Carbon cloth supported black phosphorus-FeMoO 4 composites for peroxymonosulfate-assisted photoelectrocatalytic degradation of tetracycline hydrochloride

Abstract

This research utilized the solvothermal technique to prepare carbon cloth supported black phosphorus-FeMoO₄ (CC-BP-FMO) composites, which were used for the degradation of tetracycline hydrochloride (TC-HCl) through the peroxymonosulfate (PMS)-assisted photoelectrocatalysis (PEC) route. Several characterization techniques were employed to systematically examine the morphology, elemental composition, crystal and molecule structures, magnetism properties, bonding states, energy band structure, separation efficiency of photo-induced carriers, electrochemical behaviors, reactive species, and intermediate products of TC-HCl degradation. The experimental results demonstrated that the performance of PMS-assisted PEC TC-HCl degradation by the CC-BP-FMO composites was significantly improved in comparison with photocatalysis, electrocatalysis, PMS activation, and PEC. The CC-BP-FMO composites with more exposed BP-FMO catalysts exhibited the ability to be reused for PMS-assisted PEC degradation of TC-HCl, with a large k value of 0.21 min⁻¹ and a high degradation rate of 97% after the fourth cycle. The remarkable PMS-assisted PEC property of the CC-BP-FMO composites was mainly attributed to the swift separation of photo-induced charge carriers, which hastened the creation of reactive radicals. Five possible catalytic reaction pathways existed in the CC-BP-FMO composites towards TC-HCl degradation under the PMS-assisted PEC condition. Importantly, the photo-generated electrons and $\cdot O_{2}^{-}$ radicals were advantageous for the redox reaction of Fe⁺²/Fe⁺³ and Mo⁺⁴/Mo⁺⁶. The oxygen vacancies caused by P and Mo in the BP-FMO heterojunction could suppress the carrier recombination. The degradation of TC-HCl involved the participation of oxygen vacancy, $\cdot O_{2}^{-}$ , $\cdot {SO}_{4}^{-}$ , and ∙OH radicals, and ¹O₂ nonradicals. The CC-BP-FMO composites have the potential application for treating wastewater containing organic pollutants.

Keywords

Carbon cloth photoelectrocatalysis peroxymonosulfate tetracycline hydrochloride degradation

In recent years, the advanced oxidation process (AOP) has been extensively utilized for the degradation of organic pollutants mainly due to the production of various reactive species, such as oxygen vacancy (O_V), $\cdot O_{2}^{-}$ , $\cdot {SO}_{4}^{-}$ , and ∙OH radicals, and ¹O₂ nonradicals.¹ These reactive species possess the ability to break down organic compounds into small molecules that have low toxicity or are non-toxic. The AOP can be categorized into different types based on the reaction conditions and active radicals, including photochemical oxidation, catalytic wet oxidation, acoustochemical oxidation, ozone oxidation, electrochemical oxidation, Fenton oxidation, and peroxymonosulfate (PMS) activation.² Among them, photocatalysis technology is regarded as a green processing method.³ The generation of photo-induced electrons and holes is confirmed when the photocatalyst is exposed to light of a specific wavelength. In this process, the photo-generated electrons convert dissolved oxygen into $\cdot O_{2}^{-}$ radicals, while the photo-generated holes transform water into ∙OH radicals.^4,5 However, the photocatalytic process is greatly hindered because of the tendency of photo-generated electrons and holes to recombine.⁶

Meanwhile, the electrocatalysis technique is another effective way to degrade organic pollutants. The electrocatalysts are applied by a certain voltage to generate reactive radicals, which react with organic molecules.⁵ The reactive species resulting from electrocatalysis are mainly ∙OH radicals, which have a strong oxidation potential (2.8 V versus standard hydrogen electrode (SHE)); however, some organic pollutants are not sensitive to ∙OH radicals.⁷ In order to enhance the degradation effectiveness of contaminants, the combination of electrocatalysis and photocatalysis is commonly employed owing to the synergistic effect of photoelectricity.⁸ This effect not only facilitates the separation of charge carriers, but also conserves energy.⁹ According to reports, when a semiconductor electrode is exposed to light containing photons that have higher energy than its band gap, the electrons will migrate towards the opposing electrode due to the applied voltage. This migration effectively prevents the recombination of photo-generated holes with electrons.¹⁰ In addition, the reactive substances produced through both photocatalysis and electrocatalysis collaborate to enhance the breakdown of organic contaminants.¹¹ PMS-assisted photoelectrocatalysis (PEC) can generate ∙OH and $\cdot {SO}_{4}^{-}$ radicals with high oxidizing capability and a long half-life time.¹²

As is well known, the catalyst’s activity plays a vital role in the degradation process of organic pollutants. As a type of two-dimensional (2D) layered semiconductor material, black phosphorus (BP) has a direct band gap with high charge carrier mobility (6000 cm²·V⁻¹·s⁻¹)¹³ and large electrical conductivity (102 s/m), making it the ideal candidate for the preparation of PEC materials.¹⁴ The PEC visible light response performance of BP was markedly enhanced because the hybridization of the sp³ orbital of BP made the bonds of the lone pair electrons of the P atom become unstable. Oxygen atoms have the ability to oxidize the P atom, leading to a tendency for the electrons and holes to recombine. Consequently, the efficiency of photocatalysis/electrocatalysis is somewhat hindered.¹⁵ FeMoO₄ (FMO), as a heterogeneous catalyst, is very stable in acidic or alkaline aqueous solution¹⁶ and is easy to recover because of its magnetism.¹⁷ FMO could be used to activate PMS to remove pollutants from water.¹⁸ In fact, FMO is composed of a mixed metal oxide of Fe and Mo, which possesses the exceptional redox chemical properties with 0-3 polyvalent Fe ions and 0-6 polyvalent Mo ions.¹⁹

The current study involved the preparation of composites of carbon cloth supported black phosphorus-FeMoO₄ (CC-BP-FMO) using a solvothermal method. Firstly, the utilization of the carbon cloth (CC) as the substrate in the PEC degradation of pollutants is well established due to its affordability, extensive specific surface area, excellent conductivity,²⁰ and electron transfer capability.²¹ Secondly, the porous carbon fabric with large contact surface is of benefit to accelerate the diffusion of the reactant and the fast migration of charge carriers.²² Thirdly, efficient separation of photo-generated electrons and holes is made possible by the strong conductivity of CC when a bias voltage is applied.²³ Fourthly, the CC not only accelerated the charge separation, but also provided more surface active sites.²⁴ BP has been confirmed as a p-type semiconductor material,²⁵ while FMO has been identified as an n-type semiconductor material.²⁶ Thus, a type-II BP-FMO p-n heterojunction might be formed by n-type FMO and p-type BP, which could ameliorate the charge carrier separation.²⁷ Finally, BP has high charge carrier mobility and high conductivity.²⁸ Polyvalent Fe and Mo ions are provided by FMO, which exhibits an α and β phase structure.¹⁸ In addition, composites of CC-BP-FMO containing defects in P and Mo oxygen vacancies have the potential to reduce the recombination of electrons and holes generated by light.²²

Experimental details

Materials

The CC (WOS1009) was purchased from Taiwan Carbon Energy Technology Co, Ltd. The fiber diameter and area density of the CC are 360 µm and 125 g/m², respectively. The analytical grade chemical reagents were used without further purification, including nitric acid (65–68%, HNO₃), sulfuric acid (98%, H₂SO₄), potassium permanganate (KMnO₄), ferrous chloride tetrahydrate (FeCl₂∙4H₂O), sodium molybdate dihydrate (Na₂MoO₄∙2H₂O), red phosphorus (RP, P₄), ethylenediamine (C₈H₈N₂), sodium sulfate (Na₂SO₄), potassium ferricyanide (K₃[Fe(CN)₆]), nafionperluorinated resin solution, 5,5-dimethyl-1-pyrroline n-oxide (DMPO, C₆H₁₁NO), 2,2,6,6-tetramethyl-4-piperidinone hydrochloride (TEMP, C₉H₁₉N), 2,2,6,6-tetramethylpiperidoxyl (TEMPO, C₉H₁₈NO), methanol (CH₃OH), formic acid (HCOOH), acetonitrile (CH₃CN), anhydrous ethanol (CH₃CH₂OH), potassium peroxomonosulfate (H₃K₅O₁₈S₄), and tetracycline hydrochloride (TC-HCl, C₂₂H₂₄N₂O₈∙HCl). Deionized water was used for the entire duration of the experiment.

