3.1 Basic characterization of BiVO4/MWCNTs
In this work, the preparation of BiVO4/MWCNTs is given priority, and then Ag3PO4 is deposited on the body and loaded onto the cotton fiber. Therefore, the preparation conditions of a double composite catalyst are particularly important. The N2 adsorption capacity, UV-visible diffuse reflectance spectrum, and crystal structure of BiVO4 / MWCNTs were characterized in Fig. 2. As shown in Fig. 2A, the specific surface was 1.7361 m2/g, 7.8932 m2/g, 11.3093 m2/g, 5.7543m2/g, and 3.1932m2/g when the pH value was 1, 3, 5,7 and 9, respectively. In the dark reaction stage, the adsorption effect of samples at pH = 5, pH = 7, and pH = 9 is relatively prominent due to the existence of MWCNTs in the form of freedom in the preparation process (black spots in the prepared powder part), which improves the adsorption capacity of the sample. However, in the photoreaction stage, free MWCNTs could not promote catalysis, resulting in the degradation effect of relatively good combined samples (prepared at pH = 3) under the same conditions, and the absorbance of the dye solution decreased more obviously. As shown in Fig. 2B, When the 2θ Angle was 18.5°, 35°, 46°, all the diffraction peaks of BiVO4/MWCNTs were split peaks, while when the 2θ Angle was 28.6°, 30.5°, 55.6°, these peaks were characteristic peaks of monoclinic BiVO4 system compared with JCPDS. When pH = 3 and pH = 5, the single peak diffraction peak occurs at the 2θ Angle of 26.4°. Compared with the standard diffraction card, it is found that the peak belongs to the (002) crystal plane of carbon, indicating the successful composite of MWCNTs. In addition, the absorbance of the double composite catalyst is better in the ultraviolet region and lower in the visible region. With the change in the doping degree of MWCNTs themselves, the response degree to light varies in the measured wavelength range, as shown in Fig. 2C. Finally, hv-(Ahv)2 spectra of catalysts prepared at different pH values were drawn, and the tangents of the spectra were shown in Fig. 2D. The introduction of MWCNTs did reduce the band gap width of BiVO4 and obtained good light absorption performance under the preparation of pH = 3. This demonstrates that the composite catalyst is more easily excited by visible light and gains a good modification effect.
3.2 Characteristics of materials
Figure 3A exhibits the surface morphology of BiVO4. The monomers prepared in this work are blocky in shape and form partial stacking agglomerates. Figure 3B and Fig. 3C show the surface morphology of BiVO4/MWCNTs at different magnifications, from which it can be seen that the elongated MWCNTs accumulate on the surface of BiVO4 and promote the crystal agglomeration of it to some extent. Figure 3D-3F shows the powder dispersion of Ag3PO4/BiVO4/MWCNTs. The agglomerated surface of the three composite catalysts deposited many round crystal particles, which were not shaped the same as those of BiVO4, suggesting the surface accumulation of Ag3PO4, thus meaning the successful preparation of the composite catalysts. The composite catalyst can be adsorbed on the surface of cotton fabric under hydrothermal co-bath conditions, as shown in Fig. 3G as well as 3H. At higher magnification, the shape of the catalyst can be seen to be consistent with the characterization results in Fig. 3I, proving that hydrothermal heat did not change the underlying structure of the catalyst. Figure 3J shows the results of Ag3PO4/BiVO4/MWCNTs EDS energy spectrum test, where the most obvious survey results of Bi, O elements indicate the highest content in the catalyst. In addition, other elemental energy spectra can also be detected, demonstrating that the catalysts were successfully prepared. Finally, TEM was employed to determine the morphology of different catalysts and the composition of heterojunctions. From Figs. 3K, 3L, and 3M, it could be seen that MWCNTs had been successfully introduced onto the surface of BiVO4. The lattice spacing was about 0.25 nm, corresponding to the (0 0 2) crystal plane of BiVO4.Moreover, Ag3PO4/BiVO4/MWCNTs were also tested in Figs. 3N, 3O as well as 3P. Some small circular lamellar structures which belong to the Ag3PO4 compared with the SEM test results were observed in the triple compound system. The (2 0 2) (d = 0.18nm) crystal plane of BiVO4 also existed, further validating the synthesis of the catalyst.
