3.1 Crystal structure analysis
The phase structure of ZnO before and after compounding with AgSiW and CsPW was studied by XRD. The results are shown in Fig. 1. The characteristic peaks of AgSiW at 21.08, 23.58, 25.9, 28.0, 29.96, 31.86, 33.58, 35.3, 38.5, 41.46, 42.88, 44.3, 45.66, 46.98, 49.58, 54.4, 61.18 and 63.38° are consistent with the XRD spectra in the literature[13]. The characteristic peaks of CsPW appear at 10.54, 18.3, 23.7, 26.0, 30.22, 35.52, 38.72, 43.38, 47.18, 54.78 and 61.56°, which are consistent with those reported in the literature[14, 15]. The peaks of ZnO at 31.79, 34.43, 36.26, 47.52, 56.59, 62.85, 66.31, 67.94 and 69.08° are consistent with the standard card (PDF#65-3411), corresponding to the (100), (002), (101), (102), (110), (103), (200), (112) and (201) crystal planes of ZnO respectively. The characteristic peaks related to ZnO are shown in the spectra of ZnO/AgSiW and ZnO/CsPW composite catalysts, and the characteristic peaks of AgSiW (25.86, 28.04 and 30.08°) and CsPW (26.02°) can be observed in the spectra of ZnO/AgSiW and ZnO/CsPW, but the peak intensity is weak. It may be that the crystallinity of AgSiW and CsPW decreases or AgSiW are highly dispersed on the surface of ZnO during the formation of the complex, or it may be related to the low dosage of AgSiW and CsPW[16, 17].
3.2 Morphology analysis
The morphology of the prepared nanomaterials was determined by TEM. Fig. 2 shows TEM images of ZnO, AgSiW, CsPW, ZnO/AgSiW and ZnO/CsPW. It can be seen from the figure that the prepared samples are nano materials. In Fig. 2 (a), ZnO is a nano sheet with a size of about 100-400 nm and a thickness of about 30 nm. Fig. 2 (b) shows nano AgSiW. It can be seen from the image that AgSiW is massive, the size is about 200 nm, and nano dots with uniform size are distributed on the surface and edge. Fig. 2 (c) shows the TEM image of CsPW. It can be seen from the image that CsPW is a nanosphere with a diameter of 100-200 nm and has good dispersion. As can be seen from Fig. 2 (d), when ZnO and AgSiW form a composite, the morphology of AgSiW is changed, so that the massive AgSiW becomes dispersed and evenly distributed on the surface and edge of ZnO. It can also be seen from Fig. 2 (e) that there are ZnO and CsPW in the composite system, and they are closely combined. The results in Fig. 2 show that ZnO/AgSiW and ZnO/CsPW composite catalysts were successfully prepared.
3.3 FTIR analysis
In order to further determine the structure of the composite catalyst, the infrared spectra of the samples were analyzed. In the FT-IR spectrum, the characteristic bands of Keggin anion of AgSiW appear at 1020, 980, 926 and 783 cm−1, corresponding to the stretching vibration of silver silicotungstate Si-Oa, W-Od, W-Ob-W and W-Oc-W bonds respectively. The peak at 878 cm−1 may be due to the connection between counter ion and bridge oxygen W-Ob-W[13]. The peaks of CsPW at 1081, 989, 892 and 810 cm−1 are attributed to the stretching vibration of P-Oa, W=Od, W-Ob-W and W-Oc-W, respectively[18]. The characteristic peaks of ZnO appear at 566 and 435 cm−1, corresponding to the stretching vibration of zinc oxide Zn-O[19, 20]. It can be seen from the figure that in the spectra of ZnO/AgSiW and ZnO/CsPW, the characteristic peaks of AgSiW can be observed at 988, 912, 862 and 733 cm−1, corresponding to the stretching vibration of Si-Oa, W-Od, W-Ob-W and W-Oc-W bonds respectively, and the characteristic structures of CsPW can be observed at 1081, 988, 888 and 816 cm−1, corresponding to the stretching vibration of P-Oa, W=Od, W-Ob-W and W-Oc-W, respectively. Compared with ZnO/CsPW, AgSiW in ZnO/AgSiW composite catalyst shifted to a lower wave number, which may be related to the strong interaction between AgSiW and ZnO.
