3.1.Structure and composition
The crystal structures of the CN, 0.005TCN, 0.01TCN and 0.02TCN catalysts were tested by XRD. As shown in Fig. 1 at 13.0° and 27.2°, the 0.005TCN, 0.01TCN and 0.02TCN samples have the same profile as CN with two significant diffraction peaks. The dominant peak at 27.2° corresponds to the (002) crystal plane, which is due to the interlayer stacking reflection of the conjugated aromatic system. The peak at 13.0°is the (100) plane, caused by the periodic build-up of layers in the graphite structure (Han 2018). Since no significant impurity peaks were observed in xTCN, the doping of Tb ions may not affect the crystalline phase of CN. The intensity of the diffraction peak of the 0.005TCN catalyst (27.2° ) was significantly enhanced compared to CN, indicating that the participation of Tb enhanced the degree of polymerisation and crystallisation of the g-C3N4 interlayer network. However, the intensity of the 27.2° characteristic peak decreases with increasing Tb doping content, which may be due to the graphite diffraction intensity of the tri-s-triazinyl linker becoming weaker with increasing amounts of Tb incorporation (Jin 2019).
The molecular structure information of the prepared CN, 0.005 TCN, 0.01 TCN and 0.02 TCN was investigated using FTIR spectroscopy.As shown in Fig. 2, the CN shows three main absorption regions at 805 cm− 1, 1200–1700 cm− 1 and 3171 cm− 1. The sharp characteristic peak at 805 cm− 1 is mainly caused by the bending vibration of the triazine unit. The series of peaks in the 1200–1700 cm− 1 range are the result of aromatic C-N stretching vibrations and the deposition of C = N bonds (Qi 2016). The stretching vibrations of the N-H group and the surface adsorption of water molecules give rise to a broad absorption band at 3171 cm− 1. The FTIR profiles exhibited by xTCN and CN are essentially identical, except for the centre of the band between 2290 and 2390 cm− 1 which is due to C ≡ N stretching vibrations,indicating that the C ≡ N triple bond is formed and not due to aromatic C-N stretching vibrations. The characterization results of the FTIR spectra can be explained by the addition of Tb ions breaking some of the triazine rings of g-C3N4, transforming the sp2 C-N single bond into a C ≡ N triple bond (Wang 2019).
The surface elemental composition and valence state of the 0.01TCN material was further confirmed by XPS analysis. The main peaks of the elements carbon, nitrogen, terbium and oxygen were observed through the measured spectra of 0.01 TCN, as shown in Fig. 3d. The high-resolution spectra of C1s at 284.8, 285.4, and 288.1 eV are attributed to sp2 hybridized C = C, C = and N-C = N, respectively, which are derived from coupled g-C3N4.The peak carbon atom in a pure carbon environment is 284.8 eV. The peak of 0.01 TCN at 285.4 eV is the C-NHx of the heptane molecule (x = 1, 2). The main peak of 0.01 TCN located at 288.1 eV was assigned to N-C = N, that further verifies the presence of C ≡ N, as the binding energy of C ≡ N is similar to that of C-NHx (Fig. 3a). The high resolution profile of N1s can be fitted to three peaks at 398.4 eV,399.9 eV and 400.9 eV respectively. The peaks at 398.7 eV and 399.9 eV can be identified as sp2 hybridized N, where 398.7 eV corresponds to N in the N = C bond and 399.9 eV belongs to N in the N-(C)3 bond. Due to the incomplete polymerization of melamine and the adsorption of surface water, the peak with binding energy at 401.0 eV belongs to the amino functional group (N-Hx,x = 1,2) (Fig. 3b) (Zhang 2019). The positive charge localization in the heterocycle leads to the weakest peak at 404.8 eV. The peak around 152.8 eV in the Fig. 3c belongs to Tb4d5/2, indicating the presence of Tb3+ ions; the doped Tb is present in the form of Tb(lll) (Fig. 3c) (Lu 2017).
