3.1. Photocatalytic activity
The photocatalytic NO removal of cZHS and Ag:cZHS materials under solar light are shown in Fig. 2a. The photocatalytic efficiency of Blank, cZHS, 1% Ag:cZHS, 5% Ag:cZHS, 10% Ag:cZHS, 20% Ag:cZHS, 30% Ag:cZHS materials are 13.01%, 52.14%, 53.86%, 61.96%, 67.6%, 87.30%, and 81.23%, respectively. The photocatalytic efficiency was significantly increased upon being combined with Ag NPs, indicating the effect of the SPR of Ag NPs. The photocatalytic efficiency of 20% Ag:cZHS reached 88.4% after only 7 min and remained unchanged until the end of the test. In contrast, the photocatalytic efficiency of cZHS increased slowly during the reaction and reached the highest efficiency of 60% at the end of the test. Thus, the 20% Ag:cZHS is the most efficient sample. Practically speaking, the reusability of materials is important, which was evaluated by a recycling test. As shown in Fig. 2b, the photocatalytic efficiency of 20% Ag:cZHS was decreased from 87.30–77.55% after 5 times recycling at the same condition. The result showed that the durability of the sample was promising. Besides, the conversion of NO to NO2 and green products was also calculated and shown in Fig. 2c. The apparent quantum efficiency (AQE) was calculated by Eq. (S3) to understand the effect of photons on the photocatalytic ability of Ag:cZHS and cZHS (Fig. 2d). The AQE (10− 4%) value of cZHS, 1% Ag:cZHS, 5% Ag:cZHS, 10% Ag:cZHS, 20% Ag:cZHS, 30% Ag:cZHS were 3.81, 4.59, 4.60, 5.56, 7.11, and 6.51, respectively.
3.2. Materials characterizations
Figure 3a shows the FTIR spectra of 1%, 5%, 10%, 20% and 30% of Ag:cZHS and cZHS, the tracing region in the range from 400–4000 cm− 1. The wide peak observed in 3800 cm− 1 to 2750 cm− 1 shows OH bending and stretching vibrations [12]. Besides, the sharp peak appears at 1170 cm− 1, attributed to Sn – OH deformation vibration [28]. The stretching vibration of Sn-O-Sn is observed at 779 cm− 1 and 536 cm− 1 [29]. The results obtained confirm the success of the cZHS synthesis, but the signal of Zn and Ag were difficult to detect by the FTIR analysis. Therefore, the XRD diffraction was carried out to confirm the structure and crystallization of cZHS.
The XRD patterns of all samples are shown in Fig. 3b. The results show that several characteristic peaks located at 19.7°, 22.8°, 32.7°, 36.7°, 38.5°, 40.3°, 46.9°, 52.8°, 58.3°, 63° and 73° corresponding to the (111), (200), (220), (310), (311), (222), (400), (420), (422), (511), and (531) lattice planes of the cZHS, respectively [JCPDS 74-1825]. The results indicated that the cZHS has been successfully synthesized and has crystallinity with high purification in the cube phase. Furthermore, small additional peaks were overlapping at 38.2°, 44.25°, 64.5°, and 77.41° could be attributed to the (111), (200), (220), and (311) planes of the Ag NPs in the Ag:cZHS, respectively [30]. While other studies have been in the direction of peak expansion, peak intensity decreased or even peak material loss.
3.3. Morphology of materials
The SEM images in Fig. 4 show the cube shape of the sample before and after Ag NPs loading. As shown in Fig. 4a, the cZHS were synthesized with precise cube shapes, smooth surfaces. The cZHS were dispersed well in the solvent, without any conglomeration. It can be seen in Fig. 4b that the original morphology of cZHS was changed when Ag NPs were loaded onto the surface. The shape and structure of the Ag:cZHS become distinctive. However, the change is not much, which proves that this synthesis method does not change the morphology of the substrate material. Such change in the morphology demonstrated that the Ag NPs successfully loaded onto the cZHS surface.
Figure 5 shows the morphology and structure of cZHS and 20% Ag:cZHS. As shown in Fig. 5a, b, the TEM images of the pristine cZHS with the precise edges and a length range from 300 nm – 600 nm match well with SEM images. It is observed in Fig. 5c, d that the presence of Ag NPs covered on the surface of cZHS cubes with a diameter less than 10 nm. Furthermore, the dots appearing in Fig. 5d are predicted to be Ag NPs. In contrast, in Fig. 5b, the dots of Ag NPs do not appear; instead, the nanopores were created by the F127. These nanopores were filled with Ag NPs in 20% Ag: cZHS.
