Figure 3 XPS spectra of ZnCo2O4/Ag3PO4:(a) survery scan, (b) Ag 3d, (c) O1s,(d) Zn 2p, (e) P 2p, (f) Co 2p.
The XPS of ZnCo2O4/Ag3PO4is shown in Fig. 3. Fig. 3a shows the full spectrum scan of ZnCo2O4/Ag3PO4, from which it can be seen that the product has six elementary substances: Ag, O, Zn, C, P, and Co, where C is the substrate. Fig. 3b shows the XPS spectrum peaks of Ag 3d. The peaks at 367.85 eV and 373.8 eV in Ag 3d correspond to Ag 3d5/2 and Ag 3d3/2, respectively. Ag 3d5/2 can be decomposed into two peaks at 367.75 eV and 368.20 eV, while Ag 3d3/2 can also be decomposed into two peaks at 373.8 eV and 374.4 eV. The peaks appearing at 367.75 eV and 373.8 eV belong to Ag+, while the peaks at 368.2 eV and 374.4 eV belong to Ag0 monomer, which indicates that the Ag in the catalyst exists mainly in the form of Ag+ [13, 14]. Fig. 3c shows the XPS spectrum of O1s. The whole peak can be split into three feature peaks: 530.35 eV, 530.8 eV, and 532.2 eV. Among the three characteristic peaks, 530.35 eV and 530.8 eV corresponded to Ag3PO4 and ZnCo2O4 lattices in the material, respectively. The peak at 532.2 eV manifests H2O or OH- adsorbed on the material surface [15]. Fig. 3d is the XPS analytical diagram of Zn 2p. There are two main peaks at 1021.6 eV and 1044.7 eV, corresponding to the regional of Zn 2p3/2 and Zn 2p1/2 [16, 17]. It can be seen that the peak of Zn 2p3/2 is a single peak near 1021.6 eV, which is typical of the oxidation state of Zn2+. Fig. 3e indicates the XPS peak of P 2p, corresponding to P5+ in PO3+ 4 structure at 132.45 eV. Fig. 3f is the XPS analytical diagram of Co 2p. There are two main peaks at 780.8 eV and 795.9 eV, corresponding to the regional of Co 2p1/2 and Co 2p 3/2, and the obvious satellite peaks observed at 780.8 eV are characteristic peaks of the Co3+ oxidation state [18, 19]. XPS analysis further proved that ZnCo2O4 and Ag3PO4 were compounded.
Figure 4 a UV-Vis DRS results of Ag3PO4, ZnCo2O4 and 0.1 ZnCo2O4/Ag3PO4;b Plots of (αhv)2 as a function of energy (hv) for bandgap energies of Ag3PO4, ZnCo2O4 and 0.1 ZnCo2O4/Ag3PO4.
Fig. 4a shows the UV-Vis diffuse absorption spectra of Ag3PO4, ZnCo2O4, and 0.1 ZnCo2O4/Ag3PO4catalysts. The light absorption intensity of ZnCo2O4 is stronger in the whole wavelength range, while Ag3PO4 absorbs wider in the visible light region with an absorption boundary of approximately 530 nm. The addition of ZnCo2O4 widens the Ag3PO4 light absorption range and also raises the light absorption intensity, which indicates that ZnCo2O4 has a synergistic effect with Ag3PO4.
According to the Kubelka-Munk formula, the band gap width of Ag3PO4 and ZnCo2O4 can be counted [20]:
\(\alpha hv=A\left(hv-Eg\right)^{\frac{n}{2}}\) (3)
In this equation, and h are the absorption coefficient and Planck constant, Eg is the energy band gap, V is the optical frequency, and a is the constant. n represents the optical transition type of the semiconductor. When the semiconductor has a direct band gap, n is 1, and when the semiconductor has an indirect band gap, n is 4. Since ZnCo2O4 and Ag3PO4 are direct semiconductors, n is taken as 1 [19, 21]. Fig. 4b shows the (αhv)2versus energy (hv) of band gap energy for Ag3PO4, ZnCo2O4, and 0.1 ZnCo2O4/Ag3PO4catalysts, in which the band gap widths of ZnCo2O4 and Ag3PO4 are 2.63 eV and 2.45 eV, respectively.
In order to show the carrier transfer of ZnCo2O4/Ag3PO4catalyst in the process of photocatalytic reaction, the VB and CB potentials of ZnCo2O4 and Ag3PO4 were forecasted by this formula [22]:
\(E_{\text{CB}}=X-Ee-0.5Eg\) (4)
\(E_{\text{VB}}=E_{\text{CB}}+Eg\) (5)
Where x is the absolute electronegativity of the semiconductor, Ee is the potential of the free electron relative to the standard hydrogen electrode (about 4.5 eV), Eg is the band gap width of the semiconductor, and ECB and EVB are the conduction band and valence band potentials of semiconductors, respectively. X values of ZnCo2O4 and Ag3PO4 are 5.96 eV and 6.17 eV, and the EVB values of ZnCo2O4and Ag3PO4 can be calculated to be 2.775 eV and 2.9 eV, respectively. Therefore, the ECB values of ZnCo2O4 and Ag3PO4 are estimated to be 0.145 eV and 0.45 eV, respectively.