Fabrication of a Novel CNT-COO−/Ag3PO4@AgIO4Composite with Enhanced Photocatalytic Activity under Natural Sunlight

In this study, a carboxylated carbon nanotube-grafted Ag3PO4@AgIO4 (CNT-COO−/Ag3PO4@AgIO4) composite was synthesized through an in situ electrostatic deposition method. The synthesized composite was characterized by Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), diffuse reflectance spectroscopy (DRS), and energy-dispersive X-ray spectroscopy (EDS). The electron transfer ability of the synthesized composite was studied using electrochemical impedance spectroscopy (EIS). The CNT-COO−/Ag3PO4@AgIO4 composite exhibited higher activity than CNT/Ag3PO4@AgIO4, Ag3PO4@AgIO4, and bare Ag3PO4. The material characterization and the detailed study of the various parameters thataffect the photocatalytic reaction revealed that the enhanced catalytic activity is related to the good interfacial interaction between CNT-COO and Ag3PO4. The energy band structure analysis is further considered as a reason for multi-electron reaction enhancement. The results and discussion in this study provide important information for the use of the functionalized CNT-COOH in the field of photocatalysis. Moreover, providinga new way to functionalize CNT viadifferent functional groups may lead to further development in the field of photocatalysis. This work could provide a new way to use natural sunlight to facilitate the practical application of photocatalysts toenvironmental issues.

Carbon-based substrates played an important role in supporting the photocatalytic performance of the photocatalysts due to their novel chemical, thermal, electrical, and optical properties. Carbon nanotubes (CNT), which are fabricated from sp2 carbon, have attracted researchers' attention due to their unique specific surface area (100-700 m 2 g −1 ), and electronic, optical, chemical, and thermal characteristics [12,[19][20][21][22][23][24][25]. These properties recommend CNT as a novel material for catalyst carriers and supporters in heterogeneous catalysts. Some studies have reported improved photocatalytic activity for CNT-based composites [26][27][28][29][30][31]. However, the low dispersion and the poor interfacial interaction between CNT and other materials remain the major disadvantages that limit their practical application as catalyst supporters [32][33][34]. Therefore, functionalization of CNT achieved by attaching electronegative groups or aliphatic carbon chains may increase their dispersion. Furthermore, this process can provide a more active surface on the CNT, allowing for a greater amount of catalyst materials to be loaded onto the CNT's surface.Based on the above analysis, CNT might be used as an electron capture agent to increase the activity and stability of Ag 3 PO 4 .Therefore, developing a high-efficiency photocatalyst is always a hot research topic in the photocatalytic field. In this study, CNT-COO − /Ag 3 PO 4 @AgIO 4 was synthesized with enhanced photocatalytic activity through an in situ electrostatic deposition method, which was then characterized using different techniques. The photocatalytic activity of the synthesized CNT-COO/Ag 3 PO 4 @AgIO 4 was evaluated based onthe degradation of methylene blue as the target dye under natural sunlight. In addition, the effect of the optimal CNT-COOH amount on the photocatalytic performance was also investigated. Moreover, the mechanism of photodegradation was proposed.

The Synthesis Mechanism of CNT-COO − /Ag 3 PO 4 @AgIO 4
In this work, CNT-COO − /Ag 3 PO 4 @AgIO 4 was synthesized viathe electrostatic deposition method, as shown diagrammatically in Figure 1. After the negatively charged CNT-COO was suspended in water and the Ag + ions were added, the electrostatic interaction derived the adsorption of the positively charged Ag + ions onto the negatively charged CNT-COO to form intermediate complexes of CNT-COO − /Ag + . After Na 2 HPO 4 was dropped into the mixture, the HPO 4 2− could react with Ag + ions on the surface of CNT-COO to form Ag 3 PO 4 nuclei. As the reaction proceeded, Ag 3 PO 4 particles successfully grew on the surface of CNT-COO. The addition of KIO 4 , which was added dropwise, into the solution mixture resulted in the formation of large-sized AgIO 4 particles, which were grafted by CNT-COO − /Ag 3 PO 4 to form the CNT-COO − /Ag 3 PO 4 @AgIO 4 composite.

FT-IR Analysis
The as-synthesized materials were first characterized by FT-IR, and the results areshown in Figure 2a. Non-carboxylated CNT does not have any characteristic peaks,whereas the carboxylated one (CNT-COOH) shows CO stretching vibration at about 1700 cm −1 , and the − OH groups show stretching and bending vibrations at about 3450 cm −1 and 1615 cm −1 , respectively, confirming the successful carboxylation of CNT. In