3.4. Specific surface area of the materials
The N2 adsorption isotherms of cZHS and 20% Ag:cZHS are shown in Fig. 6. The quantity adsorbed of cZHS is higher than that of the 20% Ag:cZHS. In addition, as shown in Table 1, the BET surface area of cZHS and 20% Ag:cZHS is 20.24 m2 g− 1 and 34.93 m2 g− 1, respectively. The total pore volume and average pore width of cZHS and 20% Ag:cZHS are 0.034 cm3 g− 1, 6.702 nm, and 0.0267 cm3 g− 1, 3.967 nm, respectively. These results confirm that the higher total pore volume and average pore width of cZHS are due to the presence of F127, which increases the porosity of the sample [31]. The hypothesis that Ag NPs filled the nanopores of cZHS has been determined by the lower total pore volume and average pore width of 20% Ag:cZHS.
Table 1
BET surface area, total pore volume, and average pore width of cZHS and 20% Ag: cZHS.
Samples | BET surface area (m2 g− 1) | Total pore volume (cm3 g− 1) | Average pore width (nm) |
cZHS | 20.24 | 0.034 | 6.702 |
20% Ag:cZHS | 34.93 | 0.027 | 3.067 |
3.5. Optical properties of materials
The optical absorption properties of cZHS and 20% Ag:cZHS are shown in Fig. 7. In Fig. 7 (a), the 20% Ag:cZHS could reflect photons with a wavelength in the range of UV light (300 nm > λ > 400 nm). At the wavelength from 300 nm to 400 nm, an SPR peak appears, which was generated by Ag NPs. Besides, the formation of a peak of SPR in the range of UV (200 nm < λ < 300 nm) and the DRS tail of the 20% Ag:cZHS does not coincide with the Ox axis in the range of IR. In contrast, in the cZHS, there is a peak present at around 360 nm. The morphology of this peak is bizarre and attributed to the manipulation of the DRS operator. However, after repeated measurements of DRS, we concluded that the appearance of this peak of the cZHS sample was due to the influence of F127 on the surface of the material. As shown in Fig. 7b, Kubelka-Munk plots of DRS reveals the band structure of cZHS is an indirect transition with the bandgap of 5.34 eV. The 20% Ag:cZHS has a smaller bandgap (5.1 eV), which is expected to have better photocatalytic activity.
The surface chemistry of materials is shown in Fig. 8. Figure 8a shows that the peaks around are assigned to Sn 3d, O 1s, and Zn 2p of cZHS. Furthermore, the peak around 370 eV of 20% Ag:cZHS corresponds to the Ag 3d. The HR-XPS of Sn 3d5, O 1s, and Zn 2p3 are shown in Fig. 8a, b and c, respectively. The peak at 485.3 eV, 493.8 eV, 531.8 eV, and 1020 eV corresponds to Sn 3d5/2, Sn 3d3/2, O 1s, and Zn 2p3/2, respectively. The HR-XPS of Ag 3d is shown in Fig. 8 (e); there are two peaks of Ag 3d at 367.7 and 373.6, corresponding to Ag 3d5/2 and Ag 3d3/2, respectively. In addition, XPS is a directly used tool to measure the valence-band maximum (VBM) of the materials. In Fig. 8f, the VBM of cZHS is 2.55 eV, and 20% Ag:cZHS is 4.08. The VBM of 20% Ag:cZHS is higher than the cZHS is 1.53 eV, electrons provided by Ag enhance electron transport.
3.6. Photocatalytic mechanism over the materials.
The trapping test considered the critical factors of photocatalytic degradation of NO, as shown in Fig. 9. In the trapping test, the h+, e–, and •OH were trapped by adding the KI, K2Cr2O7, and IPA scavengers, respectively. The trapping results indicate that e– and •OH contributed equally to the photocatalytic activity of 20% Ag:cZHS. The photocatalytic efficiency of the 20% Ag:cZHS was decreased dramatically by adding KI. The ESR was invested in determining the generation of radicals of 20% Ag:cZHS. Figure 9b shows that the 20% Ag:cZHS generate more •O2 radicals than •OH. These results explained that in the 20% Ag:cZHS, the e- react with O2 before forming •OH radicals.
A proposed mechanism on the photocatalytic NO removal of the Ag:cZHS under solar light is presented in Fig. 10. Under light activation, the electrons and holes are generated in the valence band of the cZHS. The photo-generated electrons spontaneously move to the conduction band of the cZHS and are isolated by the interface between the cZHS and the Ag NPs [32]. These electrons can reduce adsorbed O2 on the surface to produce •O2‾, as evidenced from the ERS results. These •O2‾ radicals could get more electrons to produce •OH species. In the meantime, the photo-generated holes in the VB oxidized adsorbed water on the surface of cZHS to •OH radicals. The •O2‾ and •OH radicals assist in the removal of NO [33]. Thus, the introduction of Ag NPs drastically enhances photocatalytic NO removal of the cZHS through the surface plasmonic effect, resulting in a better photoresponse and excellent electron-hole pairs separation.