Evaluation procedure of photocatalysts for VOCs degradation from the view of density functional theory calculations: g-C3N4 dots/graphene as an example

Various techniques have been utilized in experiments to evaluate the performance of photocatalysts and to understand the corresponding catalytic mechanism. However, it is still challenging to demonstrate the phenomenon in detail during the catalytic process in experiments to fundamentally understand the catalytic mechanism. Density function theory (DFT) is an excellent technique to solve this problem at the level of electrons and atoms. However, the evaluation procedure for photocatalysts based on DFT calculations was unclear. Thus, in this work, we propose an evaluation procedure for photocatalysts for the degradation of volatile organic compounds (VOCs) from the point of view of DFT calculations through the following aspects: (i) the band structure of photocatalysts, including the band gap, capability of photoadsorption and the positions of the conduction band (CB) and valence band (VB); (ii) the adsorption of H2O, O-2 or other oxidants; (iii) the adsorption of pollutants; (iv) the reaction pathway for pollutant degradation. To demonstrate the application of the proposed evaluation procedure, g-C3N4 dots/graphene is taken as an example to evaluate its photocatalytic performance. The results show that g-C3N4 dots/graphene has enhanced visible light absorption with significantly reduced height for photoelectron excitation and the photoelectron migrates between the parts of the heterostructure, from the valence band of the g-C3N4 dots to the graphene layer, and then to the conduction band of the g-C3N4 dots, which promotes light adsorption and induces the generation of electron-hole pairs with a low recombination rate. In addition, both the adsorbed H2O and O-2 have strong interactions with g-C3N4 dots/graphene, inducing activation to form (OH)-O-center dot and O-center dot(2)- radicals to attack and degrade the adsorbed VOCs. Therefore, a procedure to evaluate the performance of photocatalysts for VOCs degradation based on DFT calculations is proposed in this work, and the evaluation procedure is successfully applied to predict the high photocatalytic performance of g-C3N4 dots/graphene, where a unique photoelectron excitation pathway is also demonstrated within the heterostructure.