FT-IR Analysis
The as-synthesized materials were first characterized by FT-IR, and the results areshown in Figure 2a. Non-carboxylated CNT does not have any characteristic peaks, whereas the Molecules 2023, 28, 1586 3 of 14 carboxylated one (CNT-COOH) shows CO stretching vibration at about 1700 cm −1 , and the − OH groups show stretching and bending vibrations at about 3450 cm −1 and 1615 cm −1 , respectively, confirming the successful carboxylation of CNT. In the FTIR spectra of AgIO 4, a band located between 626-830 cm −1 is attributed to the vibrations between Ag-I-O atoms. The − OH group of the physically absorbed H 2 O shows stretching and bending vibrations at 3450 cm −1 and 1630 cm −1 , respectively. In the spectra of Ag 3 PO 4 , the peak located at 553 cm −1 is attributed to the bending vibration of the O=P-O. The symmetric and asymmetric stretching vibrations of P-O-P rings are found at 853 and 998 cm −1 , respectively. The absorbed H 2 O molecules are shown to have bending and stretching vibrations at 1662 and 3420 cm −1 , respectively. The P=O shows stretching vibration at 1393 cm −1 . In the spectra of CNT-COO − /Ag 3 PO 4 @AgIO 4 and Ag 3 PO 4 @AgIO 4 , all the characteristic vibration peaks of the individual materials CNT-COO − , Ag 3 PO 4 , and AgIO 4 are present, which proves the formation of the heterocomposite.

SEM Analysis
The morphologies of the synthesized composites were analyzed viaSEM, and the results are shown in Figure 3. Figure S1a,b of the Supporting Information show images of CNT before and after purification, respectively. The carboxylated CNT-COOH was shown in Figure 3a, with different lengths ranging between 100 nm and 1µm. Figure 3b shows the image of the cubic-like morphology of Ag3PO4. AgIO4 displayed a hexagonal microstructure (Figure 1c Supporting Information). In Figure 3c, it can be seen that the micro-size AgIO4 was successfully coupled with Ag3PO4 particles. Figure 1d Supporting Information shows the SEM image of CNT/Ag3PO4@AgIO4. Figure 3d,e represented a different magnification of the SEM images of CNT-COO − /Ag3PO4@AgIO4. As seen, the Ag3PO4@AgIO4 particles successfully decorated the CNT-COO surface. Moreover, the particle size decreased compared tothe individual one. This is beneficial forenhanced photocatalytic activity. The EDS element spectra of the CNT-COO − /Ag3PO4@AgIO4 composite are shown in Figure 3f, which further confirmed the presence of the C, O, Ag, P, and I elements on the as-synthesized CNT-COO − /Ag3PO4@AgIO4.

XRD Analysis
The phase purity and the crystal structure of AgIO 4 , Ag 3 PO 4 , Ag 3 PO 4 @AgIO 4 , and CNT-COO − /Ag 3 PO4@AgIO 4 composites are identified by XRD spectra, which are shown in Figure 2b. Pure Ag 3 PO 4 shows characteristic diffraction peaks that are identical to the body-centered cubic structure of Ag 3 PO 4 [35].The XRD pattern of AgIO 4 diffraction peaks can be indexed to the tetragonal structure of AgIO 4 [36]. TheXRD results indicate that the well-crystallized Ag 3 PO 4 and AgIO 4 were successfully fabricated under experimental conditions. For Ag 3 PO 4 @AgIO 4 , theXRD patterns showed diffraction peaks corresponding to Ag 3 PO4 and AgIO 4 crystal phases, proving that AgIO 4 and Ag 3 PO 4 are well coupled and that Ag 3 PO 4 @AgIO 3 has been successfully synthesized. In the pattern of CNT-COO − /Ag 3 PO 4 @AgIO 4 , a smalldecrease is observed compared with Ag 3 PO 4 @AgIO 4 , suggesting that the combination of CNT-COOH does not affect the crystalline structure and phase composition of Ag 3 PO 4 @AgIO 4 ; therefore, no apparent diffraction peak is observed for CNT-COOH in the XRD pattern of CNT-COO − /Ag 3 PO 4 @AgIO 4 . This could be due to their relatively low percentage in the composites.Alternatively, the functionalization process maynarrow those peaks due to the loss of amorphous carbon, which is in good agreement with the previous reports [23,37]. The XRD patterns confirmed the fabrication of the CNT-COO − /Ag 3 PO 4 @AgIO 4 composite.

SEM Analysis
The morphologies of the synthesized composites were analyzed viaSEM, and the results are shown in Figure 3. Figure S1a,b of the Supporting Information show images of CNT before and after purification, respectively. The carboxylated CNT-COOH was shown in Figure 3a, with different lengths ranging between 100 nm and 1µm. Figure 3b shows the image of the cubic-like morphology of Ag 3 PO 4 . AgIO 4 displayed a hexagonal microstructure ( Figure S1c Supporting Information). In Figure 3c, it can be seen that the micro-size AgIO 4 was successfully coupled with Ag 3 PO 4 particles. Figure S1d Supporting Information shows the SEM image of CNT/Ag 3 PO 4 @AgIO 4 . Figure 3d,e represented a different magnification of the SEM images of CNT-COO − /Ag 3 PO 4 @AgIO 4 . As seen, the Ag 3 PO 4 @AgIO 4 particles successfully decorated the CNT-COO surface. Moreover, the particle size decreased compared tothe individual one. This is beneficial forenhanced photocatalytic activity. The EDS element spectra of the CNT-COO − /Ag 3 PO 4 @AgIO 4 composite are shown in Figure 3f, which further confirmed the presence of the C, O, Ag, P, and I elements on the as-synthesized CNT-COO − /Ag 3 PO 4 @AgIO 4 .
The morphologies of the synthesized composites were analyzed viaSEM, and the results are shown in Figure 3. Figure S1a,b of the Supporting Information show images of CNT before and after purification, respectively. The carboxylated CNT-COOH was shown in Figure 3a, with different lengths ranging between 100 nm and 1µm. Figure 3b shows the image of the cubic-like morphology of Ag3PO4. AgIO4 displayed a hexagonal microstructure (Figure 1c Supporting Information). In Figure 3c, it can be seen that the micro-size AgIO4 was successfully coupled with Ag3PO4 particles. Figure 1d Supporting Information shows the SEM image of CNT/Ag3PO4@AgIO4. Figure 3d,e represented a different magnification of the SEM images of CNT-COO − /Ag3PO4@AgIO4. As seen, the Ag3PO4@AgIO4 particles successfully decorated the CNT-COO surface. Moreover, the particle size decreased compared tothe individual one. This is beneficial forenhanced photocatalytic activity. The EDS element spectra of the CNT-COO − /Ag3PO4@AgIO4 composite are shown in Figure 3f, which further confirmed the presence of the C, O, Ag, P, and I elements on the as-synthesized CNT-COO − /Ag3PO4@AgIO4.

