Fabrication and Characterization of Co-Doped Fe2O3 Spindles for the Enhanced Photo-Fenton Catalytic Degradation of Tetracycline.

Co-doped Fe2O3 spindles with different Co contents were successfully fabricated by a facile one-step hydrothermal method. The crystalline structure, morphology, optical properties, and chemical state of the as-prepared catalysts before and after photo-Fenton reaction were characterized. Co2+ incorporated into the Fe2O3 lattice was confirmed by the above characterizations. Also, the photocatalytic and photo-Fenton catalytic performances of the samples were evaluated by the degradation of tetracycline (TC) under visible light irradiation in the absence/presence of H2O2. The results demonstrated that Co-doped Fe2O3 spindles exhibited better catalytic degradation performance in comparison with single Fe2O3 spindles, and the sample of Co(5%)-Fe2O3 spindles displayed the highest activity and best stability. The improvement of photo-Fenton activity might be attributed to two reasons: On the one hand, Co-doped Fe2O3 spindles not only formed the Fe vacancies to reduce the band gap but also could build up an internal electric field, which inhibits electron/hole pair recombination and facilitates the transfer of photoexcited charge carriers. On the other hand, the intrinsic Co2+/Co3+ redox cycling can accelerate the circulation between Fe2+ and Fe3+ in Co(5%)-Fe2O3 spindles to facilitate H2O2 consumption and produce more ·OH radicals for TC degradation.

Introduction

The rapid development of the global energy demand and the increasing improvement of people’s living standards have resulted in environmental pollution and clean water crisis.[1,2] Water is the source of all the living beings, but the organic and inorganic compounds of various industries in water have become a serious threat.[3,4] Particularly, antibiotics have been used for some decades; until recently, the disorderly emission of antibiotics in the aquatic environment has become a global concern.[5,6] Tetracycline, one of the most representative antimicrobial agents, has been widely used in veterinary, agriculture, medical, and clinical therapy applications.[7,8] However, tetracycline has a stable aromatic structure and functional groups; a large amount of tetracycline remains in the aquatic environment due to low removal efficiency in natural conditions.[9] To treat tetracycline, various techniques including physical adsorption, filtration, biological methods, chlorine oxidation, ozonation oxidation, etc., have been developed.[10−14] Nevertheless, these methods have certain defects (without degradation, low efficiency, complicated process, high energy consumption, and operating costs).[15] Therefore, an efficient, cost-effective, and environmentally friendly technology should be developed to remove tetracycline from wastewaters.[16]

Advanced oxidation processes (AOPs) with high chemical stability and/or low biodegradability produce very active radicals at near ambient temperature and pressure and are considered as a powerful technology for the treatment of tetracycline from organic wastewaters into nontoxic products of CO2 and H2O.[17,18] AOPs are environmentally friendly methods including wet oxidation,[19] photochemical oxidation,[20] electrochemical oxidation,[21] photoelectrochemical oxidation,[22] Fenton oxidation,[23] and photo-Fenton oxidation.[24] AOPs possess common features of the strong standard reduction potential, high bimolecular reaction rate constants, and nonselective reactivity of active radicals, which are beneficial to the rapid and high-efficiency degradation of organic compounds.[25] The reaction of Fenton oxidation is one of the most cost-effective AOPs and was discovered in 1894 by Henry J. Fenton.[26] However, the method was not applied for the degradation of organic pollutants until the late 1960s.[27] The reagents of conventional Fenton oxidation consist of H2O2 and homogeneous solution of Fe2+ ions. The catalytic process of Fenton reaction consists of the oxidation of Fe2+ and the reduction of Fe3+. Also, the reaction rate of the oxidation of Fe2+ is about 6000 times that of the reduction of Fe3+.[28] So, there are three obvious shortcomings in the conventional Fenton oxidation process (the high costs and risks associated with handling, transportation, and storage of reagents, the narrow pH range of the solution, and the significant iron sludge-related environment pollution).[29]

