Figure 1 shows the XRD patterns of [email protected] sample, BiOCl (JCPDS#06-0249) and Ag (JCPDS#04-0783). It can be seen from the figure that the characteristic peaks of the [email protected] samples are located at 11.9°, 25.86°, 32.49°, 33.44°, 41.17°, 46.45°, 54.37°, and 58.6°, respectively. These characteristic peaks correspond to {001}, {101}, {110}, {102}, {112}, {200}, {211} and {212} crystal planes. The results are consistent with BiOCl, indicating that the introduction of Ag does not affect the crystal structure of BiOCl. At the same time, the characteristic peak corresponding to the {111} crystal plane of Ag appears at 38.11° after loading Ag, and gradually gets stronger with the increase of Ag amount. It indicates that the composite material contains BiOCl and Ag. In addition, it is also observed that the characteristic peak of BiOCl became narrower after loading Ag, and the intensity ratio of the crystal plane gradually decreased from the value of {110}/{001}, which may be because the thickness of BiOCl nanoplates increased after loading Ag[16].
In order to understand the microscopic morphology of the prepared [email protected] photocatalyst, the SEM characterization of the [email protected] sample was performed. The results are shown in Fig. 2. It can be seen from the figure that all samples have a lamellar structure. Among them, Fig. 2(a)-(d) are the SEM images of AB-0, AB-5, AB-10 and AB-20, respectively, as can be seen from the figure, the peeling between the BiOCl nanosheets without Ag loading The degree of separation is small, and the peeling degree between the nanosheets of the sample after Ag loading becomes larger, and the thickness is increased on the original basis. This result is consistent with the description of the XRD pattern. At the same time, with the increase of Ag content, Ag nanoparticles began to appear on BiOCl nanosheets. The presence of Ag nanoparticles could not be clearly observed in AB-5, while only a small amount of Ag particles were observed in AB-10. On the one hand, it is because the amount of AgNO3 added is small; on the other hand, it may be because the surface of the BiOCl nanosheets is smooth, and the Ag particles are not easy to adhere to the surface, and some of them exist between the sheets in the form of intercalation, so only See the presence of a small amount of Ag particles. When the loading of Ag was continued to increase to 20wt%, as shown in Fig. 2(d), Ag nanoparticles began to agglomerate.
Using transmission electron microscope to further characterize AB-10, the results are shown in Fig. 3. Among them, Fig. 3 (a) and (b) are the overall morphology of the sample. It can be seen from the figure that a small amount of Ag nanoparticles can be observed on the BiOCl nanosheets. Figure 3 (c) is an enlarged view of Ag nanoparticles. It can be seen that Ag nanoparticles are spherical and have a diameter of approximately 40 nm. At the same time, using high-resolution transmission electron microscopy (HRTEM) analysis, as shown in Fig. 3 (d), you can observe different lattice fringes, according to the spacing of the stripes to identify the crystal plane of Ag and BiOCl. Among them, the lattice fringe pitch of 0.275 nm corresponds to the {110} crystal plane of BiOCl [17]. The {111} crystal plane of Ag corresponding to the lattice fringe spacing of 0.24 nm corresponds to the {111} crystal plane in the XRD pattern [18–19]. Therefore, lattice fringes can further prove that Ag nanoparticles are successfully loaded on the surface of BiOCl nanosheets.
In addition, in order to understand the surface element composition of AB-10, EDS analysis was carried out, and the results are shown in Fig. 4. From Fig. 4 (b), the peaks corresponding to Ag, C1, O, Bi, and Au can be observed. The peak of Au is caused by gold spraying on the surface of the material before testing, indicating that the sample contains silver, bismuth, oxygen and Four elements of chlorine. Figure 4 (c)-(f) are the surface subdivision diagrams of the elements. It can be seen from the figure that the four elements are evenly distributed. The above analysis results indicate that Ag is successfully loaded on the BiOCl sample surface.