Optical Properties
The optical properties of the synthesized photocatalysts were evaluated using DRS analysis. The DRS spectra of Ag 3 PO 4 , Ag 3 PO 4 @AgIO 4, and CNT-COO − /Ag 3 PO 4 @AgIO 4 are shown in Figure 4a. The absorption edges of CNT-COO − /Ag 3 PO 4 @AgIO 4 , Ag 3 PO 4 @AgIO 4, and Ag 3 PO 4 were observed at about 586, 455, and 543 nm, respectively. The light absorption of Ag 3 PO 4 @AgIO 4 was extended in the visible light region compared with that of pure Ag 3 PO 4 . This is due to the combination of the two silver salts. The absorption edge of CNT-COO − /Ag 3 PO 4 @AgIO 4 was further extended to 586 nm with the introduction of the CNT-COO. This confirms that the CNT-COO − /Ag 3 PO 4 @AgIO 4 composite strongly enhanced the absorption of the visible light of Ag 3 PO 4 due to the good interfacial interface between CNT-COO − and Ag 3 PO 4 @AgIO 4 , which is beneficial to the enhancement of the photocatalytic activity. The band gaps of the synthesized composites are determined by plotting the transformed Kubelka-Munk function of light energy (αh Molecules 2023, 28, x FOR PEER REVIEW

Optical Properties
The optical properties of the sy analysis. The DRS spectra of Ag3PO4 shown in Figure 4a. The absorption e and Ag3PO4 were observed at abou sorption of Ag3PO4@AgIO4 was exten pure Ag3PO4. This is due to the com of CNT-COO − /Ag3PO4@AgIO4 was f the CNT-COO. This confirms that enhanced the absorption of the visib face between CNT-COO − and Ag3PO the photocatalytic activity. The band by plotting the transformed Kubelka (hʋ), (see Figure 4b). The band gaps w eV attributed to CNT-COO − /Ag3PO spectively.

Optical Pro
The optical p analysis. The DRS shown in Figure 4 and Ag3PO4 wer sorption of Ag3PO pure Ag3PO4.
Thi of CNT-COO − /Ag the CNT-COO. T enhanced the abs face between CN the photocatalyti by plotting the tra (hʋ), (see Figure 4 eV attributed to spectively. ), (see Figure 4b). The band gaps were estimated to be 2.05 eV, 2.23 eV, 2.4 eV, and 2.45 eV attributed to CNT-COO − /Ag 3 PO 4 @AgIO 4 , Ag 3 PO 4 @AgIO 4, and Ag 3 PO 4 @AgIO 4 , respectively. enhanced the absorption of the visible light of Ag3PO4 due to the good interfacial interface between CNT-COO − and Ag3PO4@AgIO4, which is beneficial to the enhancement of the photocatalytic activity. The band gaps of the synthesized composites are determined by plotting the transformed Kubelka-Munk function of light energy (αhʋ) 2 versus energy (hʋ), (see Figure 4b). The band gaps were estimated to be 2.05 eV, 2.23 eV, 2.4 eV, and 2.45 eV attributed to CNT-COO − /Ag3PO4@AgIO4, Ag3PO4@AgIO4, and Ag3PO4@AgIO4, respectively.

Electrochemical Impedance Spectroscopy
The electron-hole recombination resistance and the charge transfer ability ofthe synthesized photocatalysts were measured using electrochemical impedance spectroscopy (EIS). As shown in Figure 5, the diameters of the arc followed the order CNT-COO − /Ag3PO4@AgIO4<CNT-Ag3PO4@AgIO4< Ag3PO4@AgIO4< Ag3PO4. Therefore, it can be confirmed that CNT-COO − /Ag3PO4@AgIO4 exhibited enhanced e − /h + separation and transferability at the catalyst-electrolyte interface compared with the other composites. In addition, the good interaction between the CNT-COO and Ag3PO4 in the CNT-COO/Ag3PO4@AgIO4 composite further enhanced the electron transfer over the CNT-Ag3PO4@AgIO4, which was due to the interfacial bonding between the CNT-COO and Ag3PO4, which resultedin the higher photocatalytic activity of CNT-COO − /Ag3PO4@AgIO4.

