Synthesis and Electrochemical Characterization of g-C3N4/Ag3PO4 As n-p-Type Heterostructure Based Photoanode for Dye Degradation: Reactive Red 239 Study Case

Alvarez Gonzalez, Luisa

Hybrid photocatalysts are gaining importance due to their unique and enhanced photocatalytic activity, using urea as precursors. In the present investigation, the g-C3N4/Ag3PO4 composite was immobilized on ITO substrate to develop a photoanode catalyst, which was used in a typical photoelectrocatalytic process for Reactive Red 239 degradation. The results indicate that the g-C3N4/Ag3PO4 composite exhibit a higher photoelectrocatalytic activity for Reactive Red 239 degradation due to the charge transfer/separation properties and prolonged lifetime of charge carriers. The photoluminescence and electrochemical impedance analysis confirmed reduction in recombination of photogenerated electron and hole pairs.

In this study, g-C3N4/Ag3PO4 composites were constructed by a simple drop casting deposition method. The g-C3N4/Ag3PO4 exhibited much improved activity comparing with other classic catalysts such as g-C3N4 and Ag3PO4. The highly efficient separation of photogenerated electron-hole pairs was attributed to the construction of Z-scheme mechanism.

The g-C3N4 was prepared via thermal treatment of urea in two steps. In the first one, 12 g of urea was pun in a covered crucible and heated at 550 °C in air atmosphere for 4 h. In the second step, the resulting powder was collected and placed in an open crucible and further heated at 500 °C for 2 h to complete the reaction. The resulting brownish product was washed with distilled water to remove any residual alkaline species followed by exfoliation in water using ultrasonication. Finally, the exfoliated g-C3N4 nanosheets was centrifuged and dried at 60 °C during 24 h

In the preparation procedure of g-C3N4/Ag3PO4, certain amount of g- C3N4 nanosheet was first exfoliated in water via sonication for 1 h to produce a stable, homogeneous light brown dispersion of g-C3N4 nanosheets. Separately, silver nitrate was dissolved in distilled water, and then the solution was added to the g-C3N4 suspension. The resultant mixture was further stirred for 20 min. Then, ammonia aqueous solution was added dropwise to the mixture in order to form the silver-amino complex followed by a magnetic stirring at room temperature for 1 h. Furthermore, KH2PO4 aqueous solution was introduced drop wise into the mixture. The reaction mixture was sealed and kept under magnetic stirring at room temperature for 12 h. The resulting product was separated by centrifugation, washed with distilled water and dried overnight at 60 °C.

The g-C3N4, Ag3PO4, and g-C3N4/Ag3PO4 electrodes were prepared by coating the active materials powders on the surface of indium tin oxide (ITO) glass. Briefly, the ITO substrates were first cleaned under ultrasonic irradiation for 20 min in acetone and deionized water, respectively. Then, 40 mg of prepared powders was suspended in 50 ml of ethanolic polyethylene glycol solution and ultrasonicated for 30 min. Finally, the ITO substrates were coated with the prepared suspensions, and finally dried overnight under vacuum at 60 ◦C.

In the semiconductor characterization, different electrochemical techniques such as cyclic voltammetry, electrochemical impedance spectroscopy and anodic linear scanning were evaluated. The density of charge carriers in the semiconductor was evaluated and defined, as well as the position of the valence and conduction bands and their respective band gap.

Finally, the discoloration of the solution was evaluated from UV-VIS measurements. This analysis was performed on a problem solution that resembles the concentration of dye and salts of a textile industry effluent.