3.4 Raman spectra analysis
The Raman spectra of ZnO/AgSiW, ZnO/CsPW, ZnO, AgSiW and CsPW are shown in Fig. 4. The characteristic bands of Keggin anion of AgSiW appear at 983, 954, 883 and 559 cm−1, corresponding to the asymmetric stretching mode of W=Od, the symmetric stretching mode of W=Od and W-Ob-W and the bending vibration of O-Si-O, respectively[21–23]. The peaks of CsPW at 1009 cm−1 and 994 cm−1 are attributed to the symmetrical and asymmetric stretching modes of W=O. The bands at 906 and 537 cm−1 are attributed to the asymmetric stretching mode of W-O-W and the bending vibration mode of O-P-O, respectively[24, 25]. The peaks at 336, 439 and 583 cm−1 belong to the vibration modes of 2E2 (M), E2 (high) and E1 (LO) of ZnO[26]. When ZnO and AgSiW are combined, a significantly enlarged band compared with AgSiW is observed at 897cm−1, and there is an obvious displacement, indicating that there is a certain interaction between ZnO and AgSiW[27, 28]. However, no obvious characteristic peak of ZnO was observed in the spectrum, which may be due to the weak Raman intensity of ZnO after recombination. After ZnO and CsPW were compounded, the characteristic peaks belonging to ZnO and CsPW can be found in the spectrum, but the characteristic peak of CsPW is weaker than that of pure CsPW, which may be due to the low compounding amount of CsPW or the hydrogen bond interaction between the oxygen atom of Keggin anion and the hydroxyl group on the surface of ZnO[27].
3.5 Band gap analysis
The optical properties of ZnO/AgSiW, ZnO/CsPW, ZnO, AgSiW and CsPW were studied by measuring the UV diffuse reflectance spectra (DRS). The results are shown in Fig. 5. The illustrated part in the figure is calculated by formula (1). It can be estimated that the band gaps of ZnO/AgSiW, ZnO/CsPW, ZnO, AgSiW and CsPW are 2.53, 3.10, 3.15, 3.06 and 3.21eV respectively. It can be seen that the heterojunction system of ZnO with AgSiW and CsPW widens the absorption range of ZnO to light, and the effect of introducing AgSiW is very obvious, which also makes ZnO/AgSiW make more full use of sunlight. This result may also be directly related to the stronger photocatalytic degradation of ZnO/AgSiW than ZnO/CsPW.
(αhν)1/n = A(hν - Eg) (1)
Where, α , hν, Eg and A represent the absorption coeffificient, photon energy, band gap energy and a constant, respectively[16, 29].