Optical Properties
The optical properties of t analysis. The DRS spectra of A shown in Figure 4a. The absorp and Ag3PO4 were observed at sorption of Ag3PO4@AgIO4 was pure Ag3PO4. This is due to the of CNT-COO − /Ag3PO4@AgIO4 the CNT-COO. This confirms enhanced the absorption of the face between CNT-COO − and A the photocatalytic activity. The by plotting the transformed Ku (hʋ), (see Figure 4b). The band g eV attributed to CNT-COO − /A spectively. The electron-hole recomb synthesized photocatalysts we copy (EIS). As shown in Fi CNT-COO − /Ag3PO4@AgIO4<CN it can be confirmed that CNT-C and transferability at the cataly sites. In addition, the good CNT-COO/Ag3PO4@AgIO4 com CNT-Ag3PO4@AgIO4, which w and Ag3PO4, which res CNT-COO − /Ag3PO4@AgIO4.

Electrochemical Impedance Spectroscopy
The electron-hole recombination resistance and the charge transfer ability ofthe synthesized photocatalysts were measured using electrochemical impedance spectroscopy (EIS). As shown in Figure 5, the diameters of the arc followed the order CNT-COO − /Ag 3 PO 4 @AgIO 4 < CNT-Ag 3 PO 4 @AgIO 4 < Ag 3 PO 4 @AgIO 4 < Ag 3 PO 4 . Therefore, it can be confirmed that CNT-COO − /Ag 3 PO 4 @AgIO 4 exhibited enhanced e − /h + separation and transferability at the catalyst-electrolyte interface compared with the other composites. In addition, the good interaction between the CNT-COO and Ag 3 PO 4 in the CNT-COO/Ag 3 PO 4 @AgIO 4 composite further enhanced the electron transfer over the CNT-Ag 3 PO 4 @AgIO 4 , which was due to the interfacial bonding between the CNT-COO and Ag 3 PO 4 , which resultedin the higher photocatalytic activity of CNT-COO − /Ag 3 PO 4 @AgIO 4 .
Molecules 2023, 28, x FOR PEER REVIEW 6 of 15 Figure 5. Impedance spectrum of the synthesized composites.

Photocatalytic Degradation Results and Analysis
The photocatalytic activities of the synthesized composites were evaluated based onthe photocatalytic degradation of MB under natural sunlight irradiation. To investigate the efficient catalytic activity between different CTN-based composites, their photodegradation performance toward MB was carried out under natural sunlight. The carboxylatedCNT-based composite (CNT-COO − /Ag3PO4@AgIO4) exhibited higher activity than the non-carboxylated one (CNT/Ag3PO4@AgIO4), Figure 6a.This is because the surface electric properties of CNT-COO greatly increased the dispersity of the catalyst in the aqueous dye solution ( Figure S2a Supporting Information), which increased the contact between the dye and the catalyst. Moreover, the adsorption of the positively charged dye on the CNT-COO − /Ag3PO4@AgIO4 increased due to the electrostatic attraction between the different charges ( Figure S2b Supporting Information). Additionally, the electrostatic interaction between CNT-COO and Ag3PO4 significantly enhanced the charge transfer, which therefore decreasedthe recombination between the photogener-  COO − /Ag 3 PO 4 @AgIO 4 increased due to the electrostatic attraction between the different charges ( Figure S2b Supporting Information). Additionally, the electrostatic interaction between CNT-COO and Ag 3 PO 4 significantly enhanced the charge transfer, which therefore decreasedthe recombination between the photogenerated electron-hole pairs. This is in good agreement with the EIS results.

Simultaneous Degradation of Different Organic Dyes
The photocatalytic activity of the as-synthesized composite toward the decomposition of a mixture of organic dyes was investigated under natural sunlight. MO and Rh B with a concentration of 0.01 g/L are used as representative dyes. As clearly seen in Figure  S4 Supporting Information, nearly 90% of MO degraded within 2 min, while 60% of Rh B degraded at the same time. The results further confirm the enhanced catalytic activity of the synthesized composite.

