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Article

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

by
Abdalla A. Elbashir
1,2,*,
Mahgoub Ibrahim Shinger
3,
Xoafang Ma
4,
Xiaoquan Lu
4,
Amel Y. Ahmed
1,5,* and
Ahmed O. Alnajjar
1
1
Department of Chemistry, College of Science, King Faisal University, Al-Hofuf 31982, Saudi Arabia
2
Department of Chemistry, Faculty of Science, Khartoum University, P.O. Box 321, Khartoum 11111, Sudan
3
Department of Applied and Industrial Chemistry, Faculty of Pure and Applied Sciences, International University of Africa, Khartoum 11111, Sudan
4
Key Laboratory of Bioelectrochemistry & Environmental Analysis of Gansu Province, College of Chemistry & Chemical Engineering, Northwest Normal University, Lanzhou 730070, China
5
Chemistry and Nuclear Physics Institute, Sudan Atomic Energy Commission, P.O. Box 3001, Khartoum 11111, Sudan
*
Authors to whom correspondence should be addressed.
Molecules 2023, 28(4), 1586; https://0-doi-org.brum.beds.ac.uk/10.3390/molecules28041586
Submission received: 27 December 2022 / Revised: 28 January 2023 / Accepted: 1 February 2023 / Published: 7 February 2023
(This article belongs to the Topic Nanomaterials for Sustainable Energy Applications)
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Abstract

:
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.

1. Introduction

Since the application of semiconductors as photocatalysts was discovered [1], enormous efforts have been madeto develop and enhance their catalytic performance by discovering new photosensitized material or designing heterogeneous semiconductor photocatalysts [2,3,4,5,6,7,8,9,10,11,12]. Recently, Ag3PO4 has been discovered as a novel semiconductor photocatalyst with exclusive photocatalytic activity toward oxygen evolution and organic pollutant removal [13,14,15]. However, its highest possibility of photocorrosion stands as the main drawback that hinders its practical application [13,16,17]. Therefore, it isnecessaryto overcome this drawback by designing and constructing Ag3PO4-based composites. Recently, AgIO4 withconduction and valence bands of 3.61 eV and 0.96 eV, respectively, has beenreported as a promising visible light-responsive photocatalyst [18]. Therefore, fabricating Ag3PO4 with AgIO4 can successfully improve charge separation and transportation.
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 m2 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 Ag3PO4.Therefore, developing a high-efficiency photocatalyst is always a hot research topic in the photocatalytic field. In this study, CNT-COO−/Ag3PO4@AgIO4 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/Ag3PO4@AgIO4 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.

2. Results and Discussion

2.1. The Synthesis Mechanism of CNT-COO−/Ag3PO4@AgIO4

In this work, CNT-COO−/Ag3PO4@AgIO4 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 Na2HPO4 was dropped into the mixture, the HPO42− could react with Ag+ ions on the surface of CNT-COO to form Ag3PO4 nuclei. As the reaction proceeded, Ag3PO4 particles successfully grew on the surface of CNT-COO. The addition of KIO4, which was added dropwise, into the solution mixture resulted in the formation of large-sized AgIO4 particles, which were grafted by CNT-COO−/Ag3PO4 to form the CNT-COO−/Ag3PO4@AgIO4 composite.

2.2. Characterization

2.2.1. 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 the FTIR spectra of AgIO4, 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 H2O shows stretching and bending vibrations at 3450 cm−1 and 1630 cm−1, respectively. In the spectra of Ag3PO4, 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 H2O 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−/Ag3PO4@AgIO4 and Ag3PO4@AgIO4, all the characteristic vibration peaks of the individual materials CNT-COO−, Ag3PO4, and AgIO4 are present, which proves the formation of the heterocomposite.