3.2 Morphological analysis
The microstructural features of CN, HCN, 0.005TCN, 0.01TCN and 0.02TCN catalysts were analyzed by transmission electron microscopy. For HCN, it exhibits a completely different morphology from that of CN. It exhibits a typical laminar porous structure and many pores of different diameters are observed in the layer structure. There are several reasons for this to happen: When nitric acid solution is added to melamine, the ionized H+ is transferred to the nitrogen atoms on the melamine triazine ring, which then causes the melamine to be protonated. In the heating process, the nitrate anion connected with the protonated melamine cation will decompose at high speed to produce a large amount of nitrogen. Nitrogen can be used as a bubble template to form mesoporous structures of CN. As shown in Fig. 4, 0.005TCN, 0.01TCN and 0.02TCN have a microstructure similar to that of HCN(Fig. 4) (Liu 2016; Xie 2018a).
When the photocatalyst is energized by light, e− in the valence band is excited to jump into the conduction band, resulting in e− and h+ pairs. When e− and h+ are compounded in the bulk phase or surface of the photocatalyst, some energy is released in the form of fluorescence. Usually, the complexation rate of photogenerated e− and h+ is proportional to the fluorescence intensity, and the PL characterization was performed to study the photogenerated carrier migration and separation of the prepared TCN samples by HCN and TCN samples, as shown in Fig. 5. From the figure, it can be seen that both HCN and TCN catalysts have a broad peak at around 460 nm, that belongs to the band photoluminescence specific to the photogenerated carriers of g-C3N4 polymer, and it can be found from the figure that the separation of photogenerated e− and h+ in the photocatalyst doped with Tb elements is significantly better than that of CN (Bui 2020); The order of luminous intensity is: HCN > 0.02TCN > 0.005TCN > 0.01TCN. This implies that the separation and transport rates of photogenerated carriers are significantly increased after Tb doping, and the Tb element doping and mesoporous structure have a significant effect on improving the separation and transport of charges.
3.3 Analysis of the light absorption properties of Tb/g-C3N4
The light absorption ability of the prepared samples was measured by UV-Vis spectrophotometer,and the light absorption performance of the catalyst has a great influence on its degradation of organic pollutants. The absorption edge of pure CN can be obtained from Fig. 6 to be about 450 nm, which is consistent with the previous literature. However, the optical absorption edges of 0.005TCN and 0.02TCN were shifted to 475 nm, and 0.01TCN was further red-shifted to 490 nm (Bui 2020). In addition, 0.005TCN, 0.01TCN and 0.02TCN exhibit stronger light absorption than g-C3N4 in the visible range, which may be due to the synergistic effect of Tb doping into g-C3N4 to form a heterojunction.
3.4 Photocatalytic performance
In order to determine the optimal doping concentration of terbium in g-C3N4, the degradation experiments of TC (25 mg/L) and TYL (25 mg/L) using photocatalysts with different terbium doping concentrations under simulated solar irradiation were carried out. Figure 7 shows the photodegradation rate curves of the photocatalytic effect of solar simulated light expressed by CN, 0.01TCN, 0.02TCN and 0.005 TCN for TYL and TC, respectively. According to the blank test, TC and TYL could hardly be degraded under simulated solar irradiation without a catalyst, which means that it is difficult to photolyze TC and TYL without a photocatalyst. The photocatalytic activity of CN was relatively low after 90 min of simulated sunlight irradiation, degrading only 52.1% TYL and 64.1% TC. In order to make a more visual comparison of the photocatalytic activity of CN and xTCN; the results of the photocatalytic degradation of antibiotic by CN and TCN samples were investigated using quasi-level kinetics, the equations are as follows:
-ln(C/C0) = kt (6)
where C is the concentration of TYL and TC at irradiation time t(min); C0 is the initial concentration of TYL and TC; and k denotes the apparent reaction rate constant. CN degraded TYL and TC with k values of 0.0127 and 0.0095; 0.01TCN degraded TYL and TC with k values of 0.0271 and 0.0239.