Preparation of the CC-BP-FMO composites

Figure 1 illustrates the schematic for preparing the CC-BP-FMO composites. Prior to loading the BP-FMO composites, the CC was treated by a liquid phase oxidation method.²⁰ The CC sample was divided into small sections measuring 4 cm × 4 cm. These sections were then immersed in a solution containing 5 mL of HNO₃, 45 mL of H₂SO₄, and 3.2 g of KMnO₄ at a temperature of 50°C for a duration of 15 min. Afterward, they were rinsed with deionized water and anhydrous ethanol until the solution reached a neutral pH. Finally, the sections were dried in the air.

Figure 1.
Schematic for the preparation of the carbon cloth supported black phosphorus (BP)-FeMoO₄ composites. RP: red phosphorus.

FMO particles were synthesized using a hydrothermal technique.¹⁹ To create a suspension, a solution was prepared by dissolving 3.2 g of FeCl₂∙4H₂O in 30 mL of deionized water, followed by the addition of 30 mL of an aqueous solution of Na₂MoO₄∙2H₂O with a concentration of 0.08 M. The suspension was promptly moved to a 100 mL autoclave lined with polytetrafluoroethylene (PTFE), which was then heated to 150°C while rotating at a speed of 10 revolutions per minute. Following a 24-h reaction at 150°C, the autoclave underwent cooling to reach room temperature. Subsequently, the as-synthesized FMO particles were washed individually with deionized water and anhydrous ethanol for a duration of 30 min on three occasions. Finally, the particles were freeze-dried. When the CC sample was added in the FMO precursor solution, CC loaded with FMO particles (CC-FMO) could be obtained.

The CC-BP-FMO composites were prepared by a modified solvothermal synthesis route.²⁸ Approximately 1.5 g of RP powder was finely ground and combined with a solution of 65 mL of C₈H₈N₂. Subsequently, 0.5 g of FMO particles were added. After being magnetically stirred for 30 min, the blended mixture was transferred to an autoclave lined with polytetrafluoroethylene (PEFT), which had a capacity of 100 mL. The pretreated CC (4 cm × 4 cm) sample was subsequently added. The autoclave was heated to a temperature of 180°C and maintained at this steady temperature for a duration of 24 h. Following this, the as-obtained CC-BP-FMO composites underwent three rounds of washing with deionized water and anhydrous ethanol, each lasting 30 min. Finally, the sample was dried in the air. Furthermore, the CC sample was prepared exclusively with BP particles (CC-BP) without the addition of FMO. In addition, pure BP particles were prepared without the addition of FMO and CC, as well as BP-FMO composite particles without the addition of CC, following the aforementioned procedure.

Characterization techniques

The surface morphologies of the samples prepared were examined using a field-emission scanning electron microscope (FESEM, Tescan Mira 4). The contents of the elements were analyzed using an energy-dispersive X-ray spectrometer (EDS, Oxford Ultim Max 65) that was attached.

The samples’ crystal structures were examined using a Smart Lab 9kW X-ray diffractometer (XRD) that utilized Cu Kα radiation. The XRD was operated at 40 kV and 40 mA, with a scanning rate of 5°/min and a scanning step of 0.02°.

The samples were analyzed for their molecular structures using a Fourier transform infrared spectrometer (FTIR, Thermo Fisher Nicolet Summit X-type) utilizing the KBr pellet technique. The range of scanning was between 4000 and 400 cm⁻¹, with a total of 32 scanning instances and a resolution of 4 cm⁻¹.

The magnetic characteristics of the specimens were analyzed using a VersaLab multifunction vibrating sample magnetometer (VSM) from Quantum Design in San Diego, CA, USA. The measurements were performed at a frequency of 40 Hz and an amplitude of 2 mm.

The samples’ chemical bonding states were analyzed using an X-ray photoelectron spectrometer (XPS, AXIS SUPRA). We utilized the Al Kα radiation emitter (1486.68 eV) and maintained a vacuum level below 1.33 × 10⁻⁶ Pa. Calibration of the binding energies was performed using the C_1s peak at 284.8 eV.

The samples’ absorption spectra were measured using a Cary 5000 ultraviolet (UV)-visible spectrophotometer, scanning at a speed of 250 nm/min across the 200–800 nm range. Tauc’s Equation (1)²⁷ was used to estimate the energy band gap (E_g) by plotting (αhv)² against hv:
$α h v = A {(h v - E_{g})}^{n}^{/ 2}$
(1)

The absorption coefficient α, Plank’s constant h, light frequency v, constant A, and the value of n, which depends on the optical transition type of the semiconductor (n = 4 for an indirect semiconductor, n = 1 for a direct semiconductor), were all factors considered in this study.

An FLS980 fluorescence spectrophotometer was used to measure the steady-state photoluminescence (PL) spectra of the samples at an excitation wavelength of 280 nm.

The samples were subjected to cyclic voltammetry (CV), linear sweep voltammetry (LSV), the Mott–Schottky (M-S) curve, and electrochemical impedance spectroscopy (EIS) Nyquist curves using a CHI660E electrochemical workstation (Shanghai Chenhua Instrument Co., Ltd, Shanghai, China). The experiment utilized a conventional three-electrode setup, with a saturated calomel electrode (SCE) serving as the reference, a platinum wire as the counter, and fluorine-doped tin oxide (FTO) conductive glass coated with the powder acting as the working electrode. To fabricate the working electrode, 20 mg of the powder was dispersed in a solution mixture comprising 30 µL of naphthol and 300 µL of ethanol. The mixture was then subjected to sonication at 28 kHz and 100 W for 5 min. Subsequently, the resulting solution was coated onto the FTO conductive glass (1 cm × 4 cm) and dried at 40°C. The electrolytes were 0.2 mol/L sodium sulfate aqueous solution for CV, LSV, and the MS, and 0.1 mol/L potassium ferricyanide aqueous solution for the EIS Nyquist curves. The solution’s pH was not altered at room temperature.

The samples were analyzed on a Bruker A300 electron paramagnetic resonance (EPR) spectrometer to identify the active radicals. DMPO was used as a scavenger to detect ·OH and $\cdot {SO}_{4}^{-}$ in water and $\cdot O_{2}^{-}$ in methanol, while TEMP and TEMPO were used as the scavengers to detect ¹O₂ and O_V, respectively. The xenon light source had a power of 300 W.

The TC-HCl PEC degradation intermediate products were examined using an Ultimate 3000 UHPLC-Q Exactive high-performance liquid chromatography with tandem mass spectrometry (HPLC-MS/MS) instrument (Thermo Scientific, USA). Before conducting chromatographic analysis, the analytical liquid underwent filtration using a microporous filter membrane with a pore size of 0.22 µm. To separate the products, a C18 chromatographic column measuring 100 mm × 4.6 mm and with a particle size of 3.5 µm was utilized. The sample injection was 10 µL in volume and had a flow rate of 0.35 mL/min. The temperature of the chromatographic column was 30°C. The mobile phase consisted of an aqueous solution containing 0.1% formic acid and acetonitrile in a ratio of 80:20 (v/v). The mass spectrum parameters were as follows: the ion source was heated electrospray ionization (HESI), the spray voltage was 3.8 kV, the capillary temperature was 320°C, and the injection needle heating temperature was 350°C. Fullms/dd-ms2 top10 was the scanning mode used, with a scanning resolution of 70,000 in the first stage and a range of 50–600 m/z. The resolution for the second-stage scanning was set at 17,500, with an initial ion of 50 m/z, and collision voltages of NCE 15, 30, and 45 were applied.