3.6 XPS spectra of ZnO/AgSiW and ZnO/CsPW
Figure 6 shows the X-ray photoelectron spectroscopy (XPS) of the sample to further verify the elemental composition and chemical state of the synthetic sample. As shown in Fig. 6a, Zn 2p, Ag 3p, O 1s, W 4p, Ag 3d, C 1s, W 4d and W 4f signals are clearly displayed in the full scan measurement spectrum of ZnO/AgSiW. The low content of Zn 2p may be related to the uniform distribution of AgSiW on the surface of ZnO. The signals of Zn 2p, Cs 3d, O 1s, W 4p, C 1s, W 4d, Zn 3s, Zn 3p, W 4f and Zn 3d were clearly displayed in the full scanning measurement spectrum of ZnO/CsPW. The elemental composition of ZnO/AgSiW and ZnO/CsPW was determined by full spectrum scanning. Fig. 6b shows the XPS spectra of Zn 2p in ZnO/AgSiW, ZnO/CsPW and ZnO. Compared with the peaks of pure ZnO at 1021.63 eV (Zn 2p3/2) and 1044.63 eV (Zn 2p1/2), the Zn binding energy in ZnO/AgSiW and ZnO/CsPW samples shifted. Among them, the binding energy of Zn in ZnO/AgSiW samples shifts to the right, while the binding energy of Zn in ZnO/CsPW samples shifts to the left, indicating that there may be a certain interaction force between Zn and AgSiW and CsPW when ZnO forms a composite heterojunction system with AgSiW and CsPW. Figure 6 (c) and Figure 6 (d) show the XPS spectra of samples Ag 3d and Cs 3d respectively. The peaks at 366.88 and 372.88 eV are attributed to the 3d3/2 and 3d5/2 orbits of Ag (I), while the peaks at 724.78 and 738.68eV are attributed to the 3d3/2 and 3d5/2 orbits of Cs (I). Compared with the AgSiW and CsPW of monomers, the binding energy of Ag and Cs after composite has shifted[30]. Figure 6 (e) shows the W 4f XPS spectra of ZnO/AgSiW and ZnO/CsPW samples. In the spectrum of AgSiW, the peaks at 34.17 and 36.27 eV can be attributed to W (VI), and the peaks can be observed at 33.56 and 35.66 eV, indicating the presence of reduced W (V) in the complex. Compared with pure AgSiW, the binding energy of W shifted significantly to the right. In the spectrum of CsPW, the peaks at 36.22 and 38.36 eV can be attributed to W (VI), the peaks at 35.24 and 37.31 eV can be attributed to W (V), and the binding energy of W changes less than that of pure CsPW. Figure 6 (f) shows the O 1s XPS spectra of ZnO/AgSiW and ZnO/CsPW samples. The peaks at 528.84, 530.41 and 531.86 eV can be attributed to the lattice oxygen in ZnO, the lattice oxygen in Keggin structure (W-O-W) and Si-O bond in ZnO/AgSiW samples, respectively. The peaks at 531.06 and 532.39 eV can be attributed to the lattice oxygen and P-O bond in ZnO of ZnO/CsPW sample, respectively[31]. The above results show that when ZnO constructs a heterojunction system with AgSiW and CsPW, a strong interaction is formed, this result is also consistent with FT-IR and Raman spectra.
3.7 BET analysis
The surface area and pore structure of the samples were further analyzed by nitrogen adsorption desorption. As can be seen from Fig. 7 (a), ZnO and ZnO/AgSiW samples show type III isotherms with H3 hysteresis loops[32, 33], and ZnO/AgSiW samples show type IV isotherms with H3 hysteresis loops[34, 35], indicating that the samples have cracked mesoporous structure. The specific surface area of pure ZnO is 13.043 m2/g. When the heterojunction system was constructed with ZnO/CsPW and ZnO/AgSiW, the specific surface areas of the heterojunction catalysts were 13.032 and 8.191 m2/g, respectively. It can be seen from Fig. 7 (b) that the introduction of CsPW and AgSiW has a certain impact on the pore size of ZnO, and AgSiW has a significant impact on the specific surface area of the heterojunction system. The possible reason is that AgSiW is evenly dispersed on the surface of ZnO and strongly coupled with ZnO[36], which is consistent with the TEM results.
3.8 Photocatalytic activity
The photocatalytic activities of ZnO/AgSiW and ZnO/CsPW samples were tested with RhB as the target pollutant. Under the irradiation of simulated sunlight, the removal effect of the catalyst on dyes was evaluated under the conditions of catalyst dosage of 15 mg and dye concentration of 50 mL. According to Fig. 8 (a), the degradation rates of RhB by ZnO/AgSiW, ZnO/CsPW and ZnO are 92.3%, 72.7% and 17.2% respectively. The results show that the catalytic performance of the prepared ZnO/AgSiW and ZnO/CsPW composite catalysts is greatly improved compared with pure ZnO. Although pure AgSiW and CsPW have strong adsorption properties, they hardly have catalytic properties. However, when combined with ZnO, the composite system showed high catalytic activity, which may be because AgSiW and CsPW constructed a heterojunction system with ZnO, which improved the problem of fast recombination rate of ZnO photogenerated carriers and widened the absorption range of ZnO to light. As can be seen from Fig. 8 (b), ln(C0/Ct) has a good linear relationship with time, indicating that the photocatalytic degradation of RhB by ZnO, ZnO/AgSiW and ZnO/CsPW conforms to the pseudo first-order kinetic model:
(2)
where kapp is the apparent rate constant (min−1 ), 𝑡 is the irradiation time (min), C𝑜 and C𝑡 are the absorbance at 𝑡 = 0 min and 𝑡 = ∞[12].