Compared with the conventional Fenton process, the process of photo-Fenton oxidation can produce more hydroxyl radicals, which can enhance the degradation efficiency of organic pollutants. Also, the reduction reaction of Fe3+ to Fe2+ can be also accelerated under light irradiation to reduce the production of iron sludge.[30] The catalyst plays a crucial role in the photo-Fenton oxidation process. Fe2O3 is an n-type semiconductor with a band gap of 1.9–2.2 eV, which possesses high stability and low cost.[31] Nonetheless, Fe2O3 inherently exhibits low surface reaction rates, low carrier mobility (<1 cm2/(V·S)), high recombination rates of photogenerated charge carriers (∼10 ps), and a short hole diffusion length (2–4 nm) .[32] Many efforts have been done to overcome its shortfalls and make it an effective catalyst. Doping/incorporation of metal ions such as Ti, Zn, Ni, Sn, Al, Mg, Ga, Rh, and Zr is an effective way to alter the physical–chemical properties of Fe2O3.[33,34] Co2+ as a suitable dopant could be incorporated into the crystal lattice of Fe3+, thus easily leading to the narrowed band gap and reducing the recombination of photogenerated charge carriers.[35]

Herein, Co-doped Fe2O3 spindles were fabricated by the in situ hydrothermal method. The morphological, crystal structural, and optical properties of the as-prepared catalysts were characterized. The photocatalytic and photo-Fenton activity, stability, and reusability for tetracycline degradation were investigated under visible light illumination. Furthermore, the possible photo-Fenton mechanism over Co-doped Fe2O3 spindles for the degradation of TC was also elucidated.

Results and Discussion

XRD patterns for Fe2O3 spindles and Co(m%)-Fe2O3 (m = 2.5, 5, 7.5, and 10) spindles are shown in Figure and Figure S1. There are eight main diffraction peaks in Figure a located at 2θ values of 24.4, 33.5, 35.8, 41.2, 49.5, 54.5, 62.7, and 64.2, which match the Fe2O3 phase (JCPDS: 33-0664) and correspond with the planes (012), (104), (110), (113), (024), (116), (214), and (300), respectively.[35] No extra diffraction peaks of other impurity phases were observed, indicating the high crystallinity of the as-obtained Fe2O3 spindles. Compared with pure Fe2O3 spindles, the diffraction peaks corresponding to the Fe2O3 phase are clearly presented in the Co(m%)-Fe2O3 (m = 2.5, 5, 7.5, and 10) samples, but much decreased shapes compared to those from pure Fe2O3 spindles are demonstrated in Figure b. Also, the (104) and (110) peaks shift to a much lower angle after Co(5%) doping in Fe2O3 spindles, revealing that larger Co ions have successfully entered into the lattice of Fe2O3.[36] In addition, the Raman spectra agree well with the exact characteristic phonon vibration modes of Fe2O3 spindles in Figure S2. There are six well-defined Raman active modes such as A1g(1), Eg(1), Eg(2), A1g(2), Eu, and 2Eu situated at ∼219, 288, 400, 487, 644, and 1307 cm–1, respectively.[37] The peak intensities of Co(5%)-Fe2O3 spindles show a slight decrease compared to those of Fe2O3 spindles. This may be due to the partial substitutions of the Fe ion lattice sites by Co ions.[38] The results of Raman spectroscopy are consistent with the XRD results.

Figure 1

(a) XRD patterns and (b) enlargement of the diffraction peak positions in the range of 2θ = 32–37° of Fe2O3 spindles and Co(5%)-Fe2O3 spindles.

The morphologies of samples were characterized by FESEM with EDX analysis and HRTEM. Figures a and 3a show the morphology and microstructure of the as-synthesized Fe2O3 spindles. They exhibit a mean length of 1–1.3 μm and a width of about 320 nm. The lattice fringe analysis is presented in Figure b; the measured interplanar distance of 0.25 nm is in good accord with the d spacing of (110) planes of Fe2O3 spindles.[39] As can be seen from the images shown in Figures b and 3c,d and Figure S3, when Co ions were doped into the lattice sites of Fe2O3 spindles, the morphology and interplanar distance did not change. Furthermore, Figure c–g exhibits the EDS spectra and the elemental mapping of Co(5%)-Fe2O3 spindles. The results reveal that the as-made Co(5%)-Fe2O3 spindles is mainly composed of Fe, Co, and O elements. Obviously, Co could be uniformly distributed in the Co(5%)-Fe2O3 spindles. To further confirm the structure, the specific surface area of Fe2O3 and Co(5%)-Fe2O3 was measured by the adsorption and desorption isotherms of N2. As shown in Figure S4, the N2 sorption isotherms were determined as type IV with H3 hysteresis loops.[40] The specific surface areas of Fe2O3 and Co(5%)-Fe2O3 are about 15.0 and 15.4 m2/g, and the corresponding pore volumes are calculated to be 0.074 and 0.072 cc/g, respectively. Thus, according to the results and the previous literature, when Co is introduced into Fe2O3 spindles, there is still ambiguity about Co replacing the Fe site in the Fe2O3 lattice.[41]