2.2.2. XRD Analysis

The phase purity and the crystal structure of AgIO4, Ag3PO4, Ag3PO4@AgIO4, and CNT-COO−/Ag3PO4@AgIO4 composites are identified by XRD spectra, which are shown in Figure 2b. Pure Ag3PO4 shows characteristic diffraction peaks that are identical to the body-centered cubic structure of Ag3PO4 [35].The XRD pattern of AgIO4 diffraction peaks can be indexed to the tetragonal structure of AgIO4 [36]. TheXRD results indicate that the well-crystallized Ag3PO4 and AgIO4 were successfully fabricated under experimental conditions. For Ag3PO4@AgIO4, theXRD patterns showed diffraction peaks corresponding to Ag3PO4 and AgIO4 crystal phases, proving that AgIO4 and Ag3PO4 are well coupled and that Ag3PO4@AgIO3 has been successfully synthesized. In the pattern of CNT-COO−/Ag3PO4@AgIO4, a smalldecrease is observed compared with Ag3PO4@AgIO4, suggesting that the combination of CNT-COOH does not affect the crystalline structure and phase composition of Ag3PO4@AgIO4; therefore, no apparent diffraction peak is observed for CNT-COOH in the XRD pattern of CNT-COO−/Ag3PO4@AgIO4. 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−/Ag3PO4@AgIO4 composite.

2.2.3. 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 S1c Supporting Information). In Figure 3c, it can be seen that the micro-size AgIO4 was successfully coupled with Ag3PO4 particles. Figure S1d 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.

2.2.4. Optical Properties

The optical properties of the synthesized photocatalysts were evaluated using DRS analysis. The DRS spectra of Ag3PO4, Ag3PO4@AgIO4,and CNT-COO−/Ag3PO4@AgIO4 are shown in Figure 4a. The absorption edges of CNT-COO−/Ag3PO4@AgIO4, Ag3PO4@AgIO4, and Ag3PO4 were observed at about 586, 455, and 543 nm, respectively. The light absorption of Ag3PO4@AgIO4 was extended in the visible light region compared with that of pure Ag3PO4. This is due to the combination of the two silver salts. The absorption edge of CNT-COO−/Ag3PO4@AgIO4 was further extended to 586 nm with the introduction of the CNT-COO. This confirms that the CNT-COO−/Ag3PO4@AgIO4 composite strongly 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.

2.2.5. 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.

2.3. Photocatalytic Activity

2.3.1. 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 photogenerated electron–hole pairs. This is in good agreement with the EIS results.
To investigate the optimal CNT-COOH amount on the synthesized composites, the photocatalyticperformanceofCNT-COO−/Ag3PO4@AgIO4-2.5%, CNT-COO−/Ag3PO4@AgIO4-5%, and CNT-COO−/Ag3PO4@AgIO4-7.5% has been tested in relation tothe degradation of MB (Figure S3 Supporting Information). The results revealed that CNT-COO−/Ag3PO4@AgIO4-5% 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 Ag3PO4 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−/Ag3PO4@AgIO4-5% and its contents, Ag3PO4@AgIO4 and Ag3PO4 to further prove the photocatalytic efficiency of CNT-COO−/Ag3PO4@AgIO4-5%. As seen in Figure 6b, in the absence of the photocatalyst and/or light irradiation, the dye degradation can be ignored. CNT-COO−/Ag3PO4@AgIO4 exhibited the highest photodegradation of the dye, with nearly 100% of MB decomposingwithin 4 min, while the Ag3PO4@AgIO4 and the bare Ag3PO4 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−/Ag3PO4@AgIO4-5% composite.

2.3.2. 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.

2.3.3. 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
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%, 50%, and 25%, respectively.

2.3.4. 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.

2.3.5. 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 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.