Photocatalytic Reaction Kinetics
The kinetic behavior of the synthesized composite on the degradation of the organic dyes under light irradiation could be expressed as follows: −ln(Ct/Co) = kt (1) where k is the degradation rate constant, Co is the initial concentration of MB, and Ct is the concentration of the dye at the irradiation time of t. Figure 7 shows the regression curves of-ln(Ct/Co) versus irradiation time, indicating that the photodegradation of MB over different samples is considered to fit pseudo-first-order kinetics. The MB degradation rate constants under different conditions are shown in Table 1. As seen, CNT-COO − /Ag3PO4@AgIO4-5% composite has predominantly enhanced photocatalytic performance for the degradation of MB with a degradation rate constant of 0.877min −1 compared to CNT/Ag3PO4@AgIO4,Ag3PO4@AgIO4, and Ag3PO4, which have a rate constant of 0.4143 min −1 0.3107 min −1 and 0.1611 min −1 , respectively. To study the effect of the light intensity on the photocatalytic activity of CNT-COO − /Ag3PO4@AgIO4-5%, a set of photocatalytic experiments were carried out using a 350 W Xe lamp as a light source. As seen in Figure S5 Supporting Information, the photocatalytic activity of CNT-COO − /Ag3PO4@AgIO4-5%is increased with the increase of the light intensity, and the degradation rate constants of the dye are0.877 min −1 , 0.794 min −1 , 0.595 min −1 and 0.309 min −1 , corresponding to the light intensities of 100%, 75%, To investigate the optimal CNT-COOH amount on the synthesized composites, the photocatalyticperformanceofCNT-COO − /Ag 3 PO 4 @AgIO 4 -2.5%, CNT-COO − /Ag 3 PO 4 @ AgIO 4 -5%, and CNT-COO − /Ag 3 PO 4 @AgIO 4 -7.5% has been tested in relation tothe degradation of MB ( Figure S3 Supporting Information). The results revealed that CNT-COO − / Ag 3 PO 4 @AgIO 4 -5% exhibited the highest catalytic activity. When the content of CNT-COO increased to 7.5 mg, the degradation efficiency wasreduced. This may be due to the increasing amount of CNT-COO − shielding the surface of the photosensitive Ag 3 PO 4 from the light, and the dye may have beenisolated from direct contact with the catalyst surface.
A photocatalytic activity comparison study was carried out between CNT-COO − / Ag 3 PO 4 @AgIO 4 -5% and its contents, Ag 3 PO 4 @AgIO 4 and Ag 3 PO 4 to further prove the photocatalytic efficiency of CNT-COO − /Ag 3 PO4@AgIO 4 -5%. As seen in Figure 6b, in the absence of the photocatalyst and/or light irradiation, the dye degradation can be ignored. CNT-COO − /Ag 3 PO 4 @AgIO 4 exhibited the highest photodegradation of the dye, with nearly 100% of MB decomposingwithin 4 min, while the Ag 3 PO 4 @AgIO 4 and the bare Ag 3 PO 4 displayed degradation efficiencies of 70% and 40% within 4 min, respectively. The enhanced photocatalytic activity is attributed to the excellent charge separation and transferring of CNT-COO − /Ag 3 PO 4 @AgIO 4 -5% composite.

Simultaneous Degradation of Different Organic Dyes
The photocatalytic activity of the as-synthesized composite toward the decomposition of a mixture of organic dyes was investigated under natural sunlight. MO and Rh B with a concentration of 0.01 g/L are used as representative dyes. As clearly seen in Figure S4 Supporting Information, nearly 90% of MO degraded within 2 min, while 60% of Rh B degraded at the same time. The results further confirm the enhanced catalytic activity of the synthesized composite.

Photocatalytic Reaction Kinetics
The kinetic behavior of the synthesized composite on the degradation of the organic dyes under light irradiation could be expressed as follows: −ln(Ct/Co) = kt (1) where k is the degradation rate constant, Co is the initial concentration of MB, and Ct is the concentration of the dye at the irradiation time of t. Figure 7 shows the regression curves of-ln(Ct/Co) versus irradiation time, indicating that the photodegradation of MB over different samples is considered to fit pseudo-first-order kinetics. The MB degradation rate constants under different conditions are shown in Table 1. As seen, CNT-COO − /Ag 3 PO 4 @AgIO 4 -5% composite has predominantly enhanced photocatalytic performance for the degradation of MB with a degradation rate constant of 0.877min −1 compared to CNT/Ag 3 PO 4 @AgIO 4 ,Ag 3 PO 4 @AgIO 4 , and Ag 3 PO 4 , which have a rate constant of 0.4143 min −1 0.3107 min −1 and 0.1611 min −1 , respectively.

Catalyst Recycling
To investigate the reusability of the synthesized composite, the CNT-COO − /Ag3PO4@AgIO4-5%was repeatedly exposed to degraded MB solution. As mentioned in the photocatalytic activity study, CNT-COO − /Ag3PO4@AgIO4-5% was suspended in the dye solution and then subjected to natural sunlight illumination, and the degradation of the dye was determined as mentioned above. This process was repeated for three cycles. After each cycle, the composite was washed using deionized water and dried at 60 °C, then used for the next cycle. As seen in Figure 8, the synthesized catalyst displayed efficient reusability and stability during three successive cycles.  To study the effect of the light intensity on the photocatalytic activity of CNT-COO − / Ag 3 PO 4 @AgIO 4 -5%, a set of photocatalytic experiments were carried out using a 350 W Xe lamp as a light source. As seen in Figure S5 Supporting Information, the photocatalytic activity of CNT-COO − /Ag 3 PO 4 @AgIO 4 -5%is increased with the increase of the light intensity, and the degradation rate constants of the dye are0.877 min −1 , 0.794 min −1 , 0.595 min −1 and 0.309 min −1 , corresponding to the light intensities of 100%, 75%, 50%, and 25%, respectively.