Figure 2

SEM images of (a) Fe2O3 spindles and (b) Co(5%)-Fe2O3 spindles, SEM image of (c) individual Co(5%)-Fe2O3 spindles, (d) EDX, and the corresponding elemental mapping of (e) Fe, (f) Co, and (g) O.

Figure 3

TEM images of (a) Fe2O3 spindles and (c) Co(5%)-Fe2O3 spindles and HRTEM images showing lattice fringes of (b) Fe2O3 spindles and (d) Co(5%)-Fe2O3 spindles.

XPS reveals the elemental chemical status and composition of the as-obtained Co(5%)-Fe2O3 spindles. As shown in Figure a, it can be clearly seen that the peaks of Fe, Co, O, and C can be observed in the survey spectrum of Co(5%)-Fe2O3 spindles. The photoelectron peak for C 1s at 284 eV is probably attributed to the contamination caused by the residual carbon from the precursor solution and specimen handling or pumping oil from the XPS instrument itself.[42] For the high-resolution XPS spectra, the Fe 2p3/2 and Fe 2p1/2 peaks were located at binding energies of 710.5 and 723.8 eV, respectively. Also, the corresponding satellite peaks at 718.3 and 732.9 eV are detected. The results prove that the chemical state of the iron element is Fe3+.[43] One clear peak at 783.7 eV in Figure c can be deconvoluted into four peaks for Co2+ 2p3/2 (780.1 and 786.6 eV), Co3+ 2p3/2 (783.1 eV), and Co2+ 2p1/2 (794.1 eV).[44] The high-resolution spectrum of O 1s is shown in Figure d; the peaks located at around 529.3, 530.7, and 532.1 eV were associated with the lattice oxygen atoms, chemically adsorbed oxygen, and physically adsorbed oxygen, respectively.[45]

Figure 4

XPS survey spectra of (a) Co(5%)-Fe2O3 spindles and high-resolution XPS spectra of (b) Fe 2p, (c) Co 2p, and (d) O 1s of Co(5%)-Fe2O3 spindles.

The optical absorption property of the samples was characterized by UV–Vis absorption spectroscopy in the range of 250–750 nm. Figure S5 and Figure show the optical absorption property. As shown in the corresponding band gap of Fe2O3 spindles and Co(5%)-Fe2O3 spindles in Figure a, the as-prepared Fe2O3 spindles show a good harvesting ability in the visible light range. The corresponding band gap (Eg) was calculated by the equation αhν = A(hν – Eg), where α, h, ν, and A represent the absorption coefficient, Planck’s constant, the frequency of incident photons, and a constant, respectively.[46] The band gap of Fe2O3 spindles was measured to be ∼2 eV in Figure b. Furthermore, Co(5%)-Fe2O3 spindles exhibit enhanced light harvesting ability and the red-shift of the absorption edge. The band gap of Co(5%)-Fe2O3 spindles was measured to be ∼1.9 eV. The introduction of cobalt ions will further enhance and extend the visible absorption ability to generate more photogenerated charge carriers for higher photocatalytic performance.[36]

Figure 5

(a, b) UV–Vis absorption spectrum and transformed Tauc plots of Fe2O3 spindles and Co(5%)-Fe2O3 spindles.

