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.