Measurements of photocatalysis, electrocatalysis, PMS activation, PEC, and PMS-assisted PEC degradations of TC-HCl

TC-HCl was used as the model of organic pollutants for evaluating the performance of photocatalysis, electrocatalysis, PMS activation, PEC, and PMS-assisted PEC of as-prepared specimens. For the PMS-assisted PEC degradation of TC-HCl, a quartz reaction cell was filled with 70 mL of an aqueous solution containing TC-HCl at a concentration of 10 mg/L, and 0.05 mM PMS activator was added when the adsorption equilibrium was reached. The light source utilized was a 300 W Xenon lamp with a 420 nm filter, having an optical power density of 0.73 W/cm². The conductive glass loaded with catalysts or the CC specimen served as the working electrode, while the counter electrode was a platinum wire. The reference electrode, on the other hand, was a SCE. The reaction mixture was exposed to a xenon lamp while simultaneously applying a voltage of 1.0 V to the catalyst sample. Approximately 5 mL of degradation solution was extracted from the reaction solution at specific time. The absorbance (A_t) of the extracted solution was then measured using a T2602 spectrophotometer at the maximum absorption wavelength of 356 nm. The concentration C_t of TC-HCl could be obtained by referring to the standard calibration curve of TC-HCl (A_t = −0.02894 +0.002933C_t, R² = 0.99). Using Equation (2),⁶ the calculated rate constants (k values) of TC-HCl were determined:
$㏑ (C_{t} / C_{0}) = - k \times t$
(2)

The initial concentration of TC-HCl, denoted as C₀, and the concentration of TC-HCl at time t, denoted as C_t, were recorded.

The calculation of the degradation rate (D) of TC-HCl was determined by utilizing Equation (3) accordingly:
$D = (1 - C_{t} / C_{0}) \times 100 %$
(3)

According to the above procedure, the photocatalysis, electrocatalysis, PMS activation, and PEC degradation of TC-HCl by the CC-BP-FMO composites were conducted solely with xenon lamp irradiation at 1.0 V voltage, 0.5 mM PMS activator, and light irradiation at 1.0 V voltage, individually.

Results and discussion

Analysis of morphology and elements

Figure 2 shows the FESEM pictures illustrating the morphologies of BP, FMO, BP-FMO, and CC-BP-FMO, along with the EDS mapping and quantitative analysis findings of CC-BP-FMO. It is seen that the as-obtained BP powder is comprised of irregular flake-like particles in the micrometer range, which are randomly heaped up with each other (Figure 2(a)). The FMO powder actually consisted of both granular and lamellar particles (Figure 2(b)), and the magnified FESEM image suggests that the lamellar particle is constituted of submicron-sized spherical particles (Figure 2(c)). The BP-FMO powder is made up of micrometer-scale particles (Figure 2(d)). The enlarged image obtained from the FESEM confirms the presence of sheet-like particles on the surface of flaky particles (Figure 2(e)). Figure 2(f) displays the formation of the CC-BP-FMO composites where the carbon fiber is enveloped by a dense coating of micrometer-scale particulate matter. In the EDS mapping of BP-FMO powder (Figure 2(g)), the presence of C, N, O, P, Mo, and Fe elements has been detected. It can be deduced from the distribution and lightness of elements that FMO should be covered by BP. Furthermore, the analysis of composition reveals that the percentages of carbon, nitrogen, oxygen, phosphorus, molybdenum, and iron elements are 18.58%, 2.99%, 20%, 23.55%, 26.92%, and 7.96%, correspondingly.

Figure 2.
Field-emission scanning electron microscope images of (a) black phosphorus (BP), (b), (c) FeMoO₄ (FMO), (d), (e) BP-FMO, (f) carbon cloth (CC)-BP-FMO, and (g) energy-dispersive X-ray spectrometer mapping and quantitative analysis results of the CC-BP-FMO.

Crystal and molecular structures and magnetic property

Figure 3 shows the XRD profiles, FTIR spectra, and magnetic hysteresis curves of BP, FMO, BP-FMO, and CC-BP-FMO. As illustrated in Figure 3(a), the wide diffraction pattern of BP powder suggests that the crystallinity of the prepared BP is not ideal. The peaks located at 16.5°, 26.7°, and 34.0° correspond to the lattice planes (002), (012), and (004) of the orthorhombic phase BP, respectively. These results align with the simulated diffraction peaks of BP ((P₂) _n , cell: a 3.32Å, b 4.39 Å, c 10.52 Å; α 90°, β 90°, γ 90°) of the Cambridge Crystallographic Data Centre (CCDC) ICSD 27847 file.²⁹ The FMO powder consists of FMO in both the α-phase and β-phase, exhibiting excellent crystallization. The α-phase FMO standard card JCPDS No.22-1115 shows peaks at 14.10°, 18.62°, 23.6°, 25.3°, 28.16°, 32.05°, and 32.48°. The peaks at 23.0°, 26.2°, 26.96°, and 31.57° correspond to the β-phase FMO standard card JCPDS No.22-0628.¹⁸ For the BP-FMO powder, the typical peaks of BP, α-FMO, and β-FMO can be detected. The peak at 16.5° has moved to 15.2°, while the intensity of the peak at 23° has been greatly increased. This implies that BP is successfully complex with FMO. In addition, the CC-BP-FMO composites exhibit the characteristic peak of carbon fiber at 25.7°.

Figure 3.
The (a) X-ray diffractometer profiles, (b) Fourier transform infrared spectra, and (c) magnetic hysteresis curves of black phosphorus (BP), FeMoO₄ (FMO), BP-FMO, and carbon cloth (CC)-BP-FMO.

As revealed in Figure 3(b), the -OH group is responsible for the broad peak of BP powder, which occurs at around 3008 cm⁻¹. The 1633 cm⁻¹ peak is associated with the absorption of water and the stretching of the O=P-OH group, while the 1509 and 1401 cm⁻¹ peaks are attributed to the presence of the P=O group.³⁰ The symmetric stretching and asymmetric stretching of the PO₃ group are assigned to the peaks at 1140 and 1005 cm⁻¹, respectively.¹³ In addition, the stretching of the O-P=O group is responsible for the peak observed at 490 cm⁻¹.³¹ Adsorbed water, Mo–O–Mo stretching, and Fe–O–Mo stretching cause the peaks at 1623, 829, and 932 cm⁻¹, respectively.³² The disappearance of the P=O group is responsible for the peak at 490 cm⁻¹ in the BP-FMO composites, which is attributed to the O-P=O group of BP. Simultaneously, the FMO’s Fe-O-Mo and Mo-O-Mo groups vanish, while new peaks emerge at 1102, 1032, and 989 cm⁻¹, suggesting the formation of a chemical bond between BP and FMO through the P-Fe/Mo linkage.

As depicted in Figure 3(c), the magnetization of FMO is greater than that of BP-FMO due to the fact that BP is classified as a non-magnetic substance.³³ With the increase in magnetic field strength, the magnetization of ferromagnetic FMO powder experiences a rapid initial increase, followed by a gradual increase beyond 5000 Oe. Notably, there is no observation of saturated magnetization, indicating the existence of anti-ferromagnetism in FMO.¹⁷ Based on the hysteresis curves shown in the insert of Figure 3(c), the FMO exhibits a coercive field of 0.97 Oe and a remnant magnetization of 52.48 emu/g. In contrast, the magnetization of BP-FMO powder increases gradually as the magnetic field strength increases. The corresponding coercive force and remnant magnetization are 0.035 Oe and 37.52 emu/g, respectively.