XPS can be used to characterize the elemental composition and element valence state of composite catalysts. Figure 4A and 4B are the full spectra of the XPS survey of the samples. The figures illustrate that the prepared composite catalyst contains basic elements such as Bi, V, O, Ag, P, and C, which preliminarily verifies the successful composite BiVO4/MWCNTs and Ag3PO4/BiVO4/MWCNTs. In addition, Fig. 5C is the characterization of the Bi in the composite catalyst, from which it can be seen that its binding energies are at 158.4 eV and 164.5 eV, respectively, which are due to Bi 4f7/2 and formed by the presence of Bi 4f5/2. The potential of Ag3PO4/BiVO4/MWCNTs was shifted to the left by 0.35ev compared with that of BiVO4/MWCNT, indicating that there was a charge interaction between Ag3PO4 and the dual composite catalysts. (Sun et al. 2022b). In addition, Figs. 4D and 4E further verify the existence of V-O bonds in the composite catalysts. According to their binding energy assignments, it can be shown that V and O in the composite catalysts exist in the form of VO43−(Zhao et al. 2020b). The binding energy peak of the C element (284.8eV) in Fig. 4H. This is due to the presence of C-C bonds in the sample(Mei et al. 2022, Wu et al. 2022). Therefore, Figs. 4C, 4D, 4E, and 4H fully demonstrate the existence of BiVO4 and MWCNTs in the samples (Khazaee et al. 2021, Sun et al. 2022a). The deposition of Ag3PO4 can be seen in Figs. 4F and 4G. Figure 4F is the characterization of the Ag element in the sample, and its peak binding energies are at 368.4eV and 374.2eV, respectively, which proves that the valence of the Ag element in the composite catalyst is + 1, Fig. 4G is the characterization of P 2P and the peak energy at 133.4eV verifies the existence of P-O(Chen et al. 2022a, Ma &Cheng 2022). In conclusion, XPS characterization shows the existence of each element in the composite catalyst and its valence form, which can further confirm the successful preparation of Ag3PO4/BiVO4/MWCNTs (Chen et al. 2022a).
Figure 5A casts the N2 adsorption-specific surface area test for different composite catalyst components. The specific surface area of the composite catalysts gradually increases with the increase of the catalyst composite components, proving that their adsorption in the dark reaction stage will be strengthened. Then, electrochemical basis tests were performed for different catalysts. Figure 5B is the results of the EIS impedance test. The radius of curvature of the impedance curve for the monomeric bismuth vanadate is the largest, while the radius of curvature for the triple composite catalyst turns out to be the smallest, indicating that the increase in composite composition bears a negative effect on the interfacial resistance of the catalyst. Then the photocurrent of the catalyst was measured for every 20s intervals of switching light time, as shown in Fig. 5C. The maximum photocurrent of Ag3PO4/BiVO4/MWCNTs in the figure ensures the superior photogenerated carrier separation efficiency of the catalyst, which is consistent with the results of the PL photoluminescence spectroscopy test in Fig. 5D. Under the excitation wavelength of 315nm, the composites obtained similar stronger diffraction peaks at wavelength differences of no more than 5 nm. The more components, the lower the diffraction peaks, suggesting that the fluorescence effect of the material gradually decreases, which is negatively correlated with the separation efficiency of photogenerated carriers. Figure 5E as well as Fig. 5F exhibit the UV-vis diffuse reflectance spectra and hv-(Ahv)2 spectra for the three catalysts, respectively. The increase of the composite composition of the catalysts can significantly change the UV absorption and redshift of the semiconductor together with reducing its band gap width, which sets up its utilization of the solar spectrum and good photocatalytic performance.
3.3 Degradation analysis of dyes and heavy metal ions
The degradation curves of BiVO4, BiVO4 / MWCNTs, Ag3PO4/BiVO4/MWCNTs, and Ag3PO4/BiVO4/MWCNTs @ Cotton for reactive blue KN-R under different preparation conditions were shown, respectively(Fig. 6A-6C), to determine the optimal preparation conditions of composite catalytic cotton fabrics. The experimental results showed that the composite catalyst could be effectively loaded on the cotton fiber when the pH value was 3, the reaction time was 6 h, the system temperature was 160°C, and the mass ratio of Ag3PO4: BiVO4/MWCNTs was 1: 2. What is more, the decolorization rate of the dye reached more than 90% within 60 min, indicating the high efficiency of Ag3PO4/BiVO4/MWCNTs. Figure 6D is the further verification of the photodegradation performance of the final functional fabric. By comparing the degradation curve of the reaction without light for a long time and under the condition of light, it is determined that the catalytic degradation performance of functional cotton fabrics is mainly from photocatalysis, rather than the pseudo-degradation caused by physical adsorption. Under the optimal preparation conditions of catalysts, this study characterizes the catalytic performance of different catalysts under the same degradation environment. Figure 6E shows the degradation of reactive blue KN-R. The results show that with the increase of composite components, the photodegradation efficiency of the catalyst for dye liquor gradually increases, and the Ag3PO4/BiVO4/MWCNTs@Cotton functional cotton fabric retains three of the decolorization effect of the composite component catalyst is that the decolorization rate of the dye solution is 92% within 100 minutes; Fig. 6F shows the degradation of Cr (VI). With the increase of the composite component, the final content of Cr (VI) gradually decreases. The removal rate of Cr (VI) by Ag3PO4/BiVO4/MWCNTs@Cotton functional cotton fabric can reach 90%. This shows that the functional cotton fabric can solve the problem of easy agglomeration of powder catalysts, as well as realize catalyst loading and retain the catalytic performance of powder catalysts. Finally, Fig. 6G and 6H are the before and after comparisons of reactive blue KN-R and Cr conversion.