The photocatalytic performance of [email protected] samples was evaluated by degrading ARB under visible light. The experimental results are shown in Fig. 5. Among them, Fig. 5 (a) is a graph of the degradation of ARB by the sample. It can be seen from the figure that the degradation effect of Ag-loaded samples on ARB is significantly higher than that of BiOCl, and with the increase of Ag content, the degradation rate of ARB of samples first increases and then decreases. Among them, the degradation effect of AB-10 is significantly better than other samples. After 240 minutes of visible light irradiation, the degradation rates of ARB were 85%, and the degradation rates of AB-5 and AB-20 were 62% and 71%, respectively. At the same time, we can also see from the SEM spectrum of the sample that the Ag nanoparticles in AB-20 have agglomerated, which is not conducive to the photocatalytic degradation reaction, so the photocatalytic efficiency of AB-20 has been reduced. In AB-0, the degradation efficiency is only 21.2%, so it can be determined that the optimal loading of Ag is 10wt%. The UV-visible absorption spectrum of the solution during ARB degradation is shown in Fig. 5 (b). It can be seen that ARB has two characteristic absorption peaks, located at 516 nm and 326 nm, respectively. The absorption peak at λ = 326 nm in the ultraviolet region is a characteristic absorption peak of the naphthalene ring. The absorption at 516 nm is caused by the π→π* transition of the -N = N- bond. In addition, as the photocatalytic reaction time increases, the two characteristic absorption peaks at 326 nm and 516 nm gradually weaken, indicating that the matrix of the naphthalene ring or other unsaturated conjugated system is destroyed [20]. Additionally, the large specific surface area of the samples is beneficial to dye adsorption, light absorption, separation of photogenerated carriers, etc., which are all important factors affecting photocatalytic performance. The results of nitrogen adsorption isotherm show that as Ag content increases, the adsorption amount of the [email protected] sample increases first and then decreases. AB-10 has the maximum adsorption amount. The specific surface area calculated by the BET simulation is 65.8 m2/g. This is the other reason why AB-10 has superior properties than other samples.
In addition, the kinetic principle of ARB degradation of [email protected] samples was investigated, and the results are shown in Fig. 6. It can be seen from Fig. 6 (a) that -ln(C/C0) has a good linear relationship with the reaction time t (C0 and Ct are the initial concentration of ARB and the concentration at time t), which can determine the degradation reaction follow the first order kinetic equation. Figure 6 (b) is a bar graph of the reaction rate constant of the [email protected] sample to degrade ARB. It can be seen from the figure that the degradation rate constants of AB-0, AB-5, AB-10 and AB-20 are 6.6×10− 4, 3.45×10− 3, 6.58×10− 3 and 4.18×10− 3 respectively. It can be concluded from this that when the loading of Ag is 10wt%, the sample has better photocatalytic performance.
The UV-Vis DRS spectrum of the [email protected] sample is shown in Fig. 7. It can be seen from the figure that the absorption edge of AB-0 is located at 350 nm in the ultraviolet region, and there is little absorption in the visible range. The sample loaded with Ag showed strong and extensive light absorption in the entire visible light region, and the absorption intensity was significantly higher than AB-0. A new broad absorption band can be clearly seen around 473 nm, and the absorption intensity gradually increases with the increase of Ag content. The significant absorption in the visible region may be due to localized surface plasmon resonance (LSPR) absorption of silver nanoparticles [21–22]. It can be proved that the loading of Ag can successfully widen the light absorption range of BiOCl, and also shows that the [email protected] composite material can be used as an excellent visible light photocatalyst.
The photoluminescence spectrum of [email protected] was used to analyze the separation efficiency of photogenerated electrons and holes. At an excitation wavelength of 280 nm, the PL spectrum of the sample changes as shown in Fig. 8. It can be seen from the figure that the peak value of the sample decreases first and then increases with the increase of Ag content. Among them, the fluorescence intensity of AB-10 is significantly lower than that of other samples. Because the peak of the photoluminescence spectrum can reflect the photo-electron-hole recombination efficiency on the surface of the photocatalytic material, the low PL intensity means that the photo-electron-hole recombination probability is low, the life is long, and the concentration of e− and h+ species participating in the redox reaction high, and the effective action time is longer, these indicators correspond to the high photocatalytic activity of the material [23]. After loading Ag on BiOCl, the peak intensity of the PL spectrum is significantly reduced, indicating that Ag can be used as a capture center for photogenerated electrons, promote the effective separation of photogenerated electrons-holes, and improve photocatalytic performance [24]. From the above analysis, it can be seen that AB-10 has the highest photogenerated carrier separation efficiency and the strongest photocatalytic activity, which is consistent with the photodegradation results of ARB.