Bonding states

Figure S1 in the supporting material (SM) displays the XPS full spectra and high-resolution spectra of C_1s, N_1s, O_1s, P_2p, Mo_3d, and Fe_2p elements for the CC-BP, CC-FMO, and CC-BP-FMO. It is clear from Figure S1(a) that the CC-BP consists of C, N, O, and P components, while the CC-FMO comprises C, O, Mo, and Fe components. The CC-BP-FMO contains the components of carbon, nitrogen, oxygen, phosphorus, molybdenum, and iron. The presence of BP, FMO, and BP-FMO can be observed on the surfaces of the CCs.

It is noticed from Figures S1(b1)–(b4) that regarding the CC-BP, the C_1s, N_1s, O_1s, and P_2p spectra can be fitted into four, three, five, and five peaks, respectively. The C_1s spectrum displays binding energies of 288.72, 286.75, 285.18, and 284.83 eV, which correspond to the C=O, C-O, C-N, and C-C bonds, respectively. The N_1s spectrum shows binding energies of 401.86, 400.77, and 399.16 eV, which correspond to the N-H, N-C, and N-P bonds, respectively. The P-O-P, O-P=O, O-C, O=C, and P₂O₅ bonds are associated with the binding energies of 533.72, 532.68, 531.60, 530.94, and 528.92 eV in the O_1s spectrum, indicating the oxidation of BP. The P_2p spectrum attributes the peaks at 134.37, 133.35, 132.34, 131.15, and 130.25 eV corresponding to P₂O₅, O-P=O, P-O-P, P-N, and P_2P (P-P) bonds, respectively.³⁴

It is noted from Figures S1(c1)–(c4) that with respect to the CC-FMO, the C_1s, O_1s, Mo_3d, and Fe_2p spectra are fitted into three, four, two, and six peaks, individually. The C=O, C-O, and C-C bonds are respectively assigned to the peaks in the C_1s spectrum at 288.45, 286.31, and 284.78 eV. The O_1s spectrum exhibits peaks at 532.28, 531.24, 530.10, and 529.39 eV, which correspond to the O-C, O=C, O-Fe, and O-Mo bonds, respectively. The Mo_3d spectrum exhibits peaks at 235.48 and 232.36 eV, which arise from the Mo⁶⁺_3d3/2-O and Mo⁶⁺_3d5/2-O bonds. These peaks have a splitting width of 3.2 eV, suggesting the existence of the Mo⁶⁺ oxidation state.³⁵ The Fe_2p spectrum shows peaks at 724.56 and 711.13 eV, which are associated with the Fe²⁺_2p1/2-O and Fe²⁺_2p3/2-O bonds, respectively.³⁶ The 727.59 and 714.33 eV peaks represent the Fe³⁺_2p1/2-O and Fe³⁺_2p3/2-O bonds, respectively.³⁷ Furthermore, the 732.86 and 719.41 eV peaks arise from the Fe_2p1/2 and Fe_2p3/2 satellite peaks of elemental Fe, respectively.³⁸

As displayed in Figures S1(d1)–(d6), in relation to the CC-BP-FMO, the C_1s high-resolution XPS spectrum is fitted with five peaks, encompassing the binding energies of C=O, C-O, C-N, and C-C bonds. Moreover, a new C-Mo/Fe bond is formed at the binding energy of 283.37 eV. The high-resolution XPS spectrum of N_1s is analyzed and found to have five peaks that include N-H, N-C, and N-P bonds. In addition, two new bonds, N-Mo and N-Fe, are formed at binding energies of 396.77 and 394.73 eV, respectively. Similarly, the O_1s high-resolution XPS spectrum is separated into six peaks. In addition to the presence of O-C and O=C bonds, the detection of P-O-P, O-P=O, and P₂O₅ bonds is observed, as well as the O-Fe/Mo bond of FMO. The P_2p high-resolution XPS spectrum is fitted into five peaks. One can observe the P₂O₅, O-P=O, P-O-P, and P-N bonds, while the P_2p (P-P) bond becomes undetectable. Meanwhile, a new P-Mo/Fe bond is formed at the binding energy of 128.50 eV. Four peaks divide the Mo_3d high-resolution XPS spectrum. The peaks corresponding to the binding energies of Mo⁶⁺_3d3/2-O, Mo⁶⁺_3d5/2-O, Mo⁴⁺_3d3/2-O, and Mo⁴⁺_3d5/2-O are observed at 236.01, 232.66, 234.0, and 230.65 eV, respectively. The Fe_2p high-resolution XPS spectrum is fitted into six peaks. In addition to the elemental Fe’s Fe_2p1/2-C/N/P and Fe_2p3/2-C/N/P satellite peaks, the binding energies of Fe²⁺_2p1/2-C/N/O/P and Fe²⁺_2p3/2-C/N/O/P bonds experience a shift to 721.29 and 707.92 eV, correspondingly. The energies required to bind Fe³⁺_2p1/2-C/N/O/P and Fe³⁺_2p3/2-C/N/O/P atoms are altered to 726.37 and 712.92 eV, correspondingly. Thus, the oxidation-reduction process involving Fe³⁺/Fe²⁺ and Mo⁶⁺/Mo⁴⁺ promotes the photocatalytic reaction of BP-FMO.³⁵

Hence, it can be inferred that BP forms chemical bonds with FMO via the C-Mo/Fe, N-Mo/Fe, and P-Mo/Fe linkages.

Energy band structure

Figures 4(a) and (b) display the absorption spectra and corresponding graphs of (αhv)² versus hv for the BP, FMO, and BP-FMO powders, as well as the CC-BP-FMO composites. The evidence presented in Figure 4(a) makes it clear that the BP powder exhibits a high capacity for absorbing UV rays and visible light, specifically at a wavelength of 690 nm.³⁹ FMO powder exhibits superior spectral response capability compared to BP powder, which is advantageous for the degradation of organic pollutants through photodegradation.⁴⁰ The light absorption performance of BP-FMO powder is between BP and FMO, primarily influenced by BP. When the BP-FMO powder is loaded on CC, the optical property of the CC-BP-FMO composites with no obvious absorption edge is mainly determined by the carbon fibers. Because both BP and FMO are direct semiconductor materials,⁶ n = 1. Consequently, relying on the Tauc plot showcasing (αhv)² versus hv in Figure 4(b), the estimated E_g values of BP, FMO, BP-FMO, and CC-BP-FMO are 1.68, 1.49, 1.62, and 1.31 eV, respectively. Consequently, the CC-BP-FMO composites exhibit the lowest E_g value, suggesting that the production of photo-generated electron (e⁻) and hole (h⁺) pairs is facilitated.

Figure 4.
The (a) absorption spectra and (b) estimated band gap values of black phosphorus (BP), FeMoO₄ (FMO), BP-FMO, and carbon cloth (CC)-BP-FMO, and (c) valence band X-ray photoelectron spectrum inserted with the Mott–Schottky curve and energy band schematic of CC-BP-FMO. NHE: normal hydrogen electrode.

