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)