2.3.6. 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 of AgIO4 can be transferred to the VB of Ag3PO4 due to the potentials of the CB and VB positions of AgIO4 (CB = 0.96 eV and VB = 3.61 eV) [18] are lower than those of the Ag3PO4 (CB = 0.45 eV and VB = 2.85 eV) [13,39].Therefore, the electrons concentrate on the CB of AgIO4. However, the CB of AgIO4 is more positive than the reduction of the oxygen E0 (O2/O2•−) (0.13 eV, vs. NHE) [40]. Therefore, the O2•−could not be generated. However, the scavenger study proved the significant effect of the O2•−on the dye degradation process, whichindicatesthat the photocatalytic activity mechanism of CNT-COO−/Ag3PO4@AgIO4 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 Ag3PO4 and AgIO4, many defects can be aggregated. Therefore, the quasi-continuous energy levels can be formed in the contact interface Ag3PO4–AgIO4. 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, Ag3PO4 and AgIO4 have a matching surface edge. Thus, Ag0can be formed at the contact interface between Ag3PO4 and AgIO4. Therefore, during the photocatalytic reaction, the Ag3PO4–AgIO4 contact interface develops to be the recombination centerof the photoexcited electrons from CB of AgIO4 and the holes from VB of Ag3PO4. As a result, the photoexcited electrons on the CB of Ag3PO4can 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].
From the above results, it isclear thatthe presence of CNT-COO− on the as-synthesized composite and the multi-electron–hole species generated by Ag3PO4 and AgIO4 played a significant role in the improvement of the photocatalytic activity of CNT-COO−/Ag3PO4@AgIO4. The main photocatalytic reaction for the photodegradation of the organic dyes can be described by the following equations:
Photocatalyst + hʋhVB + eCB
hVB+ + dyeCO2 + H2O+ intermediates
hVB+ + H2O•OH + H+
O2 + e CB- O2•
O2− + dye CO2 + H2O+ intermediates
As explained in Figure 10b, after the photocatalyst is exposed to sunlight, the CNT-COO−/Ag3PO4@AgIO4 composite generates multi-electron/holes (Equation (2)). AgIO4 excites electrons to its CB, leaving holes in the VB;these electrons recombine with the holes generated by Ag3PO4 in the contact interface between the two semiconductors. The photoexcited electron on the CB of Ag3PO4 further migrates to the functionalized CNT. As a result, the holes concentrate in the VB of AgIO4 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 O2 to form superoxide radicals (O2•−) (Equation (5)). Finally, these photogenerated species can effectively degrade MB into CO2, H2O, 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−/Ag3PO4@AgIO4 composite should be attributed to the significant coeffects between the Ag3PO4@AgIO4 nanoparticle and CNT-COO. First, an Ag3PO4 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 Ag3PO4@AgIO4 to the dye molecules under light irradiation enhanced the photocatalytic efficiency.
Second, the interlayer between Ag3PO4@AgIO4 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−/Ag3PO4@AgIO4 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].

3. Materials and Methods

3.1. 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 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 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.

3.2. 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) HNO3 (70%)/H2SO4 (98%) for 8 h at room temperature. Afterwards, it was centrifuged at 11,000 rpm and dried at 70 °C for 12 h.

3.3. Synthesis of CNT-COO−/Ag3PO4@AgIO4

The CNT-COO−/Ag3PO4@AgIO4 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 AgNO3 was added to the CNT desperation, which was stirred for 4 h in dark conditions. Afterward, 0.8 g/20 mLof Na2HPO4 was added drop by drop. After 1 h of reaction, a solution of KIO4 (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.

3.4. Synthesis of CNT/Ag3PO4@AgIO4

For the synthesis of CNT/Ag3PO4@AgIO4composite, non-carboxylated CNT was sonicated in water for 2 h to obtain a water dispersion of CNT. Then, 1.2 g/20 mLof AgNO3 was added to the CNTs’ desperation under stirring for 4 h in dark conditions. Afterward, 0.8 g/20 mLof Na2HPO4 was added drop by drop. After 1 h of reaction, a solution of KIO4 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, Ag3PO4@AgIO4 and Ag3PO4 were synthesized following the same procedure without using CNT for the synthesis of Ag3PO4@AgIO4, and CNT and KIO4 for the synthesis of Ag3PO4.

3.5. 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.

3.6. 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.
The detection of the active species during the photocatalytic reaction was performed using p-benzoquinone (B.Q), disodium ethylene diamine tetra acetate (Na2-EDTA), and tert-butanol (t.B) as superoxide radical (•O2), holes (h+) and hydroxide radical (•OH) scavengers, respectively, followed by the photocatalytic activity test.