Figure 4(c) displays the XPS spectrum of the CC-BP-FMO composite’s valence band, accompanied by the M-S curve and schematic of the energy band structure. It is deduced from the valence band XPS spectrum that the valence band (E_VB) position of the CC-BP-FMO is estimated to be 0.22 eV.⁴¹ According to the Tauc equation, the E_g value of the CC-BP-FMO is 1.31 eV. By utilizing the equation E_g =E_CB − E_VB, the normal hydrogen electrode (NHE)-determined conduction band (E_CB) position of the CC-BP-FMO is computed as −1.09 eV. The CC-BP-FMO is classified as an n-type semiconductor due to the upward inclination of the tangent line extended from the M-S curve.⁴² According to the demonstration, BP is classified as a p-type semiconductor,¹⁶ whereas FMO is categorized as an n-type semiconductor.²¹ The composite material’s electronic properties are primarily influenced by FMO. Thus, the flat band potential (E_F) of CC-BP-FMO is −0.87 eV with respect to the NHE, which is equivalent to its Fermi energy level. Since the semiconductor’s E_CB value is 0.2 eV lower than its Fermi energy level, the E_CB of the CC-BP-FMO composites is calculated to be −1.07 eV, which is in close proximity to the −1.09 eV inferred from the valence band XPS spectrum. According to the energy band structure diagram of the CC-BP-FMO, it is suggested that the photo-generated electrons at E_CB have the ability to reduce O₂ in water to $\cdot O_{2}^{-}$ radicals due to the fact that the conduction band, which has a value of −1.09 eV, is lower than the potential of O₂/ $\cdot O_{2}^{-}$ couples, which is −0.33 eV. On the other hand, the photo-induced holes at E_VB cannot oxidize H₂O to ·OH radicals because the valence band, with a value of 0.22 eV, is lower than the potential of ∙OH/OH⁻ couples, which is 1.99 eV.⁴³

Separation efficiency

Figure S2 in the SM displays the PL spectra and EIS Nyquist curves of BP, FMO, BP-FMO, and CC-BP-FMO in their steady-state. Indeed, as the PL intensity decreases, the duration of photo-generated carriers generated by the catalysts increases.⁶ As presented in Figure S2(a), in comparison to FMO, the PL intensity of BP powder, which exhibits a peak approximately at 366 nm, is significantly greater. The formation of the BP-FMO heterojunction by combining BP with FMO results in a PL intensity that falls between BP and FMO, suggesting a decreased likelihood of charge carrier recombination. When the BP-FMO powder is loaded on CC, the PL intensity of the CC-BP-FMO composites is reduced and its PL spectrum is almost overlapped with that of FMO. This implies the charge carrier recombination is effectively restrained. The migration of photo-generated carriers on the CC-BP-FMO is mainly facilitated by the advantageous conduction capacity of CC.⁴⁴ Moreover, the close interaction between organic pollutants and BP-FMO powders, which are applied on the fiber surface, facilitates the rapid oxidation and decomposition of the pollutants. Therefore, the CC-BP-FMO composites exhibit the best catalytic performance.

It is apparent from Figure S2(b) that the Nyquist curves of BP, FMO, and BP-FMO powders consist of a half-circle in the high-frequency region and a declining line in the low-frequency region. The semicircle of the CC-BP-FMO composites is almost lost. The charge transfer process is represented by the semicircle, while the semi-infinite Warburg diffusion process is represented by the straight line.³⁴ The semicircle radius of the BP-FMO is between BP and FMO, suggesting that the charge carriers are prone to migrate after the construction of the BP-FMO heterojunction.⁴¹ The resistances for transferring charges of BP, FMO, BP-FMO, and CC-BP-FMO are measured to be 18.7, 32, 44.8, and 7.0 Ω, correspondingly. Clearly, the CC-BP-FMO exhibits the lowest impedance in comparison to the other samples, resulting in a rapid charge transfer. Furthermore, the CC-BP-FMO possesses the largest linear slope at the low-frequency region, indicating the fast Warburg diffusion rate of charge carriers.

Electrochemical performance

Figure 5 displays the test outcomes for the LSV, Tafel slope, and CV of the BP, FMO, BP-FMO, and CC-BP-FMO. As seen in Figure 5(a), at a potential of 0.8 V compared to the reversible hydrogen electrode (RHE), the estimated current densities of BP, FMO, BP-FMO, and CC-BP-FMO are 0.004, 0.034, 0.011, and 0.135 mA/cm², respectively. The CC-BP-FMO exhibits a current density that is 12.27 times higher than that of the BP-FMO, indicating that the presence of CC as a substrate facilitates the swift movement of charge carriers.⁴⁵ As depicted in Figure 5(b), the Tafel slope of FMO is calculated to be 16.89 mV/dec, larger than 15.76 mV/dec of BP-FMO and 11.64 mV/dec of BP. In contrast, the CC-BP-FMO exhibits the lowest Tafel slope, measuring 8.58 mV/dec. The low Tafel gradient suggests that the electrical components on the CC-BP-FMO can be depleted rapidly, resulting in a rapid rate of charge kinetics.⁴⁶ The presence of conductive carbon fibers significantly enhances the separation of photo-generated electrons and holes produced by BP-FMO when the BP-FMO powder is applied to the CC surface. Thus, the recombination probability of charge carriers is reduced to certain degree, and the interfacial charge transfer ability is fairly enhanced.⁴⁷ As illustrated in Figures 5(a) and (c), based on the LSV polarization curves of the CC-BP-FMO before and after 500 cycles in a neutral environment, as well as the corresponding CV curves, the current density of the CC-BP-FMO decreases by 34% (from 0.135 to 0.089 mA/cm²) under the 0.8 V versus the RHE condition after 500 cycles. This means that the stability of the CC-BP-FMO photoelectrode needs to be improved.³⁶

Figure 5.
The (a) linear sweep voltammetry polarization curves, (b) Tafel slopes of black phosphorus (BP), FeMoO₄ (FMO), BP-FMO, and carbon cloth (CC)-BP-FMO, and (c) cyclic voltammetry curves of the CC-BP-FMO composites for 500 cycles. RHE: reversible hydrogen electrode.

Catalytic properties

Figure 6 displays the results of examining the degradation of TC-HCl to evaluate the behaviors of photocatalysis, electrocatalysis, PMS activation, PEC, and PMS-assisted PEC. To begin with, the mass ratio of BP to FMO for the BP-FMO composites is optimized in Figures 6(a) and (b) under visible light. The FMO powder does not show any noticeable photodegradation of TC-HCl, while the BP powder exhibits only a limited ability to photodegrade TC-HCl. Nevertheless, the photodegradation capability of the BP-FMO powder is significantly improved. When the mass ratio of BP to FMO is 3:1, the k value of the BP-FMO (3:1) is calculated as 0.38 × 10⁻² min⁻¹, larger than 1.73 × 10⁻⁴ min⁻¹ for BP and 1.13 × 10⁻⁴ for FMO. Thus, the BP-FMO (3:1) powder is loaded on the CC to prepare the CC-BP-FMO composites.

Figure 6.
The (a) photocatalytic degradation of tetracycline hydrochloride (TC-HCl) by black phosphorus (BP), FeMoO₄ (FMO), and BP-FMO with different mass ratios of BP to FMO and (b) corresponding plots of ln(C₀/C_t) versus irradiation time; (c) photo-, electro-, peroxymonosulfate (PMS)-, photoelectro-, and PMS-assisted photoelectro-catalytic degradations of TC-HCl by the carbon cloth (CC)-BP-FMO and (d) corresponding plots of ln(C₀/C_t) versus time; (e) cyclic PMS-assisted PEC degradation of TC-HCl by the CC-BP-FMO. PEC: photoelectrocatalysis.