As shown in Fig. 7, Fig. 7A-7D simulates the degradation ability of composite catalytic cotton fabric in different environments by changing the inorganic salt concentration, dye concentration, dye pH value, and surfactant concentration of the degraded dye solution. The inorganic salts and the concentration of surfactants have the effect of promoting first and then inhibiting the catalytic effect of functional cotton fabrics, which is considered to be related to the dyeing promotion of dyes and the critical micelle concentration of surfactants. The effect of dye liquor concentration and pH value on the decolorization rate is negatively correlated. The higher the concentration, the greater the pH value, and the lower the catalytic effect, which may be related to the concentration of the catalyst itself along with acid and alkali resistance. These test results can provide theoretical references for the practical application of composite catalysts.
The cycle performance of the composite catalyst is also the focus of this study. The functional cotton fabric can effectively degrade reactive blue KN-R within 150 minutes, and the decolorization rate is maintained at about 90% in Fig. 8A. The degradation can be cycled 5 times, and the degradation effect of the sixth time is also maintained at 80%; Fig. 8B is the repeatability test of the composite catalyst for the Cr (VI) removal experiment. The results show that the target fabric can achieve 5 cycles of Cr (VI) degradation, and the degradation effect can reach 85% within 100min above. This shows that functional cotton fabrics can alleviate the problems of powder catalyst recollection and difficult reuse, and have good versatility. Figure 8C casts the XPS survey spectrum of the functional fabric before and after cycling, which indicates that no loss of elements occurred before and after cycling. In addition, the leaching of the composite catalyst on the fabric can be obtained by measuring the weight loss rate of the functional fabric, as shown in Fig. 8D. The weight loss rate of the functional fabric after 5 cycles was 9.6%, suggesting that most of the catalyst was retained, which is consistent with the results of cyclic degradation. Figure 8E is the surface morphology of the fabric after 5 cycles of degradation, which proved that there is still a lot of catalyst loading. Figure 8F is the macroscopic comparison before and after the degradation. Some dyes settle on the surface of the fabric, which is considered to be caused by the adsorption performance of the functional fabric for multiple cycles of degradation.
3.4 Simulation experiment
Combined with the dyeing principle of the overflow dyeing machine, a simulation test was carried out on the sewage treatment performance of functional fabrics to verify the applicability of functional fabrics, as shown in Fig. 9. Wastewater is transferred to the overflow drum under the action of the water pump, and the functional fabric is acted on by the spray device. Then, the wastewater is subjected to the first catalytic treatment under the action of ultraviolet rays followed by the initially treated wastewater being transferred to the overflow machine through the action of the transport pipe. In the process, the functional fabric rotates in a conveyor belt-like manner under the irradiation of ultraviolet light, and the wastewater is treated twice by catalysis. Finally, the wastewater after the secondary treatment is collected for subsequent dyeing and finishing.
3.5 Possible photodegradation mechanism
The feasibility of the experimental materials can be verified by theoretical calculation. In this work, GGA-PBE of CASTEP was used to optimize the structure of the experimental subject BiVO4, and its bandwidth and state density were calculated. The results are shown in Fig. 10A-10C. Figure 10A calculates its bandwidth, and the calculated result is 2.532eV, which is 0.8% different from the above experimental result of 2.51eV. It is within the allowable error range and proves that BiVO4 is successfully synthesized. The results of state density calculation showed that the Fermi energy level of BiVO4 is in the interval of DOS value 0, and it has good covalent bond performance, which verifies the semiconductor performance of BiVO4. Finally, Fig. 10D shows the catalyst heterojunction structure optimized by GGA-PBE, and the combination of BiVO4 and Ag3PO4 can be seen from the side and front respectively.
