirmed, which demonstrated the piezoelectric polarization was the dominant factor in the piezocatalysis of g-CN. This conclusion was also verified by the step-by-step performance decrease of the bulk g-CN during the successive four piezocatalytic runs, where the ultrasound treatment promoted the delamination of g-CN. In addition, superoxide (·O) radical, hydroxyl (·OH) radical and polarized positive charge were determined to be main active species, and accordingly the bulk g-CN had the highest ·OH and ·O concentrations, as well as the highest piezocurrent response. This work reveals the main role to affect the piezocatalytic performance of g-CN, and also provides a possible strategy to design piezocatalysts with optimized piezocatalytic activity.