4. Conclusions

A novel CNT-COO−/Ag3PO4@AgIO4 was synthesized through electrostatic deposition of Ag3PO4 on the surface of the carboxylated carbon nanotube (CNT-COOH), followed by the growth of AgIO4. Compared with CNT/Ag3PO4@AgIO4, CNT-COO−/Ag3PO4@AgIO4 exhibited enhanced optical and charge transfer properties. The optimal CNT-COOH content on CNT-COO−/Ag3PO4@AgIO4 was investigated; the composite with 5 mg of CNT-COOH exhibited the highest photocatalytic activity.Moreover, CNT-COO−/Ag3PO4@AgIO4 demonstrated enhanced photocatalytic performance compared to that of CNT/Ag3PO4@AgIO4, Ag3PO4@AgIO4, and Ag3PO4 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.

Supplementary Materials

The following supporting information can be downloaded at: https://0-www-mdpi-com.brum.beds.ac.uk/article/10.3390/molecules28041586/s1, The following supporting information: Figure S1: SEM image of (a) nonpurifiedCNT, (b) purified CNT, (c) AgIO4, (d) CNT/Ag3PO4@AgIO4. Figure S2: (a) Image of the dispersion of (A) CNT/Ag3PO4@AgIO4 and (B) CNT-COO/Ag3PO4@AgIO4, in MB aqueous solution. (b) The adsorption of MB over: (A) CNT/Ag3PO4@AgIO4and (B) CNT-COO−/Ag3PO4@AgIO4composites. Figure S3: The photodegradation of MB over CNT-COO/Ag3PO4@AgIO4with different CNT-COO contents. Figure S4: The photodegradation of different organic dyes over CNT-COO−/Ag3PO4@AgIO4-5%, under sunlight irradiation. Figure S5: Regression curves of −ln(Ct/Co) versus irradiation time for CNT-COO−/Ag3PO4@AgIO4-5% under different light intensities (A) 100%, (B) 75%, (C) 50%, (D) 25%.

Author Contributions

Conceptualization, A.A.E. and M.I.S.; methodology, M.I.S.; software, X.L.; validation, X.M., M.I.S. and A.A.E.; formal analysis, M.I.S.; investigation, X.L.; resources, X.M., writing—original draft preparation, M.I.S.; writing—review and editing, A.Y.A.; visualization, A.O.A.; supervision, A.A.E.; project administration, M.I.S. funding acquisition, A.Y.A. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Deanship of Scientific Research at King Faisal University, Ambitious Researcher [GRANT2,266].

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All the data of this research were uploaded within the manuscript and in the supporting information file.

Acknowledgments

The authors acknowledge the Deanship of Scientific Research at King Faisal University, for the financial support under Ambitious Researcher [GRANT2,266].

Conflicts of Interest

The authors declare no conflict of interest.

