3.1 Morphology and structure of CAT
Fig. 2 showed the SEM images of the synthesized CAT composites with different proportions. When the mass ratio was 20:1, the SEM image of CA sample was shown in Fig 2a, and the amount of AgNPs coated on CNTs surface was less, that was, most CNTs surface has no AgNPs. This was mainly because the content of treated nano silver was too low to effectively adhere to the surface of CNTs. When the mass ratio increased to 10:1, as shown in Fig. 2b, the number of silver particles on the surface of CNTs increased significantly. However, when the proportion of CNTs/Ag was increased to 5:1, as shown in Fig. 2c, the coating effect on CNTs does not further improve. This was mainly because too much nano-silver was easy to cause agglomeration, resulting in poor loading effect. Comparing the morphologies, it was considered that the CNTs /Ag was 10:1, the CA composite with uniform surface coating of AgNPs could be obtained. When the content of CA was 10%, only a small amount of CA material could be seen from Figure 2d, which indicates that TiO2 was not uniformly dispersed on the surface of CA, which might be due to the low content of CA, which could not provide sufficient active sites, which makes TiO2 particles agglomerate together. When the content of CA was 15% and 20%, as shown in Fig. 2e and 2f, the outline of carbon tubes was clearer, and TiO2 loaded on the surface was not agglomerated on a large scale, indicating that TiO2 particles were well attached to the surface of samples. In order to further determine the dispersion uniformity of the composite material, the energy spectrum analysis of CAT samples with 10% CA content was tested. The EDX spectrum of nanocomposite was depicted in Fig. 3, confirming the presence of Ti and AgNPs. The sample showed that TiO2 was evenly dispersed, and AgNPs coated on CNTs surface well, the three components were evenly distributed.
The peak position and strength of the sample FTIR could be used to reflect the changes of functional groups on the surface of the prepared nanocomposites. As shown in Fig. 4a, CNTs showed a characteristic strong vibration band at 3430cm-1, which was attributed to the stretching of -COOH group and OH adsorbed water molecules. The bands at 1420cm-1 were attributed to the OH deformation vibration of -COOH group. The bands at 1640cm-1were attributed to the C=O tensile vibrations. The bands at 1212-1175cm-1 and 1112-1038cm-1 were attributed to -C-O tensile vibrations. These peaks confirmed the introduction of the -COOH group in the acid treatment of CNTs(Natarajan et al. 2017). In CAT materials, the strength of vibration bands at 3430 and 1623cm-1 weakened as they move towards the low wave number at 3424 and 1567cm-1, indicating that the deformation vibration of O-H was replaced by the deformation vibration of Ti-OH on the surface of CNTs. The wide absorption bands between 1000 and 500cm-1 were attributed to the bending vibrations of the Ti-O-Ti and Ti-O-C bonds. The shift of the C-O bond's absorption band from 1093 to 1115cm-1 was also attributed to the tensile vibration of the Ti-O-C bond(Azzam et al. 2019).
The crystal structure of the composite samples was characterized by XRD. Used to determine whether the mixing process has an effect on the crystal structure of the sample. As shown in Fig. 4b, there were obvious diffraction peaks at 2θ of 38.1°, 44.2°, 64.40° and 77.4°, corresponding to the (111), (200), (220) and (311) crystal faces of Ag (JCPDS No.04-0783).There were obvious diffraction peaks at 2θ of 25.3°, 38.6°, 48.1°, 53.90°, 55.1° and 62.7°, corresponding to the (101), (004), (200), (105), (211) and (204) crystal faces of anatase phase TiO2 (JCPDS No. 21-1272), respectively. In the XRD pattern of CA samples, when 2θ was 25.8°, the characteristic diffraction peak of CNTs was observed, which corresponds to the (002) crystal plane of typical graphite sheet(Zhou et al. 2020). However, there was no diffraction peak corresponding to CNTs in the XRD pattern of CAT samples, which might be the proximity between the main characteristic peak of CNTs at 25.8° and the main peak of anatase TiO2 at 25.3°, resulting in the overlapping of diffraction peaks and the increase of peak width(Ahmad et al. 2017; Zhou et al. 2020). The diffraction peak of Ag was obvious in the XRD pattern of CAT samples, and we could see the diffraction peaks of Ag and TiO2 were very close (Ag (111) and TiO2 (004)) (Chaudhary et al. 2017; Tan et al. 2017; Zhang et al. 2019). The results clearly indicated that the prepared CAT sample had photocatalytic reaction sites.
