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

Graphitic carbon nitride (g-C3N4) 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-C3N4, 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-C3N4 was much lower than those of exfoliated g-C3N4 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-C3N4 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-C3N4. This conclusion was also verified by the step-by-step performance decrease of the bulk g-C3N4 during the successive four piezocatalytic runs, where the ultrasound treatment promoted the delamination of g-C3N4. In addition, superoxide (·O2-) radical, hydroxyl (·OH) radical and polarized positive charge were determined to be main active species, and accordingly the bulk g-C3N4 had the highest ·OH and ·O2- concentrations, as well as the highest piezocurrent response. This work reveals the main role to affect the piezocatalytic performance of g-C3N4, and also provides a possible strategy to design piezocatalysts with optimized piezocatalytic activity.

Tribocatalysis of homogeneous material with multi-size granular distribution for degradation of organic pollutants.

Recently, tribocatalysis driven by mechanical energy has been developed by rubbing two kinds of different materials. In this work, we firstly demonstrated that the friction of the single material also could initiate the tribocatalysis for degrading organic dyes. Under magnetic stirring, the multi-size granular polytetrafluoroethylene (PTFE) particles were triboelectrically charged, among which the collision between large and small particles would cause high energy electrons on large particles to transfer to small ones. These triboelectric charges on PTFE particles could react with adsorbed oxygen molecules or water to generate reactive oxygen species, and then promoted the degradation process of organic dyes together with oxidant holes. We further investigated the experimental parameters, such as stirring speed, size and quantity of stirring bar, to optimize the tribocatalytic performance. What's more, the PTFE tribocatalysis possessed high durability for multiple recycling runs with > 90% degradation efficiency of Rhodamine B, as well as well universality for eliminating other pollutants. Finally, we proposed a plausible tribocatalytic mechanism of multi-size granular PTFE according to the detected reactive oxygen species and the determined intermediates. This study provides new insights into tribocatalysis, and demonstrates that the single material with different particle sizes can also be used as catalyst to drive tribocatalytic process.

Piezoelectric polarization promoted spatial separation of photoexcited electrons and holes in two-dimensional g-C3N4 nanosheets for efficient elimination of chlorophenols.

Graphitic carbon nitride (g-C3N4) has been proved to be a potential photocatalyst for environment purification, but the high recombination rate of photogenerated carriers leads to the low photocatalytic efficiency. Herein, we report the enhanced degradation of chlorophenols by 2D ultrathin g-C3N4 nanosheets with intrinsic piezoelectricity through photopiezocatalysis strategy. Under the simultaneous visible-light irradiation and ultrasonic vibration, the 2D g-C3N4 presented improved removal efficiency for elimination of 2,4-dichlorophenol (2,4-DCP) with an apparent rate constant of 6.65 × 10-2 min-1, which was 6.7 and 2.2 times of the photocatalysis and piezocatalysis, respectively. The improved removal efficiency was attributed to the sufficient separation of free charges driven by the ultrasound-induced piezoelectric field in the 2D g-C3N4, which was demonstrated by the enhanced current response under photopiezocatalysis mode. Additionally, the photopiezocatalysis of 2D g-C3N4 was proved to possess well universality for removing different chlorophenols, as well as high durability and dechlorination efficiency. Finally, a possible photopiezocatalytic mechanism for removal of 2,4-DCP was proposed based on the electron paramagnetic resonance (EPR) technique and the determination of intermediates through liquid chromatography-mass spectrometry (LC-MS) analysis. This work provides a promising strategy for the design of energy-conversion materials towards capturing solar and mechanical energy in ambient environment.

Phenanthroline bridging graphitic carbon nitride framework and Fe (II) ions to promote transfer of photogenerated electrons for selective photocatalytic reduction of Nitrophenols.

Nitrophenols (NPs) are widely used in industries and highly toxic to ecological environment and human health. Because aminophenols (APs) are important chemicals, catalytic reduction of NPs via efficient and environment-friendly strategies is of great importance. Herein, we developed a green photocatalysis route to efficiently convert NPs to APs using a Fe (II) modified graphitic carbon nitride (g-C3N4) photocatalyst, where phenanthroline units were employed to bridge Fe (II) and carbon nitride framework. The optimized sample P-CN-8-Fe presented significantly improved absorption of visible light, separation of photogenerated charges and carrier transportation in comparison with the pristine g-C3N4 and the modified samples of CN-Fe and P-CN-8. Accordingly, the P-CN-8-Fe showed a high conversion (97%) of p-nitrophenol (p-NP) to p-aminophenol (p-AP) under 2 h visible light irradiation, and meanwhile possessed high photocatalytic durability. Its high activity was also demonstrated through photocatalytic reduction of other NPs and nitrobenzene compounds. Finally, a possible mechanism was proposed for the photocatalytic reduction of p-NP by P-CN-8-Fe. This work provides an effective approach to prepare Fe (II) modified g-C3N4 through the bridging effect of phenanthroline group, which is a potential visible light driven photocatalyst for reduction of nitrobenzene derivatives.