To further understand the charge separation and transfer efficiency in photoreactive materials, photoelectrochemical measurements are investigated. The transient photocurrent performances of Fe2O3 spindles and Co(m%)-Fe2O3 (m = 2.5, 5, 7.5, and 10) spindles are shown in Figure . The transient photocurrent of Co(m%)-Fe2O3 (m = 2.5, 5, 7.5, and 10) spindles is remarkably enhanced compared with that of Fe2O3 spindles, which indicates that there are a larger number of free photogenerated carriers under visible light illumination. However, it also shows that there is a decrease in the transient photocurrent on the Co(m%)-Fe2O3 (m = 7.5 and 10) spindle photoanode. Moreover, electrochemical impedance spectroscopy (EIS) was used to study the charge-transfer properties of the catalysts. Among all materials, Co(5%)-Fe2O3 spindles possessed the smallest radius in Nyquist diagrams, reflecting the lowest charge transfer impedance at the electrode–electrolyte interface.

Figure 6

Transient photocurrent response and Nyquist plots of Fe2O3 and Co(m%)-Fe2O3 (m = 2.5, 5, 7.5, and 10).

The photocatalytic activities of TC degradation over Fe2O3 spindles and Co(m%)-Fe2O3 spindles (m = 2.5, 5, 7.5, and 10) were investigated under visible light irradiation, and the experiment error was within 5%. The degradation efficiencies of TC in the dark are very low. As shown in Figure S6a, it is clearly seen that Co(5%)-Fe2O3 spindles exhibit much higher photocatalytic activity than pure Fe2O3 spindles and Co(m%)-Fe2O3 spindles (m = 2.5, 7.5, and 10). The photocatalytic degradation kinetics follows a pseudo-first-order reaction model, −ln(C/C0) = kt, where k, C0, and C are the apparent reaction rate constant (min–1), initial equilibrium concentration, and instant concentration at reaction time t, respectively.[47] Based on the reaction kinetic equation, Co(5%)-Fe2O3 spindles exhibit the optimum photocatalytic efficiency (k = 0.00492 min–1) for the degradation of TC (Figure S6b). To further investigate the photo-Fenton performance of Fe2O3 spindles and Co(m%)-Fe2O3 spindles (m = 2.5, 5, 7.5, and 10), the TC degradation performance is also measured and presented in Figure . The photo-Fenton activity of Co(5%)-Fe2O3 spindles in Figure a is much higher than those of Fe2O3 spindles and Co(m%)-Fe2O3 spindles (m = 2.5, 7.5, and 10). With the calculated first-order kinetics, the degradation rates of Fe2O3 spindles and Co(m%)-Fe2O3 spindles (m = 2.5, 5, 7.5, and 10) are 0.00855, 0.01173, 0.01871, 0.01063, and 0.00984 min–1, respectively. The degradation rate of TC through the photo-Fenton-like reaction over Co(5%)-Fe2O3 spindles is almost 3.8 times higher than that of the photocatalytic reaction (Figure b).

Figure 7

(a) Photo-Fenton activities of Fe2O3 spindles and Co(5%)-Fe2O3 spindles for TC degradation with H2O2 under visible light irradiation, (b) plots of ln(C0/C) versus time for TC, (c) stability test, and (d) active species trapping experiments of Co(5%)-Fe2O3 spindles.

In addition, the mineralization ability of the Fe2O3 spindle and Co(5%)-Fe2O3 spindle photocatalysts during TC degradation was investigated by TOC measurements. As can be seen in Figure S7, TOC removal reached 4.5 and 45.7% for Fe2O3 spindles and Co(5%)-Fe2O3 spindles after 60 min, respectively. The less TOC removal can be attributed to the decomposition of TC molecules into intermediate metabolites.[48] Obviously, doping with a certain amount of Co can significantly improve the photo-Fenton degradation efficiency of Fe2O3 spindles. However, excessive doping of Co in Fe2O3 will act as a charge carrier recombination center and has adverse effects on the photogenerated carrier separation, which will result in a negative effect on TC degradation.