Catalyst Recycling
To investigate the reusability of the synthesized composite, the CNT-COO − / Ag 3 PO 4 @AgIO 4 -5%was repeatedly exposed to degraded MB solution. As mentioned in the photocatalytic activity study, CNT-COO − /Ag 3 PO 4 @AgIO 4 -5% was suspended in the dye solution and then subjected to natural sunlight illumination, and the degradation of the dye was determined as mentioned above. This process was repeated for three cycles.
After each cycle, the composite was washed using deionized water and dried at 60 • C, then used for the next cycle. As seen in Figure 8, the synthesized catalyst displayed efficient reusability and stability during three successive cycles.

Catalyst Recycling
To investigate the reusability of the synthesized composite, the CNT-COO − /Ag3PO4@AgIO4-5%was repeatedly exposed to degraded MB solution. As mentioned in the photocatalytic activity study, CNT-COO − /Ag3PO4@AgIO4-5% was suspended in the dye solution and then subjected to natural sunlight illumination, and the degradation of the dye was determined as mentioned above. This process was repeated for three cycles. After each cycle, the composite was washed using deionized water and dried at 60 °C, then used for the next cycle. As seen in Figure 8, the synthesized catalyst displayed efficient reusability and stability during three successive cycles.

Photocatalytic Activity Mechanism
To investigate the effect of reactive species (h + , O2 •− , and • OH) on the photodegradation of organic pollutants, reactive species trapping experiments were carried out.It

Photocatalytic Activity Mechanism
To investigate the effect of reactive species (h + , O 2 •− , and • OH) on the photodegradation of organic pollutants, reactive species trapping experiments were carried out.It should be noted that 1 mM of Na 2 -EDTA, BZQ, and tert-butanol were used for scavenging h + , O 2 •− , and • OH, respectively. The experimental results in Figure 9 revealed that the addition of the scavengers to the reaction systemreduced the dye degradation efficiency, whereas, the introduction of BZQ and Na 2 -EDTA into the reaction system reduced the photocatalytic activity of the CNT-COO − /Ag 3 PO 4 @AgIO 4 -5% composite. Therefore, in this case, the photocatalytic degradation of the organic dyes over the CNT-COO − /Ag 3 PO 4 @AgIO 4 -5% composite mainly depends on the h+, the O 2 •− , and the • OH.
Molecules 2023, 28, x FOR PEER REVIEW 9 of 15 should be noted that 1 mM of Na2-EDTA, BZQ, and tert-butanol were used for scavenging h + , O2 •− , and • OH, respectively. The experimental results in Figure 9 revealed that the addition of the scavengers to the reaction systemreduced the dye degradation efficiency,whereas, the introduction of BZQ and Na2-EDTA into the reaction system reduced the photocatalytic activity of the CNT-COO − /Ag3PO4@AgIO4-5% composite. Therefore, in this case, the photocatalytic degradation of the organic dyes over the CNT-COO − /Ag3PO4@AgIO4-5% composite mainly depends on the h+, the O2 •− , and the • OH.

Possible Mechanism
The photocatalytic mechanisms of semiconductor composites can be illustrated by the Z-scheme theory and the heterojunction energy band theory [38]. Figure 10 shows the possible e-/h+ transfer pathways based on the two mechanisms. Under natural sunlight irradiation, both Ag3PO4 and AgIO4 could be excited to form the e − /h + pairs. According to the proposed heterojunction energy-band theory mechanism in Figure 10a, the electrons in the CB of Ag3PO4 can be transferred to the CB of AgIO4 easily, while the holes in the VB

Possible Mechanism
The photocatalytic mechanisms of semiconductor composites can be illustrated by the Z-scheme theory and the heterojunction energy band theory [38]. Figure 10 shows the possible e − /h+ transfer pathways based on the two mechanisms. Under natural sunlight irradiation, both Ag 3 PO 4 and AgIO 4 could be excited to form the e − /h + pairs. According to the proposed heterojunction energy-band theory mechanism in Figure 10a, the electrons in the CB of Ag 3 PO 4 can be transferred to the CB of AgIO 4 easily, while the holes in the VB of AgIO 4 can be transferred to the VB of Ag 3 PO 4 due to the potentials of the CB and VB positions of AgIO 4 (CB = 0.96 eV and VB = 3.61 eV) [18] are lower than those of the Ag 3 PO 4 (CB = 0.45 eV and VB = 2.85 eV) [13,39].Therefore, the electrons concentrate on the CB of AgIO 4 . However, the CB of AgIO 4 is more positive than the reduction of the oxygen E0 (O 2 /O 2 •− ) (0.13 eV, vs. NHE) [40]. Therefore, the O 2 •− could not be generated. However, the scavenger study proved the significant effect of the O 2 •− on the dye degradation process, whichindicatesthat the photocatalytic activity mechanism of CNT-COO − /Ag 3 PO 4 @AgIO 4 composite could not be illustrated by the heterojunction energy-band theory. The mechanism proposed by the Z-scheme theory described in Figure 10b can be obtained intwo ways. First, atthe contact interface between Ag 3 PO 4 and AgIO 4 , many defects can be aggregated. Therefore, the quasi-continuous energy levels can be formed in the contact interface Ag 3 PO 4 -AgIO 4 . This led to the contact interface exhibiting the conductor properties, such as the formation of the Ohmiccontact [38]. Second, as observed in the SEM analysis, Ag 3 PO 4 and AgIO 4 have a matching surface edge. Thus, Ag 0 can be formed at the contact interface between Ag 3 PO 4 and AgIO 4 . Therefore, during the photocatalytic reaction, the Ag 3 PO 4 -AgIO 4 contact interface develops to be the recombination centerof the photoexcited electrons from CB of AgIO 4 and the holes from VB of Ag 3 PO 4 . As a result, the photoexcited electrons on the CB of Ag 3 PO 4 can be easily transferred to the CNT, leading to enhanced charge separation. This is in good agreement with the EIS, DRS, and photocatalytic activity study results. Therefore, the Z-scheme theory provides more supportstothe photocatalytic mechanism pathway of our catalyst. The same illustration has also been proved in previous studies [41][42][43].