3.2 Degradation performance of CAT on CR wastewater
In order to explore the photocatalytic performance of CAT sample under visible light, the treatment effect of CNTs/Ag with different proportions of CAT and the ability of CAT with different CA content to degrade CR wastewater were tested under visible light, and the results were shown in Figure 5. The uniformity of CA composites was affected by different CNTs/Ag ratios. As shown in Fig. 5a, CR wastewater was completely degraded within 150 minutes when CNTs/Ag ratio was 10:1. This was because the CA composite with AgNPs coated on the surface was uniform. Therefore, in the prepared CAT sample with CA treated content of 15%, AgNPs were better dispersed on the surface of the sample, so that the role of Ag as an induced electron could be better exerted, the electron-space separation efficiency of TiO2 could be improved, and the degradation effect of TiO2 on CR wastewater could be promoted(Espino-Estévez et al. 2016). Different treated amounts of CA had a direct impact on CAT degradation of CR wastewater, as shown in Fig. 5b. Due to the separation of photogenerated electrons by Ag as visible photosensitizer and CNTs, CAT sample degradation rate of CR wastewater could reach 90% when CA treated amount was 10%. With the increase of CA treated amount, the treatment effect first increased and then decreased. The maximum removal effect was obtained when the content of CA was 15%, and 100mL of CR wastewater with a concentration of 100mg/L could be degraded in 140min. When the mass fraction of CA was 20%, the treatment effect was not as good as 15% CA. The possible reason was that the excessive CA content reduces the uniformity of the TiO2 coating, affects the absorption of photons by TiO2, produces a shielding effect, and affects the degradation efficiency. The removal rate of CR wastewater in CT samples with mass fraction of 15% prepared by CNTs and TiO2 was 82%. The reason was that TiO2 nanoparticles were attached to the sidewall of CNTs, which made TiO2 and CNTs in a good bonding state at the interface, and the formed Ti-O-C bond separates the photogenerated hole-electron pair and reduces the band gap width. Therefore, the catalytic activity of CT samples was improved, the absorption of visible light was significantly enhanced, and the photocatalytic performance of composite materials was improved(Zhao et al. 2020). In addition, on the one hand CNTs could be used as a carrier of photocatalytic reaction to block photo-generated hole-electron recombination, and on the other hand, Ti-O-C could be formed to expand the interface area and improve the degradation effect of CR wastewater in CT samples under visible light, when 15%CT samples degrade CR wastewater (Nguyen et al. 2016).
3.3 Number of reuses for CAT degradation on CR wastewater
To study the stability of CAT samples with CNTs/Ag ratio of 10:1 and CA treated ratio of 15%, repeated cyclic degradation experiments were carried out, and the results were shown in Fig.6. After the catalytic degradation was finished, the catalyst was repeatedly washed with deionized water, and the catalytic degradation experiment was carried out again in the same environment, repeated five times. It could be found that the degradation efficiency of 15% CAT remained stable in the first four experiments, and decreased slightly in the fifth experiment, which indicated that 15% CAT had high catalytic performance and excellent stability.
3.4 Photocatalytic degradation mechanism of CAT
In order to further determine the degradation mechanism of CAT composites, FTIR and XRD of CAT samples with 15% CA treated after reaction were measured. As shown in Fig. 7a, CAT showed a characteristic strong vibration band at 3424 and 1567cm-1, and the tensile strength of Ti-OH group and OH adsorbed by water molecules decreases, which might be the reaction between activated electron holes and adsorbed water or OH- to form highly active superoxide radical ions, thus achieving the effect of catalytic degradation(Jung et al. 2015). After the end of the reaction, the absorption bands of CAT at 3424 and 1567cm-1 might be related to the formation of OH formed by hydrogen bonds between the benzene ring on the surface of CNTs and organics containing oxygen functional groups(Zhao et al. 2018). The bending vibration strength of the Ti-O-Ti and Ti-O-C bonds in the wide absorption band between 1000 and 500cm-1 was weakened, and the tensile vibration strength of the -C-O bond in the absorption band between 1112-1038cm-1 was weakened. This was attributed to the adsorption of CR wastewater onto CNTs by complexation with oxygen-containing functional groups. Therefore, the adsorption mechanism of CNTs for CR wastewater might include the electrostatic adsorption on the sample surface, π-π interaction and the complexation of oxygen-containing functional groups of the adsorbents(Wang et al. 2015). The bending vibration intensity of T-O-C bond was weakened, which indicated that Ag was mainly used as photosensitizer in the photocatalysis of samples, which played an important role in the efficient photocatalytic activity of TiO2 under visible light. It was found from Fig. 7b that the structure of CAT composite had not changed obviously during the photocatalytic reaction, which indicated that TiO2, Ag and CNTs were only carriers of photo-generated electron transfer in the photocatalytic reaction, and their own structure had not changed. The oxidation-reduction reaction with organic pollutants was mainly caused by superoxide radicals with high activity(El Mragui et al. 2021). CNTs mainly played a role in the separation of photogenerated electrons. Due to the local surface plasmonic resonance effect of Ag , it could not only improve the separation of photogenerated charge carriers, but also generate hot electrons transfer to TiO2 and induce photocatalytic reaction, which improved the utilization rate of TiO2 for visible light(Chen et al. 2021). Therefore, the photocatalytic reaction of the prepared CAT ternary composites under visible light may be as follows: visible light resonates with Ag, and electrons are injected into the TiO2 conduction band. At the same time, the Ti-O-C bond formed by TiO2 and CNTs reduces the band gap width, so that the electrons in the TiO2 conduction band were excited, and the excited electrons were further transferred by a well-conducted Ag and CNTs to promotes the separation of electron holes. When electrons and holes flow, a Schottky potential barrier was formed at the interface between Ag and TiO2, which inhibits the recombination of electron holes in the degradation process and further improves the catalytic activity of CAT materials in visible light.