In-situ photocalorimetry-fluorescence spectroscopy studies of RhB photocatalysis over Z-scheme g-C3N4@Ag@Ag3PO4 nanocomposites: A pseudo-zero-order rather than a first-order process

Much effort has been expended recently to apply photocatalysis to address the environmental crises and energy shortage crises. However, it is controversial to demonstrate that photocatalytic kinetics follows a pseudo-first-order process by common spectroscopy, because it only reflects the variation of chromophoric groups vs. time in the initiating photocatalysis, ignoring the subsequent intermediates oxidation. In this study, we used a photocalorimetry-fluorescence spectrum, coupling of a photocalorimeter and a laser-induced fluorescence spectroscopy, to demonstrate it is a pseudo-zero-order rather than a first-order process. It shows the feasibility to investigate not only photocatalytic kinetics but also reaction pathway and rate-determining step. During RhB photocatalysis over g-C3N4@Ag@Ag3PO4, three main heat changes occurred: (ab) the photoresponse of Rhodamine B molecules and the photocatalyst, consequently generating both hydroxyl and superoxide radicals, (bc) the balance between endothermic photoresponse and exothermic RhB photodegradation by O-2(center dot-)/(OH)-O-center dot and finally (cd) a stable exothermic period of RhB photodegradation. Specifically, the photocatalysis underwent an initial endothermic reaction and a subsequent exothermic stage, and eventually maintained a stable exothermic rate of (0.2345 +/- 0.0971) mJ s(-1) with a pseudo-zero-order process. With Ag NPs working as a bridge, the g-C3N4@Ag@Ag3PO4 system followed a Z-scheme mechanism, simultaneously generating both hydroxyl and superoxide radicals. Therefore, N-demethylation and chromophore cleavage rapidly occurred within 10 min. Then the ring cleavage and the oxidation of intermediates which were the rate-determining step eventually dominated, corresponding to the cd stage with a pseudo-zero-order process, and resulting in a pseudo-zero-order rather than a first-order process in RhB photocatalysis over g-C3N4@Ag@Ag3PO4. (C) 2017 Elsevier B.V. All rights reserved.