Scheme 1 Schematic diagram of photocatalytic mechanism of ZnCo2O4/Ag3PO4.
Based on the aforesaid analysis, the reaction mechanism of the ZnCo2O4/Ag3PO4photocatalytic degradation process was proposed. The valence band potential energy level of Ag3PO4 is about 2.9 eV, and the conductive band potential energy level is about 0.45 eV, so its band gap is 2.45 eV. The conduction band potential level of ZnCo2O4 is 0.145 eV, the valence band potential level is 2.775 eV, and its band gap width is 2.63 eV. Both ZnCo2O4 and Ag3PO4 can be excited by visible photons to form electron-hole pairs (Eq. (6-7)). With the accumulation of photogenerated electrons in the CB of Ag3PO4, Ag3PO4 was photoetched, which makes part of the Ag+ converted to singlet silver (Eq. (8)) [13]. Silver nanoparticles can absorb visible photons and form photoexcited electron hole pairs (Eq. (9)). [23]. Photogenerated electrons formed in Ag nanoparticles are captured by dissolved oxygen to form superoxide anions (O- 2· ) (Eq. (10)). The strong oxidation property of O- 2· degrades MO to produce carbon dioxide and water (Eq. (11)). Meanwhile, the photogenerated electrons in the conduction band of ZnCo2O4 combine with the photogenerated holes generated by Ag nanoparticles to prevent further corrosion, making the catalyst itself self-stabilizing. The holes left in the valence bands of Ag3PO4 and ZnCo2O4 directly decomposed the MO oxidation to water and CO2 (Eq. (12)). This is consistent with the active factor capture results.
\(\text{Ag}_{3}PO_{4}+hv\rightarrow e^{-}+h^{+}\) (6)
\(\text{ZnCo}_{2}O_{4}+hv\rightarrow e^{-}+h^{+}\) (7)
\(\text{Ag}^{+}+e^{-}\rightarrow Ag\) (8)
\(Ag+hv\rightarrow e^{-}+h^{+}\) (9)
\(e^{-}+O_{2}\rightarrow O_{2}^{-\ }\mathbf{}\ \) (10)
\(O_{2}^{-\ }\mathbf{}+\text{RhB}\rightarrow\ \text{CO}_{2}+H_{2}O\)(11) \(h^{+}+\text{RhB}\rightarrow\text{CO}_{2}+H_{2}O\) (12)