The calculated kapp values of ZnO/AgSiW, ZnO/CsPW and ZnO are 0.0403, 0.0203 and 0.0028 min−1 respectively, and the reaction rate constants of ZnO/AgSiW and ZnO/CsPW are 14.4 and 7.3 times that of ZnO, respectively. The results show that the two composite catalysts have stronger catalytic activity than pure ZnO.
3.9 Recyclability of photocatalysts
In addition to the efficiency of photocatalysts, the stability and recoverability of photocatalysts are also important indicators to evaluate whether they can be applied to practice. In order to explore the stability and recoverability of photocatalyst, it was studied by cyclic experiment. As shown in Fig. 9, under simulated sunlight, the removal rates of RhB by ZnO/AgSiW and ZnO/CsPW for the first time are 91.3% and 73.7% respectively. After the two catalysts are recycled for 3 times, the removal rates of RhB are 84.8% and 65.3% respectively. They still have high photocatalytic activity, which proves the stability and recoverability of the complex as a catalyst.
3.10 Reaction mechanism
To confirm the effective separation of photogenerated carriers, the transient photocurrent response and electrochemical impedance were measured. Fig. 10 (a) shows the transient photocurrent response of ZnO, ZnO/AgSiW and ZnO/CsPW under visible light irradiation. As can be seen from the figure, when the light is switched in three cycles, the generated photocurrent is stable and repeatable. Under visible light irradiation, it can be clearly seen that the photocurrent density of ZnO/AgSiW and ZnO/CsPW is higher than that of ZnO, and the higher the photocurrent density, the lower the efficiency of photoinduced charge recombination[37, 38], which means that the charge separation of the two heterojunction heterojunction catalysts has been improved and the charge separation rate of the composite catalysts has been greatly improved. The Nyquist curve is shown in Fig. 10 (b). Generally, the arc radius on the EIS Nyquist diagram corresponds to the charge transfer resistance (Rct), while the small arc radius represents the low charge transfer resistance[39, 40]. Obviously, ZnO/AgSiW and ZnO/CsPW composite catalysts have lower charge transfer resistance than ZnO, which means that the formed composite system can effectively improve the charge transfer between interfaces. This result is consistent with the photocurrent response, which shows that the composite system formed by ZnO, AgSiW and CsPW can significantly improve the separation efficiency of photogenerated carriers, improve the charge transfer between interfaces, and then improve the photocatalytic degradation performance.
3.11 Free radical capture experiment
Figure 11 is a free radical capture experiment to determine the active substances that play a major role in the photocatalytic process. P-benzoquinone, isopropanol and methanol were used as capture agents for superoxide anion radical, hydroxyl radical and hole. It can be seen from the left half of Figure 11 that the addition of p-benzoquinone has the greatest impact on the photocatalytic activity of ZnO/AgSiW composite catalyst, followed by methanol, indicating that superoxide anion free radicals and holes play the main catalytic role in the photocatalytic process, while the addition of isopropanol has relatively little impact on the photocatalytic activity, indicating that holes also hinder the degradation reaction, But not the main active substance. It can be seen from the right half of Figure 11 that the addition of isopropanol and methanol has the greatest impact on the photocatalytic activity of ZnO/CsPW composite catalyst, indicating that hydroxyl radical and hole are the main active substances in the photocatalytic reaction process, and also plays a certain inhibitory role after the addition of p-benzoquinone, indicating that superoxide anion radical is also produced in the photocatalytic process.