Chemicals
AgNO3 was purchased from Tianjin Sailing Chemical Reagent Technology Co. Ltd., sodium hydrogen phosphate (dibasic and monobasic) was purchased from Aladdin Co. Ltd., disodium ethylene diamine tetra acetate (Na2-EDTA) was purchased from Yan-taishiShuangshuang Chemical Co. Ltd., tert-butanol was purchased from Tian-jinshiKaixin Chemical Co. Ltd., p-benzoquinone (BZQ) was purchased from Tian-jinshiGuangfu Chemical Reagent Institute, MB was purchased from TianjinshiTianxin Fine Chemical Industry Development Center, and HNO3 and H2SO4 were purchased from Sinopharm Chemical Reagent Co. Ltd. All the chemicals were used as purchased without further treatment, except for CNT, which was first purified by refluxing in nitric acid.

Optical Properties
The optical properties of the synthesized photo analysis. The DRS spectra of Ag3PO4, Ag3PO4@AgIO shown in Figure 4a. The absorption edges of CNT-CO and Ag3PO4 were observed at about 586, 455, and sorption of Ag3PO4@AgIO4 was extended in the visib pure Ag3PO4. This is due to the combination of the t of CNT-COO − /Ag3PO4@AgIO4 was further extended the CNT-COO. This confirms that the CNT-COO − enhanced the absorption of the visible light of Ag3P face between CNT-COO − and Ag3PO4@AgIO4, which the photocatalytic activity. The band gaps of the syn by plotting the transformed Kubelka-Munk function (hʋ), (see Figure 4b). The band gaps were estimated t eV attributed to CNT-COO − /Ag3PO4@AgIO4, Ag3P spectively.
hVB + eCB (2)  As explained in Figure 10b, after the photocatalyst is exposed to sunlight, the CNT-COO − /Ag 3 PO 4 @AgIO 4 composite generates multi-electron/holes (Equation (2)). AgIO 4 excites electrons to its CB, leaving holes in the VB;these electrons recombine with the holes generated by Ag 3 PO 4 in the contact interface between the two semiconductors. The photoexcited electron on the CB of Ag 3 PO 4 further migrates to the functionalized CNT. As a result, the holes concentrate in the VB of AgIO 4 and the electrons concentrate on the functionalized CNT. Then, the h + can directly interact with the dye (Equation (3)), and may also decompose water molecules into • OH radicals (Equation (4)). Meanwhile, the electrons on the surface of CNT can be scavenged by the dissolved O 2 to form superoxide radicals (O 2 •− ) (Equation (5)). Finally, these photogenerated species can effectively degrade MB into CO 2 , H 2 O, and other intermediates (Equation (6)) [44][45][46][47].
As discussed above, it can be concluded that the enhancement of the photocatalytic activity of the CNT-COO − /Ag 3 PO 4 @AgIO 4 composite should be attributed to the significant coeffects between the Ag 3 PO 4 @AgIO 4 nanoparticle and CNT-COO. First, an Ag 3 PO 4 particle with a narrower band gap (2.05 eV) and exceptional absorption in the visible and near UV regions qualifies the composite with a high capability for harvesting light. In addition, the more efficient transfer of photogenerated electrons from excited Ag 3 PO 4 @AgIO 4 to the dye molecules under light irradiation enhanced the photocatalytic efficiency.
Second, the interlayer between Ag 3 PO 4 @AgIO 4 and CNT-COO can prevent the recombination of thephotoexcited electron-hole pairs. Third, the existence of H-bonds and the non-covalent intermolecular π-π conjugation between the MB and the CNT-COO − /Ag 3 PO 4 @AgIO 4 composite can significantly improve the adsorption of the dye molecules and provide increasing interfacial contact. Therefore, offering more active adsorption sites and photocatalytic reaction centers is beneficial for the enhancement of photocatalytic performance [48,49].

Chemicals
AgNO 3 was purchased from Tianjin Sailing Chemical Reagent Technology Co., Ltd., sodium hydrogen phosphate (dibasic and monobasic) was purchased from Aladdin Co., Ltd., disodium ethylene diamine tetra acetate (Na2-EDTA) was purchased from YantaishiShuangshuang Chemical Co., Ltd., tert-butanol was purchased from TianjinshiKaixin Chemical Co., Ltd., p-benzoquinone (BZQ) was purchased from TianjinshiGuangfu Chemical Reagent Institute, MB was purchased from TianjinshiTianxin Fine Chemical Industry Development Center, and HNO 3 and H 2 SO 4 were purchased from Sinopharm Chemical Reagent Co., Ltd. All the chemicals were used as purchased without further treatment, except for CNT, which was first purified by refluxing in nitric acid.