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Molecules 28 01586 g001 550
Figure 1. The scheme illustrates the synthesis method of the CNT-COO−/Ag3PO4@AgIO4 composite.
Figure 1. The scheme illustrates the synthesis method of the CNT-COO−/Ag3PO4@AgIO4 composite.
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Figure 2. (a) The FT-IR spectra of the as-synthesized composites, (b) the XRD spectra of the as-synthesized.
Figure 2. (a) The FT-IR spectra of the as-synthesized composites, (b) the XRD spectra of the as-synthesized.
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Figure 3. SEM image of (a) CNT-COOH, (b) Ag3PO4, (c) Ag3PO4@AgIO4, (d) CNT/Ag3PO4@AgIO4, (e) CNT-COO−/Ag3PO4@AgIO4, (f) EDS analysis of CNT-COO−/Ag3PO4@AgIO4.
Figure 3. SEM image of (a) CNT-COOH, (b) Ag3PO4, (c) Ag3PO4@AgIO4, (d) CNT/Ag3PO4@AgIO4, (e) CNT-COO−/Ag3PO4@AgIO4, (f) EDS analysis of CNT-COO−/Ag3PO4@AgIO4.
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Figure 4. (a) DRS spectra of the synthesized materials, and (b) the plot of (αhʋ)2 versus energy (hʋ).
Figure 4. (a) DRS spectra of the synthesized materials, and (b) the plot of (αhʋ)2 versus energy (hʋ).
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Figure 5. Impedance spectrum of the synthesized composites.
Figure 5. Impedance spectrum of the synthesized composites.
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Figure 6. The photodegradation degradation of MB over (a) CNT-COO−/Ag3PO4@AgIO4 and CNT/Ag3PO4@AgIO4 (b) Ag3PO4, Ag3PO4@AgIO4, and CNT-COO−/Ag3PO4@AgIO4, under sunlight irradiation.
Figure 6. The photodegradation degradation of MB over (a) CNT-COO−/Ag3PO4@AgIO4 and CNT/Ag3PO4@AgIO4 (b) Ag3PO4, Ag3PO4@AgIO4, and CNT-COO−/Ag3PO4@AgIO4, under sunlight irradiation.
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Figure 7. Regression curves of-ln(Ct/Co) versus irradiation time for (A) CNT-COO−/Ag3PO4@AgIO4-5%, (B) CNT/Ag3PO4@AgIO4, (C) Ag3PO4@AgIO4, (D) Ag3PO4.
Figure 7. Regression curves of-ln(Ct/Co) versus irradiation time for (A) CNT-COO−/Ag3PO4@AgIO4-5%, (B) CNT/Ag3PO4@AgIO4, (C) Ag3PO4@AgIO4, (D) Ag3PO4.
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Figure 8. Stability and recycling of the CNT-COO−/Ag3PO4@AgIO4-5% composite.
Figure 8. Stability and recycling of the CNT-COO−/Ag3PO4@AgIO4-5% composite.
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Figure 9. The effect of reactive species on the degradation of MB over CNT-COO−/Ag3PO4@AgIO4-5% composite under simulated light irradiation.
Figure 9. The effect of reactive species on the degradation of MB over CNT-COO−/Ag3PO4@AgIO4-5% composite under simulated light irradiation.
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Figure 10. The proposed photocatalytic mechanism through which CNT-COO−/Ag3PO4@AgIO4 composite degrades organic dyes based on (a) heterojunction energy band theory and (b) Z-scheme theory under visible light irradiation.
Figure 10. The proposed photocatalytic mechanism through which CNT-COO−/Ag3PO4@AgIO4 composite degrades organic dyes based on (a) heterojunction energy band theory and (b) Z-scheme theory under visible light irradiation.
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Table
Table 1. MB degradation rate constants over the synthesized composites.
Table 1. MB degradation rate constants over the synthesized composites.
CompositeDegradation Rate Constant (min−1)
CNT-COO−/Ag3PO4@AgIO4-5%0.877
CNT/Ag3PO4@AgIO40.4143
Ag3PO4@AgIO4,0.3107
Ag3PO40.1611
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Elbashir, A.A.; Shinger, M.I.; Ma, X.; Lu, X.; Ahmed, A.Y.; Alnajjar, A.O. Fabrication of a Novel CNT-COO−/Ag3PO4@AgIO4Composite with Enhanced Photocatalytic Activity under Natural Sunlight. Molecules 2023, 28, 1586. https://0-doi-org.brum.beds.ac.uk/10.3390/molecules28041586

AMA Style

Elbashir AA, Shinger MI, Ma X, Lu X, Ahmed AY, Alnajjar AO. Fabrication of a Novel CNT-COO−/Ag3PO4@AgIO4Composite with Enhanced Photocatalytic Activity under Natural Sunlight. Molecules. 2023; 28(4):1586. https://0-doi-org.brum.beds.ac.uk/10.3390/molecules28041586

Chicago/Turabian Style

Elbashir, Abdalla A., Mahgoub Ibrahim Shinger, Xoafang Ma, Xiaoquan Lu, Amel Y. Ahmed, and Ahmed O. Alnajjar. 2023. "Fabrication of a Novel CNT-COO−/Ag3PO4@AgIO4Composite with Enhanced Photocatalytic Activity under Natural Sunlight" Molecules 28, no. 4: 1586. https://0-doi-org.brum.beds.ac.uk/10.3390/molecules28041586

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