It can be noted from the figure that the photocatalytic activity of xTCN increases and then decreases with the increase of the molar concentration of Tb doping,and the possible reasons for this situation are discussed below. As the rare earth trivalent cation (Tb3+) has an incompletely occupied 4f orbital, it is thermodynamically feasible for Tb3+ to be reduced by photogenerated electrons. Therefore, the use of Tb3+ as a trapping agent is effective in trapping the electrons on VB and restoring them to Tb2+. Subsequently, the Tb2+ ion is oxidised back to Tb3+ by O2 in the ambient solution.
Tb3++e−→Tb2+ (7)
Tb2++O2→Tb3++·O2− (8)
Therefore, by doping with an appropriate concentration of Tb3+ ions can be used as an electron trapping agent, which not only promotes the rapid transfer of photogenerated electrons, but also reduces the complexation of electrons with holes, thus the quantum efficiency is increased. However, when the molar mass ratio of the doping of Tb(NO3)3·5H2O exceeded the optimum value of 0.01%, the degradation rate of the antibiotic decreased,as shown in Fig. 7. This situation is explained by the reduction of Tb3+ to Tb2+ by electrons, which means that an excess of terbium ions acts as a complex centre for electron-hole pairs, reducing the number of photogenerated electrons that would otherwise react with oxygen molecules to form superoxide radicals. At the same time, too much terbium ions can occupy the active sites on the catalyst surface, which is not conducive to improving photocatalytic activity. Therefore, doping with the right amount of terbium ions can improve the photocatalytic activity of the photocatalyst.
3.5 Stability evaluation
For photocatalysts, their stability and reproducibility are important factors affecting its reagent application. The stability as well as the reproducibility of the prepared 0.01TCN catalysts were confirmed by running the degraded antibiotics three times under the same conditions. The relative concentration (C/C0) of the antibiotics solution under simulated solar irradiation is shown in Fig. 8 as a function of time repeated over three cycles. the antibiotics photodegradation efficiency did not change significantly over the three cycles. The results show that the 0.01TCN photocatalyst has very good stability in the photocatalytic degradation process.
3.6 Toxicity test of leaching solution
The mean root length (AL) and seed germination rate (SR) of Chinese cabbage seeds were obtained by seed germination experiment, and the germination index was calculated according to formula 1. Figure 9a shows the germination conditions of different catalyst leaching solution as culture medium. In the seed germination experiment, the average root length (Al), germination percentage (Sr) and germination index (SGI) of Chinese cabbage seeds are shown in Table 1. It can be seen from Table 1 that the germination index of Chinese cabbage seeds cultivated with different catalyst leaching solutions fluctuated little, and the influence of different catalyst leaching solutions on the seed germination index was negligible. Figure 9b shows the growth of Chinese cabbage seedlings with different catalyst leaching solutions added to the culture medium. It can be seen from Fig. 9c that different catalyst leaching solutions have no inhibitory effect on Chinese cabbage seedlings and their chlorophyll content is almost the same, which does not affect the absorption and utilization of light. Therefore, it can be concluded that CN and TCN catalysts have little effect on seed germination.