The arc radius of the AC impedance graph can reflect the size of the material's charge transfer resistance. The smaller the radius, the smaller the resistance and the higher the charge transfer efficiency[25]. Therefore, electrochemical impedance spectroscopy can be used to evaluate the interface charge transfer ability of photocatalytic materials. The electrochemical impedance spectrum of the [email protected] sample is shown in Fig. 9. It can be seen from the figure that the electron transmission impedance of the sample loaded by Ag is less than BiOCl, which shows that Ag loading can make the sample have a higher interface charge transfer ability, photogenerated the carrier separation efficiency is greatly improved, which is beneficial to improve the photocatalytic performance. Among them, the arc radius of AB-10 is the smallest, indicating that the sample has higher charge transfer efficiency and photocatalytic activity, which corresponds to the degradation results of ARB.
The free radicals produced by the degradation of ARB under visible light irradiation of AB-10 were detected by ESR. The results show that ‧OH, ‧O2−, and h+ are the three kinds of active materials, simultaneously being produced and participating in the degradation reaction during the photocatalytic reaction. Therefore, in order to further explore the mechanism of photocatalytic degradation of ARB in the samples, an active species capture experiment was carried out on AB-10. Isopropyl alcohol (IPA), ammonium oxalate (AO) and 1,4-benzoquinone (BQ) were used as traps for •OH, h+ and •O2−, respectively. The samples without the capture agent were compared, and the experimental results shown in Fig. 10 were obtained. It can be seen from the figure that after adding IPA, AO and BQ, the photocatalytic degradation of ARB by AB-10 has been inhibited to varying degrees. Among them, AO had the greatest inhibitory effect on the degradation reaction. After 240 minutes of exposure, the degradation rate of ARB was only 5.96%, while the degradation rate of adding IPA and BQ were 71.6% and 60.32%, respectively. Therefore, it can be inferred that during the entire photocatalytic reaction process: the role played by the three active substances is h+ >>•O2−>•OH.
Based on the above analysis, we discussed the possible mechanism of [email protected] photocatalytic degradation of ARB, as shown in Fig. 11. Under visible light irradiation, due to the higher band gap value, the BiOCl nanosheets cannot be excited to generate photo-generated electron-holes. After the Ag nanoparticles are loaded onto the BiOCl surface, the entire system can be excited by visible light. Under visible light, Ag nanoparticles will produce an SPR effect. The electrons excited by the plasma are first concentrated on the surface of the Ag nanoparticles, and then the enriched electrons are quickly injected into the conduction band of BiOCl, so that they are left on the Ag nanoparticles. It can oxidatively degrade h + of ARB. After the electrons on the Ag nanoparticles migrate to the conduction band of BiOCl, photogenerated e- and h+ react with O2 and H2O adsorbed on the surface of the material to generate active radicals: •O2− and •OH, and react with ARB to achieve photodegradation. Therefore, in the [email protected] system, the visible light photocatalytic activity of the entire system is enhanced due to the plasma resonance effect generated by Ag nanoparticles.
In order to study the reusability of the [email protected] sample, four repeated experiments were carried out on the AB-10 sample. The experimental results are shown in Fig. 12. In the first experiment, the degradation efficiency of ARB was 85%. With the increase of repeated use, the degradation efficiency of AB-10 to ARB decreased slightly, but it remained above 80%. Therefore, the experimental results of stability analysis show that the [email protected] photocatalyst prepared in this paper has good photocatalytic stability and can be used repeatedly.