It is noticed from Figures 6(c) and (d) that the CC-BP-FMO exhibits a certain level of TC-HCl adsorption, with a k value of 4.59 × 10⁻⁴ min⁻¹. The electrocatalytic degradation k value of TC-HCl by the CC-BP-FMO is 15.0 × 10⁻⁴ min⁻¹, slightly larger than 13.8 × 10⁻⁴ min⁻¹ of pure CC. This suggests that the CC significantly contributes to the electrocatalytic degradation process of TC-HCl. The CC-BP-FMO (with a k value of 11.6 × 10⁻⁴ min⁻¹) exhibits a lower efficiency in the photocatalytic degradation of TC-HCl compared to the electrocatalytic degradation of TC-HCl. Nevertheless, the composite of CC-BP-FMO greatly enhances the degradation performance of TC-HCl when exposed to photoelectric conditions. The degradation rate and k value of TC-HCl are determined to be 53.2% and 4.75 × 10⁻³ min⁻¹ after a duration of 120 min. These values show a 4.1-fold increase for photocatalysis and a 3.2-fold increase for electrocatalysis. The application of an external voltage is believed to enhance the separation, transfer, and migration of photo-generated electrons and holes. Specifically, the CC-BP-FMO has the ability to inhibit the recombination of electrons and holes.⁸

In addition, the k value of the CC-BP-FMO in the PMS activation degradation experiments is 7.46 ×10⁻³ min⁻¹, larger than 4.75 × 10⁻³ min⁻¹ of PEC degradation due to the reaction between the catalyst and its internal groups. When PMS activation is cooperated with the PEC system to degrade TC-HCl, the degradation rate surprisingly reaches 100% within 40 min, and the k value is as high as 0.21 min⁻¹, indicating that the PMS-assisted PEC system significantly enhances the degradation ability of the CC-BP-FMO composites to organic pollutants.

It is noted from Figure 6(e) that after four consecutive degradation cycles of TC-HCl under the PEC system, the degradation rate decreases by 6.4% compared to the initial degradation rate. This suggests that the CC-BP-FMO possesses good PEC stability. More importantly, the degradation rate of the PMS-assisted PEC system is still as high as 97% after four consecutive cycles of TC-HCl degradation. It is again confirmed that the CC-BP-FMO has an excellent TC-HC degradation effect and good stability under the PMS-assisted PEC condition.

We compare the PMS-assisted PEC degradation of TC-HCl by the CC-BP-FMO with previous studies in Table S1 in the SM. Except for the superior PEC degradation performance of $FTO - Ag / N - {TiO}_{2}^{48}$ by adding H₂O₂, the poly(3,4-ethylenedioxythiophene) (PEDOT)-modified polyvinylidene fluoride (PVDF) membrane,⁸ BiVO₄/Mo-WO₃,⁹ FTO-BiOI/MnO₂,¹¹ CFs/TiO₂/MoS₂,²³ and $CC - {Co}_{3} O_{4} / {TiO}_{2}^{}$ ²⁴ exhibit high PEC degradation capabilities to TC-HCl with degradation rates of 71–100% and k values from 0.0114 to 0.03 min⁻¹ within 60–180 min. The degradation time can be reduced to 20–30 min for the PMS activation degradation of TC-HCl by ${Cu}_{2} O / {CoFe}_{2} O_{4}^{}$ ⁴⁹ and Co₃O₄@reduced graphene oxide (rGO).⁵⁰ There is only one research report showing that FTO-CoFe₂O₄-BiVO₄ is used to degrade 60 mL of 20 mg/L TC-HCl solution with a 300 W xenon lamp, 0.6 V voltage, and 0.5 mM PMS activator.¹² After 60 min of degradation, the degradation rate and k value are 89.1% and 0.037 min⁻¹, respectively, and the degradation rate decreases to 70.2% after five cycles. Obviously, the CC-BP-FMO composites (with a small amount of 5 mg BP-FMO) are able to degrade 70 mL of 10 mg/L TC-HCl within 40 min under a 300 W xenon lamp, 1.0 V voltage, and 0.05 mM PMS activator conditions, producing a high degradation rate of 100% and a large k value of 0.21 min⁻¹. After four cycles, the degradation rate is kept at 97%. Therefore, the catalysis experiments reveal that the PMS-assisted PEC degradation is a promising approach for highly efficient removal of TC-HCl from aqueous solution by the CC-BP-FMO composites.

Reactive species

The EPR technique is used to identify the reactive species of $\cdot O_{2}^{-}$ , $\cdot {SO}_{4}^{-}$ , ∙OH,¹O₂, and O_V using DMPO, TEMP, and TEMPO as trapping agents, and the findings are presented in Figure 7. The energy band structure of the CC-BP-FMO shows that the photo-generated electron has the ability to interact with dissolved O₂, resulting in the production of $\cdot O_{2}^{-}$ radicals.⁶ As displayed in Figure 7(a), the EPR signal of $\cdot O_{2}^{-}$ is detected on the CC-BP-FMO without light irradiation, suggesting that the BP-FMO components can chemically activate PMS. Under visible light, the signal peaks associated with $\cdot O_{2}^{-}$ tend to increase, indicating that more $\cdot O_{2}^{-}$ radicals are produced. It is noticed from Figure 7(b) that the signal peaks related to $\cdot {SO}_{4}^{-}$ and ∙OH show little change, indicating that the light illumination has almost no influence on the production of $\cdot {SO}_{4}^{-}$ and ∙OH radicals induced by PMS. The EPR signal of ¹O₂ nonradicals in Figure 7(c) is recognized on the CC-BP-FMO at dark and is slightly enhanced after exposure to visible light, reflecting a synergistic effect of photocatalysis and PMS activation.⁵¹ It is observed from Figure 7(d) that the oxygen vacancies are detected under both dark and illumination conditions. The presence of defects and oxygen vacancies in BP-FMO are confirmed by the strong peak at g = 2.003 induced by elemental P⁵² and the weak peak at g = 1.915 caused by elemental Mo.⁵³ Hence, the PMS-assisted PEC of the CC-BP-FMO can induce the excitation of photo-generated e⁻ and h⁺ pairs, resulting in the generation of $\cdot O_{2}^{-}$ , $\cdot {SO}_{4}^{-}$ , and ∙OH radicals and ¹O₂ nonradicals. It is reasonable to expect a synergy of the radical and nonradical coupling process along with O_V during TC-HCl degradation. The main reactive species include $\cdot {SO}_{4}^{-}$ , $\cdot O_{2}^{-}$ , and ¹O₂.

Figure 7.
The electron paramagnetic resonance spectra of (a) $\cdot O_{2}^{-}$ , (b) $\cdot {SO}_{4}^{-}$ , ∙OH, and (c) ¹O₂, and (d) the oxygen vacancy for the black phosphorus-FeMoO₄ powder. PMS: peroxymonosulfate.

Degradation pathways of TC-HCl

Figure S3 in the SM illustrates the potential degradation pathway of TC-HCl by the CC-BP-FMO composites under PMS-assisted PEC conditions, and the corresponding mass spectra of intermediates at different retention times are listed in Figure S4. The CC-BP-FMO can generate a significant number of h⁺ and ∙OH active species. These active species then attack the TC-HCl molecules adsorbed on the composite surface, leading to their gradual decomposition into different low molecular weight organic substances. This decomposition occurs through a series of consecutive reactions, including hydroxylation, oxidation, demethylation, deacylation, and deamination.⁵⁴ Specifically, the P2 deprotonated product, which has a mass-to-charge ratio (m/z) of 417 (Figure S4(a)), is derived from the TC-HCl hydrolysate P1 with m/z = 445 through the elimination of the N-dimethyl group due to the comparatively low energy of the C-N bond.⁸ The formamide group is subsequently exfoliated to produce an intermediate product of P3 with m/z = 388 (Figure S4(b)).⁵⁵ Afterward, the clusters of amino, hydroxyl, and methyl vanish to create the protonated outcome of P4 with m/z = 340 (Figure S4(c)). Subsequently, the detachment of hydroxyl and dimethyl groups from P4 leads to the generation of the intermediate product of P5 with m/z = 300 (Figure S4(d)), which is then followed by the ring cleavage. Afterward, the alkyl group is removed from P5 to produce the intermediate product of P6 with m/z = 274 (Figure S4(e)).⁵⁶ The intermediate P6 is then oxidized with a loss of carboxyl group to produce P7 with m/z = 242 (Figure S4(f)), which is oxidized to generate the low molecular weight fragmentations of P8 with m/z = 222 and P9 with m/z = 150 (Figures S4(g) and (h)). After that, the fragmentations of P10 with m/z = 132 and P11 with m/z = 118 (Figures S4(i) and (j)) are produced via the ring-opening reaction, carbonyl reduction, and benzene ring cracking, and finally decomposed into CO₂ and H₂O.