3.6 Photocatalytic reaction mechanism
In general, the type of radicals originating from photoexcited electrons and holes directly determines the photocatalytic degradation mechanism. So we have revealed the mechanism of the photocatalytic reaction by examining the main oxides produced in the degradation reaction. ·O2, ·OH, H2O2 and h+ are the main oxide species involved in the photocatalytic reaction process.Therefore,in order to identify the active species that mainly participate in the reaction during catalysis, different bursting agents are added to the reaction solution to neutralise the free radicals (Jiang 2017). The main active species during the degradation reactions were studied by using isopropyl alcohol (IPA), ammonium oxalate (AO), benzoquinone (BQ) and Fe(II)-EDTA as trapping agents for ·OH, h+, ·O2− and H2O2. as shown in Fig. 10. The degradation of TC decreased from 83.5–10.8%,78.4%,76.2% and 47.2% after the addition of BQ, IPA, AO and Fe(II)-EDTA respectively,indicating that the addition of BQ had the greatest effect on the degradation reaction of antibiotics, followed by Fe(II)-EDTA; finally AO and IPA had the least effect,thus indicating that ·O2− was the main active species. The slight decrease in degradation rate caused by IPA, Fe(II)-EDTA and AO implies that ·OH, H2O2 and h+ are also involved in the degradation reaction. The order of importance of the oxide species is: ·O2− > H2O2 > ·OH > h+; so ·O2− and H2O2 radicals play an important role in the photocatalytic process and act as the main oxide species.
Based on the above photocatalytic performance tests, and free radical capture experiments, a preliminary explanation of the reaction mechanism of Tb3+/g-C3N4 photocatalysis is presented in Fig. 11. According to previous reports CN has a CB and VB of -1.4 eV and + 1.3 eV (Guo 2019) respectively. Since the reduction potential of photogenerated electrons is more negative than the redox potential of O2/·O2−(0.33 eV), O2 is more easily reduced by electrons to form ·O2−. The redox potentials of ·OH/OH− (+ 1.99 eV) and ·OH/H2O (+ 2.27 eV) are more positive than the potential of the highest occupied molecular orbital of CN (Yang 2019), so h + located in VB cannot oxidize H2O or OH− to produce ·OH that ·OH is produced from ·O2−. During the photocatalytic degradation process, we found that during the adsorption equilibrium phase,TCN catalysts were able to provide a greater abundance of photocatalytic active sites compared to CN due to the mesoporous structure of the TCN catalysts enabling them to expose more geometric surfaces, resulting in a larger specific surface area.In addition, the presence of a mesoporous structure increases the specific surface area,which improves the adsorption and diffusion processes during the reaction. 0.01TCN catalysts with a high specific surface area enable molecules such as pollutants or oxygen to come into contact with the catalyst surface more effectively, while generating more efficient capture of photogenerated e− and h+ for oxidation or reduction reactions, thus improving the photocatalytic effect. The hollow pores in the mesoporous structure allow the incident light to be refracted and scattered several times inside the catalyst, allowing more contact with the incident light and improving the utilisation of the light. Mesoporous structures can also reduce carrier complexation by reducing the migration distances of electrons and holes and speeding up the transfer of photogenerated e− and h+ to locations at the edge of the catalyst surface (Xie 2018b). Based on the above analysis,a photocatalytic mechanism for the photocatalytic degradation of antibiotics by 0.01TCN catalyst is proposed. As shown in the figure,e− in the valence band of the 0.01TCN sample is excited to jump to the conduction band, forming e− and h+ pairs. When the g-C3N4 polymer is doped with Tb, a Tb impurity energy level is formed between its conduction band and valence band,which means that e- in the 0.01TCN sample can both jump to the Tb impurity energy level and transfer to the conduction band of g-C3N4, which facilitates the separation of e− and h+ (Zhou 2019). A portion of the e− located in the conduction band and the Tb(III) impurity energy level then moves to the catalyst surface, which in turn reduces the O2 molecules adsorbed on the catalyst surface to ·O2−.Some of these ·O2− react with H+ to form H2O2 and ·OH. A series of free radicals are generated,which finally decompose pollutants such as TYC into H2O, CO2 (Shi 2015) and other inorganic substances to reduce pollution. Compared to pure g-C3N4, the impurity energy level generated by Tb in the 0.01TCN sample provides an alternative transfer pathway for e−, which more effectively inhibits the compounding of photogenerated electron holes, thus improving the photocatalytic activity of the sample.