On this basis, the possible mechanisms of photocatalytic degradation of RhB by ZnO/AgSiW and ZnO/CsPW were proposed. The band gaps of ZnO, AgSiW and CsPW can be estimated to be 3.15, 3.06 and 3.21 eV respectively according to the UV Vis diffuse reflectance spectra. The measured potential and Ag/AgCl were converted to the normal hydrogen electrode (NHE) scale by equation (3) [41]:
ENHE=EAg/AgCl+0.197 (4)
Therefore, the CB of ZnO, AgSiW and CsPW can be estimated by Mott-Schottky, which are -0.34, 0.76 and -0.01 eV respectively. The VB of ZnO, AgSiW and CSPW can be calculated according to formula (4)[42], which are 2.81, 3.29 and 3.09 eV respectively.
EVB = ECB+Eg (4)
According to the estimated conduction band and valence band positions, the possible mechanism diagrams of photocatalytic degradation of RhB by ZnO/AgSiW and ZnO/CsPW were constructed. As shown in Fig. 13, it is assumed that ZnO and AgSiW form a Z-scheme heterojunction. Under the irradiation of simulated sunlight, electrons are excited from VB of ZnO to CB, and photogenerated holes are left in VB. Because the CB of ZnO is more negative than O2/·O2− potential (-0.046V vs. NHE)[41, 43], electrons on CB of ZnO can form ·O2− with oxygen, and the formed ·O2− can directly react with RhB molecule in water.
In AgSiW, the electrons are transferred from VB to CB and combined with the holes on VB of ZnO. This electron transfer can reduce the opportunity of electron hole recombination formed in ZnO and improve the photocatalytic activity. Because the VB (+ 2.81V vs. NHE) of ZnO is more positive than that of OH−/·OH and H2O/·OH (+ 1.99 V vs. NHE and + 2.27 V vs. NHE), the holes left on VB of AgSiW will be combined with H2O and converted into ·OH.
According to the capture experiment, only a small part will react with H2O and convert into ·OH, and the remaining holes will directly participate in the degradation of RhB. The other inference is inconsistent with the free radical capture experiment, because when the electrons on CB of ZnO are transferred to the CB of AgSiW and accumulated, the CB (+0.76V vs. NHE) of AgSiW is positive than the O2/·O2− potential (-0.046V vs. NHE). AgSiW is a deep electron trap, which makes it difficult for electrons to jump out of AgSiW, so that the conversion from O2 to ·O2− is blocked[44].
According to the free radical capture experiment, holes and hydroxyl radicals play a major role in the photocatalytic process, and superoxide anion radicals also play a certain inhibitory role. As shown in Fig. 14, assuming that a Z-scheme heterojunction is formed between the samples, when the electrons on VB of ZnO and CsPW are excited and migrated to CB under the irradiation of sunlight, holes are left on VB, and the electrons on CB of CsPW migrate to the VB of ZnO. Because the potential of ZnO (-0.34 V vs. NHE) is more negative than that of O2/·O2− (-0.046 V vs. NHE), O2 can combine with the electrons on CB of ZnO to form ·O2−, while the potential of VB of CSPW (+ 2.81v vs. NHE) is positive than that of OH−/·OH and H2O/·OH(+ 1.99 V vs. NHE and + 2.27 V vs. NHE). Therefore, some holes on VB of CSPW can react with water to form hydroxyl radicals, and the remaining holes directly play the role of oxidative degradation of RhB. If the type II heterojunction is formed, when the electrons on the CB of ZnO are transferred to the CB of CSPW, because the potential of the CB of CSPW is positive than the potential of O2/·O2− (-0.046 V vs. NHE), as a shallow electron trap, the electrons are not easy to react with O2 to generate superoxide anion radicals, which is inconsistent with the experimental results of radical capture. Therefore, it is inferred that a Z-scheme heterojunction is formed between ZnO and CSPW.