To explore the active substances in the process of dye degradation, free radical capture experiments on dyes were conducted using DMPO. The detection results are shown in Figs. 11A and 11B. Figure 11A shows the capture of OH· free radicals. EPR is shown as a wavy line in the figure under dark conditions, indicating that OH· does not exist. After the light was turned on, the peak spectrum of OH· free radical increased significantly, indicating that a certain amount of OH· free radical was produced under the light condition. Figure 11B shows the EPR spectrum of DMPO-·O2−. The change of its peak value is similar to that of OH· free radical, proving that the catalyst can produce a good catalytic effect under light conditions. As shown in Fig. 11C, different scavengers were selected (including hole scavenger sodium oxalate (Na-OA), free radical ·O2− scavenger benzoquinone (BQ), and free radical ·OH scavenger tert-butanol (t-BuOH) in the degraded solution along with the dye without scavenger is added as a blank sample. The composite catalyst can achieve a decolorization rate of more than 95% for reactive blue KN-R under visible light irradiation without adding any scavenger. After adding the hole scavenger sodium oxalate (Na-OA), the decolorization rates of the functional fabric and the composite catalyst to the dye were reduced to 28% and 18%, respectively, indicating that photogenerated holes are the main active free radicals in this reaction. In addition, the addition of free radical ·O2− scavenger benzoquinone (BQ) can also reduce the decolorization effect to 1/3 of the original, meaning that ·O2− is also the main active substance in this reaction. Finally, the addition of t-BuOH did not have a great influence on the decolorization effect of the experimental samples, which stated that ·OH was not the main active substance of the system. As shown in Fig. 11D, the addition of MWCNTs can increase the wave-absorbing properties of the monomer BiVO4, whose absorption edges are at 505nm and 520 nm, respectively. The forbidden bandwidth of the catalyst can be calculated according to the absorption edge, and the specific calculation method is as follows:
E g= 1239.6 / λ g (1)
E g is the forbidden band width (band gap energy) of the sample, λ g is the wavelength value at the intersection of the extension line of the diffuse reflection side and the horizontal axis. According to (1), the band gap energies of BiVO4 and BiVO4/MWCNTs can be obtained to be 2.51 eV and 2.38 eV, respectively.
As shown in Fig. 12, taking BiVO4/MWCNTs(BM)as a whole and depositing Ag3PO4 on this basis is the main goal of this experiment. According to previous reports, the CB of Ag3PO4 is 0.29 eV, VB is 2.6 eV(Gao et al. 2018), and the modified BM is still an N-type semiconductor. Considering its valence band and conduction band potential, it can form a type II heterojunction with Ag3PO4. Under the irradiation of visible light, both BM and Ag3PO4 can generate photogenerated carriers. Due to the formation of a type II heterojunction between the two, the holes in the valence band of BM can be transferred to the valence band of Ag3PO4, and the photogenerated electrons generated by Ag3PO4 are rapidly transferred to the conduction band of the BM conductor under the action of the heterojunction. The pollutant molecules can directly react with the photogenerated holes to generate CO2 and H2O and other harmless substances, which is the main process of the composite catalyst to degrade the dye. In addition, the transferred photogenerated electrons can be captured by heavy metal ions, and Cr (VI) is thus reduced to Cr (III). The remaining photogenerated electrons can react with reactive oxygen species in water to form ·O2−, ·O2− further react with pollutants in wastewater to generate CO2 and H2O. The possible reaction formulas in the photocatalysis process are as follows:
$$\text{B}\text{M}+\text{h}\text{v}\to {\text{h}1}^{+}+{\text{e}1}^{-}$$
2
$${\text{O}\text{H}}^{-}+{\text{h}1}^{+}\to ·\text{O}\text{H}$$
3
$$·\text{O}\text{H}+\text{d}\text{y}\text{e}\to {\text{C}\text{O}}_{2}+{\text{H}}_{2}\text{O}$$
4
$${ \text{A}\text{g}}_{3}{\text{P}\text{O}}_{4}+\text{h}\text{v}\to {\text{h}2}^{+}+{\text{e}2}^{-} (5)$$
$${\text{C}\text{r}}^{6+}+{\text{e}2}^{-}\to {\text{C}\text{r}}^{3+}$$
6
$${ \text{h}1}^{+}\to {\text{h}2}^{+}+\text{d}\text{y}\text{e}\to {\text{C}\text{O}}_{2}+{\text{H}}_{2}\text{O} (7)$$
$${\text{e}2}^{-}\to {\text{e}1}^{-}+{\text{O}}_{2}\to {·\text{O}}_{2}^{-}+\text{d}\text{y}\text{e}\to {\text{C}\text{O}}_{2}+{\text{H}}_{2}\text{O}$$
8