Bonding strength

Figure S5 in the SM displays the FESEM images depicting the surface morphology of the CC-BP-FMO composites prior to and following ultrasonic cleaning, along with the corresponding EDS element quantitative analysis outcomes. Observations reveal that the CC’s surface is adorned with numerous tiny particles, while nearly every fiber is uniformly covered with BP-FMO particles. These relatively large particles are constituted of tiny particles that are closely adhered on fiber surfaces, as confirmed by the insert of Figure S5(a). Following the process of ultrasonic cleaning, there is a notable decrease in the quantity of particles present on the CC surface, with a considerable number being eliminated from the surfaces of the fibers. However, a few particles are still tightly attached to fiber surface (Figure S5(b)). According to the EDS findings, the proportions of nitrogen, oxygen, phosphorus, molybdenum, and iron in the CC-BP-FMO decrease to different degrees following water-based ultrasonic cleaning (Figures S5(c) and (d)). While the BP-FMO particles have the capability to be affixed onto the CC surface through hydrothermal conditions, further efforts are required to enhance the adhesion strength between BP-FMO particles and carbon fiber.

Reaction mechanism

Figure 8 presents the proposed schematic diagram illustrating the catalytic reaction mechanism of TC-HCl degradation by the CC-BP-FMO composites. Under light irradiation, the BP-FMO heterojunction undergoes photocatalysis, resulting in the generation of photo-generated e⁻ and h⁺ in the E_CB and E_VB, respectively (Equation (4)). Figure 4(b) shows the calculated E_g values for BP and FMO, which are 1.68 and 1.49 eV, respectively. According to reports, the E_VB values for BP and FMO are situated at 1.0 and 1.37 eV at the NHE, respectively. Accordingly, the estimated values for the corresponding E_CB values are −0.68 and −0.12 eV at the NHE, respectively.^6,57 It is inferred that the BP-FMO heterojunction might be a type II semiconductor heterostructure. Namely, the e⁻ in the E_CB of BP migrates to the E_CB of FMO and the h⁺ in the E_VB of FMO transfers to the E_VB of BP.⁵⁸ Since the E_CB potential of the CC-BP-FMO (−1.09 eV) is less than that of O₂/ $\cdot O_{2}^{-}$ couples (−0.33 eV), the presence of $\cdot O_{2}^{-}$ radicals can be achieved by reducing the dissolved O₂ in water. However, ∙OH radicals cannot be produced because the E_VB potential (0.22 eV) of the CC-BP-FMO is smaller than 1.99 eV of ∙OH/OH⁻ couples.⁵⁹ As a result, the CC-BP-FMO has three pathways for the photodegradation of TC-HCl. The produced $\cdot O_{2}^{-}$ radicals (Path I-1, Equation (5)) break down TC-HCl and interact with H₂O to generate ∙OH radicals (Path I-2, Equation (6)) which break down TC-HCl. At the same time, the produced h⁺ reacts with $\cdot O_{2}^{-}$ radicals to create ¹O₂ nonradicals (Path I-3, Equation (7)), which break down TC-HCl.⁴³ Besides, the Fe²⁺ ions of BP-FMO react with H₂O₂, which can be classified as a Fenton reaction, to produce ∙OH radicals to decompose TC-HCl, and Fe²⁺ ions are oxidized to Fe³⁺ ions. Simultaneously, Fe³⁺ ions undergo reduction to form Fe²⁺ ions through the action of H₂O₂, resulting in the formation of ∙O₂H. Fe²⁺ cations undergo further reaction with O₂ to produce $\cdot O_{2}^{-}$ radicals, which decompose TC-HCl. Simultaneously, Fe²⁺ cations are oxidized to Fe³⁺ ions. Thus, an oxidation-reduction reaction cycle of Fe²⁺/Fe³⁺ is constructed (Equations (8)–(10)).⁴⁰ Nevertheless, the degradation of TC-HCl by the CC-BP-FMO is unsatisfactory. One factor is because the photo-generated electron–hole pairs are likely to quickly undergo recombination. One more factor is that the illumination only reaches one surface of the carbon fabric, leaving its opposite side in the dark. Moreover, some of the particulate catalysts are embedded between carbon fibers without light illumination.

Figure 8.
The proposed catalytic reaction mechanism for the tetracycline hydrochloride (TC-HCl) degradation by the carbon cloth-black phosphorus (BP)-FeMoO₄ (FMO) composites under light, electricity, and peroxymonosulfate activation conditions.

Regarding the electrocatalysis, the accumulation of electrons at the cathode of CC-BP-FMO caused by the applied voltage results in the formation of an electric field due to the potential difference between the cathode and the anode of the CC-BP-FMO.⁴⁸ The electrons react with H₂O to produce ∙OH radicals (Path II, Equation (11)).⁶⁰ It is verified that ∙OH radicals can be produced on the anode of a semiconductor caused by the oxidation of H₂O in acid and neutral media.³ So, the ∙OH radicals generated by electrocatalysis can decompose TC-HCl.⁵ The VSM findings indicate that BP-FMO particles exhibit magnetism, which aids in the accumulation of TC-HCl on the CC-BP-FMO surface, thereby enhancing the electrical catalytic degradation of TC-HCl.⁶¹ However, the TC-HCl electrocatalytic degradation by the CC-BP-FMO is not good enough, primarily because the amount of BP-FMO heterojunction loaded on CC surface is too small, resulting in a low concentration of active radicals.

With respect to the PEC, the photo-generated e⁻ and h⁺ are produced on the CC-BP-FMO under light irradiation. There exist some electrons in carbon fibers generated by the external voltage. These electrons can attract the photo-generated h⁺ to accumulate at the interface between the carbon fiber and BP-FMO heterojunction and then neutralize the photo-generated h⁺. The photo-generated e⁻ will transfer to the CC-BP-FMO surface. Consequently, the successful segregation of e⁻ and h⁺ pairs generated by light is accomplished, leading to the abundant generation of $\cdot O_{2}^{-}$ radicals to expedite the redox reactions of Fe²⁺/Fe³⁺ and Mo⁶⁺/Mo⁴⁺.⁶² In addition, the electrocatalysis also promotes the decomposition of TC-HCl by generating ∙OH radicals. The $\cdot O_{2}^{-}$ radicals are the dominant reactive species rather than ∙OH radicals, which is determined by the p-n heterostructured CC-BP-FMO (Figure 4(c)).