Preparation of Carboxylated CNT (CNT-COOH)
For the preparation of CNTs-COOH, CNT was first purified by refluxing in diluted nitric acid for 4 h. It was then sonicated in a mixture of 1:3 (v/v) HNO 3 (70%)/H 2 SO 4 (98%) for 8 h at room temperature. Afterwards, it was centrifuged at 11,000 rpm and dried at 70 • C for 12 h.

Synthesis of CNT-COO − /Ag 3 PO 4 @AgIO 4
The CNT-COO − /Ag 3 PO 4 @AgIO 4 composite was synthesized viathe room-temperature chemical fabrication method. Typically, carboxylated CNT (CNT-COOH) wassonicated in water for 30 min to obtain a water dispersion of CNT-COOH. Then, 1.2 g/20 mL of AgNO 3 was added to the CNT desperation, which was stirred for 4 h in dark conditions. Afterward, 0.8 g/20 mLof Na 2 HPO 4 was added drop by drop. After 1 h of reaction, a solution of KIO 4 (0.04 g/20 mL) was added in drops, and the reaction contents were kept inthe same conditions for another 1h. Then, the precipitate was collected viafiltration and dried at 60 • C for 12 h.

Synthesis of CNT/Ag 3 PO 4 @AgIO 4
For the synthesis of CNT/Ag 3 PO 4 @AgIO 4 composite, non-carboxylated CNT was sonicated in water for 2 h to obtain a water dispersion of CNT. Then, 1.2 g/20 mLof AgNO 3 was added to the CNTs' desperation under stirring for 4 h in dark conditions. Afterward, 0.8 g/20 mLof Na 2 HPO 4 was added drop by drop. After 1 h of reaction, a solution of KIO 4 was added in drops, and the reaction contents were kept at the same condition for another 1 h. Then, the precipitate was collected viafiltration and dried at 60 • C for 12 h. For comparison, Ag 3 PO 4 @AgIO 4 and Ag 3 PO 4 were synthesized following the same procedure without using CNT for the synthesis of Ag 3 PO 4 @AgIO 4 , and CNT and KIO 4 for the synthesis of Ag 3 PO 4 .

Material Characterization
X-ray diffraction (XRD) measurements were carried out at room temperature using a Bruker D8-Advance X-ray powder diffractometer with Cu-Kα radiation (λ = 1.5406 A) in the 2θ range of 10 • C to 80 • C. The morphology and the composition were characterized using ascanning electron microscope (SEM) with an EDS system. The UV-visible diffuse reflectance (DRS) spectra were obtained on a Shimadzu UV-2450 spectrophotometer. The Fourier transform infrared (FT-IR) spectra of the samples were recorded on a Bruker Vertex 70 FT-IR spectrophotometer using the KBr method. The electron transfer properties of the synthesized composites were studied using an electrochemical impedance spectrometer (EIS) VMP2 multi-potentiostat with the Zsimpwin program.

Evaluation of the Photocatalytic Activity
The study of the photocatalytic activity of the synthesized photocatalysts was carried out using the MB solution at room temperature under natural sunlight. Briefly, 25 mg of the prepared photocatalyst was mixed with 50 mLof MB (0.01 g/L) andthen sonicated for 10 min to establish adsorption-desorption equilibrium. Afterward, the mixture was exposed to natural sunlight. During the illumination, 5 mLaliquots were collected at a given time interval andthen centrifuged (10,000 rpm, 10 min) to remove the photocatalyst. The concentrations of MB solutions were determined by the UV-visible spectrophotometer at 664 nm. Additionally, the effect of light intensity on the photocatalytic activity was tested using a 350 W Xe lamp as a light source.

Conclusions
A novel CNT-COO − /Ag 3 PO 4 @AgIO 4 was synthesized through electrostatic deposition of Ag 3 PO 4 on the surface of the carboxylated carbon nanotube (CNT-COOH), followed by the growth of AgIO 4 . Compared with CNT/Ag 3 PO 4 @AgIO 4 , CNT-COO − /Ag 3 PO 4 @AgIO 4 exhibited enhanced optical and charge transfer properties. The optimal CNT-COOH content on CNT-COO − /A g3 PO 4 @AgIO 4 was investigated; the composite with 5 mg of CNT-COOH exhibited the highest photocatalytic activity.Moreover, CNT-COO − /Ag 3 PO 4 @AgIO 4 demonstrated enhanced photocatalytic performance compared to that of CNT/Ag 3 PO 4 @AgIO 4 , Ag 3 PO 4 @AgIO 4 , and Ag 3 PO 4 under natural sunlight irradiation. The enhanced photocatalytic activity is attributed to the best light harvesting and inductionof electron-hole pair separation and transfer. The results of this study provide important information for the use of functionalized CNT-COOH in the field of photocatalysis. Furthermore, they present a new way to functionalize CNT usingdifferent functional groups, which may lead to further development in the field of photocatalysis. Moreover, this work could providea new way for using natural sunlight to facilitate the practical application of photocatalysts in environmental issues.