There exist five possible degradation pathways in the PMS-assisted PEC system. According to reports, the greater the electronegativity of an element, the more powerful its capacity to draw in electrons. The XPS findings suggest that FMO comprises Fe³⁺, Fe²⁺, Mo⁶⁺, and Mo⁴⁺ components. The Pauling electronegativities of Fe³⁺ and Fe²⁺are 1.96⁶³ and 1.83,⁶⁴ while the Pauling electronegativities of Mo⁶⁺ and Mo⁴⁺ are 2.35 and 2.24,^63,64 respectively. The electron affinity potential of Fe is 0.151 eV, smaller than the 0.7472 eV of Mo.⁶⁵ Thus, the Mo element attracts electrons more easily than the Fe element. The order of obtaining electrons of the Fe and Mo elements with different valences is as follows: Mo⁶⁺ > Mo⁴⁺ >-Fe³⁺ > Fe²⁺. The PMS activation by Fe²⁺ can be realized through a rapid oxidation process, thus producing Fe³⁺. The Mo ions are helpful for the conversion of Fe³⁺ to Fe²⁺, thus facilitating the Fe³⁺/Fe²⁺ cycle. FMO contains a certain amount of Mo⁴⁺, which is also involved in the activation of PMS, leading to the production of Mo⁶⁺. The consumed Mo⁴⁺ is regenerated after PMS reduction (Path III-1, Equations (12)–(14)).⁵¹ Upon light excitation, ${HSO}_{5}^{-}$ acting as an electron acceptor can capture partial electrons in the E_CB of the CC-BP-FMO to produce ∙OH and $\cdot {SO}_{4}^{-}$ radicals (Path III-2, Equations (15) and (16)).¹² The $\cdot O_{2}^{-}$ radicals produced by photoexcitation can interact with ${HSO}_{5}^{-}$ to generate ∙OH and $\cdot {SO}_{4}^{-}$ radicals (Path III-3, Equation (17)).⁵³ The photo-generated holes in the E_VB of the CC-BP-FMO can oxidize ${HSO}_{5}^{-}$ to produce $\cdot {SO}_{5}^{-}$ radicals (Path III-4, Equation (18)) with high oxidation potential.¹² The $\cdot {SO}_{5}^{-}$ radicals quickly combine with H₂O to generate ¹O₂ nonradicals (Equation (19)).⁵¹ In addition, the O_V induced by P and Mo in the CC-BP-FMO can activate ${HSO}_{5}^{-}$ to produce $\cdot O_{2}^{-}$ (Path III-5, Equation (20)).⁶⁶ More importantly, as confirmed by Pauling electronegativity, the Mo⁶⁺ ions can oxide Fe²⁺ ions to Fe³⁺ ions, while Mo⁶⁺ itself is reduced to Mo⁴⁺.³⁵ In the meantime, Mo⁴⁺ ions have the ability to convert Fe³⁺ ions into Fe²⁺ ions, while Mo⁴⁺ undergoes oxidation to Mo⁶⁺ (Equations (21) and (22)). Hence, the presence of Mo⁶⁺/Mo⁴⁺ can enhance the recycling and regeneration of Fe²⁺/Fe³⁺, accelerating the generation of $\cdot O_{2}^{-}$ , $\cdot {SO}_{4}^{-}$ , and ∙OH radicals, thereby enhancing the PMS-assisted PEC system. Consequently, the TC-HCl pollutant undergoes swift decomposition into intermediates that subsequently convert into CO₂ and H₂O through the action of O_V, $\cdot O_{2}^{-}$ , $\cdot {SO}_{4}^{-}$ , and ∙OH radicals, and ¹O₂ nonradicals (Equation (23)), which is confirmed by the ESR results. The main reactive species are $\cdot {SO}_{4}^{-}$ and $\cdot O_{2}^{-}$ radicals because of the shorter reaction pathway compared with other radicals. Furthermore, the contact between TC-HCl and the catalyst⁶⁷ can enhance the surface adsorption energy of the CC-BP-FMO through the presence of O_V. Under visible light, O_V is easier to maintain a high concentration to achieve a dynamic and stable equilibrium, and can be timely replenished during PMS activation, which makes the CC-BP-FMO have a high removal rate and good cycle stability to TC-HCl⁶⁶:
$CC - BP - FMO + h v \to CC - BP - FMO (h^{+} + e^{-})$
(4)

$O_{2} + e^{-} \to O_{2}^{-}$
(5)

$\cdot O_{2}^{-} + H_{2} O \to \cdot OH + H^{+}$
(6)

$\cdot O_{2}^{-} + h^{+} \to {}^{1}O_{2}$
(7)

${Fe}^{2 +} + H_{2} O_{2} \to {Fe}^{3 +} + {OH}^{-} + \cdot OH$
(8)

${Fe}^{3 +} + H_{2} O_{2} \to {Fe}^{2 +} + \cdot O_{2} H + H^{+}$
(9)

${Fe}^{2 +} + O_{2} \to {Fe}^{3 +} + \cdot O_{2}^{-}$
(10)

$CC - BP - FMO + H_{2} O \to CC - BP - FMO (\cdot OH) + H^{+} + e^{-}$
(11)

${Fe}^{2 +} + {HSO}_{5}^{-} \to \cdot {SO}_{4}^{-} + {OH}^{-} + {Fe}^{3 +}$
(12)

${Mo}^{4 +} + {HSO}_{5}^{-} \to \cdot {SO}_{4}^{-} + {OH}^{-} + {Mo}^{6 +}$
(13)

${Mo}^{6 +} + {HSO}_{5}^{-} \to \cdot {SO}_{5}^{-} + H^{+} + {Mo}^{4 +}$
(14)

${HSO}_{5}^{-} + h v (e^{-}) \to {OH}^{-} + \cdot {SO}_{4}^{-}$
(15)

${HSO}_{5}^{-} + h v (e^{-}) \to \cdot OH + {SO}_{4}^{2 -}$
(16)

$\cdot O_{2}^{-} + {HSO}_{5}^{-} \to \cdot OH + {SO}_{4}^{2 -}$
(17)

${HSO}_{5}^{-} + h v (h^{+}) \to H^{+} + \cdot {SO}_{5}^{-}$
(18)

$\cdot {SO}_{5}^{-} + H_{2} O \to {}^{1}O_{2} + {HSO}_{4}^{-}$
(19)

$O_{v} (P, Mo) + {HSO}_{5}^{-} \to \cdot O_{2}^{-} + {HSO}_{4}^{-}$
(20)

${Fe}^{2 +} + {Mo}^{6 +} \to {Fe}^{3 +} + {Mo}^{4 +}$
(21)

${Fe}^{3 +} + {Mo}^{4 +} \to {Fe}^{2 +} + {Mo}^{6 +}$
(22)

$\cdot O_{2}^{-} (\cdot {SO}_{4}^{-}, \cdot OH,^{1} O_{2}, O_{v}) + TC - HCl \to {CO}_{2} + H_{2} O$
(23)

Conclusions

In summary, we investigated the catalytic reaction mechanism of CC-supported BP-FMO composites via synergism of PEC and PMS activation using TC-HCl as the pollutant model and analyzed the potential degradation route of TC-HCl. When the mass ratio of BP to FMO was 3:1, the CC-BP-FMO composites exhibited a favorable photocatalysis performance in the degradation of TC-HCl. Under the conditions of 300 W xenon lamp irradiation, 1.0 V voltage, and 0.05 mM PMS activator, the CC-BP-FMO achieved a k value of 0.21 min⁻¹ and a degradation rate of 100% to TC-HCl, which was superior to photocatalysis, electrocatalysis, PMS activation, and PEC. What is more, the degradation rate is still as high as 97% after four cycles. The CC proved advantageous in efficiently separating the photo-induced charge carriers of BP-FMO, thereby greatly enhancing the degradation of TC-HCl. The degradation of TC-HCl involved five possible degradation pathways in the PMS-assisted PEC system and 10 intermediate compounds. The EPR findings verified that $\cdot {SO}_{4}^{-}$ , $\cdot O_{2}^{-}$ , and ¹O₂ were the primary reactive species, while O_V and ∙OH radicals assisted in the degradation of TC-HCl. The CC-BP-FMO had a reduced band gap in contrast to BP-FMO, which was advantageous for the production of charge carriers. The electrochemical findings confirmed that the CC-BP-FMO exhibited a current density 12.27 times higher than the BP-FMO under identical conditions, while the CC-BP-FMO displayed a Tafel slope of 8.58 mV/dec. This implied the advantage of CC as the conduction of carriers. Significantly, in the PMS-assisted PEC system, the oxidation-reduction reaction involving Fe⁺²/Fe⁺³ and Mo⁺⁴/Mo⁺⁶ played a crucial role in generating reactive substances that enhanced the decomposition of TC-HCl. This work provides a promising method for treating wastewater containing organic pollutants.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Natural Science Foundation of China (no. 51873169).

ORCID iD

Hui Zhang

References

Zhao

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