The reusability and chemical stability of the catalyst are also important for the practical applications. As shown in Figure c, the removal rate of TC by Co(5%)-Fe2O3 spindles shows no remarkable difference for four successive cycles under similar conditions, indicating that the catalyst of Co(5%)-Fe2O3 spindles has good stability. To ascertain the mechanism of photocatalytic degradation reaction, some active species to TC degradation were identified and characterized through the implementation of trapping experiments. The experiment was executed by applying AO (5 mM) as a hole scavenger (h+), IBA (10 mM) as a quencher for the hydroxyl radical (·OH), and BQ (1 mM) as a scavenger for the superoxide radical (·O2–), respectively.[9] It can be seen from Figure d that the photocatalytic degradation efficiency had a slight inhibition due to the addition of AO, indicating that hole scavengers were not the major active species. However, the efficiency of TC removal had a moderate inhibition by the addition of BQ, and the addition of IBA had a considerable influence on TC removal efficiency. The results also meant that the species of hydroxyl radical (·OH) played important roles as the main oxidative species in the process of TC degradation.

The spin-trapping ESR/DMPO technique was further employed to detect the active species in our photocatalytic and photo-Fenton systems. As shown in Figure S8 and Figure , all ESR experiments were performed under both dark and light conditions. As shown in Figure S8, there are no DMPO-·O2– and DMPO-·OH signals being detected under dark conditions, while there is only the DMPO-·OH signal peak (1:2:2:1) in the EPR spectra of Fe2O3 spindles. However, the peaks of DMPO-·O2– (1:1:1:1) and DMPO-·OH were detected in the EPR spectra of Co(5%)-Fe2O3 spindles under visible light irradiation. These results indicate that the active species of ·O2– and ·OH can be produced in the photocatalytic reaction system of Co(5%)-Fe2O3 spindles for TC degradation, while the active species of ·OH can only be produced in the photocatalytic system of Fe2O3 spindles. As shown in Figure , the additional ·O2– and ·OH radicals could be generated due to the presence of H2O2 in the photo-Fenton reaction system, while the ESR signal intensity of DMPO-·O2– and DMPO-·OH increased with the increase in visible irradiation time. Compared to the EPR experiment of Fe2O3 spindles, the signal intensity of DMPO-·O2– was a little stronger in the EPR experiment of Co(5%)-Fe2O3 spindles. However, the signal intensity of DMPO-·OH was a significant enhancement in the EPR experiment of Co(5%)-Fe2O3 spindles. The ESR results show that ·OH radicals as the main active species are produced when H2O2 reacts with Co(5%)-Fe2O3 under visible light. Also, the above conclusions are also in accord with the active radical quenching experiment in Figure d.

Figure 8

ESR spectra of Fe2O3 spindles and Co(m%)-Fe2O3 (m = 2.5, 5, 7.5, and 10) spindles for DMPO-·O2– and DMPO-·OH in the photo-Fenton system.

To explore the intermediate products of the catalyst of Co(5%)-Fe2O3 in the degradation of TC, the composition of the reaction solution was analyzed by the HPLC–MS technology. The spectrum peak of TC and the corresponding spectrum peaks of intermediate organic molecules of the reaction solution are shown in Figure S9. According to the previous experimental analysis and related literature,[49] the degradation products and degradation pathways of TC are proposed in Figure . Under visible light irradiation, TC (m/z = 445) first undergoes a hydroxylation reaction to generate intermediates m/z = 458 and m/z = 475. Additionally, the TC is oxidized (dehydration reaction) into the products m/z=431 and m/z=427Subsequently, a parallel attack mode induced photo-Fenton reactions (a methylation reaction and deamidation) and generated intermediate products m/z = 406, m/z = 407.5, m/z = 392, and m/z = 384. The above intermediate products further participated in ring opening, the removal of H2O, and the deamination reaction and were converted to m/z = 373, m/z = 340, m/z = 318, m/z = 278, and m/z = 225. As the reaction continues, these unstable intermediate products are further oxidized into low-molecular-weight organic molecules. Finally, the organic molecules are further mineralized into small-molecule substances such as CO2, H2O, and NH4+.

Figure 9

Proposed photo-Fenton catalytic degradation pathways of TC.

Based on the above analyzed and discussed results, the probable reaction mechanism of the photo-Fenton degradation of TC by Co(5%)-Fe2O3 spindles was put forward (Scheme ). H2O2 played the important role of an oxidizing agent to facilitate the degradation reaction. First, the photogenerated electrons are transferred from the valence band to the conduction band under visible light irradiation, with the h+ created in the valence band. However, some of the electrons could recombine with the photogenerated holes.[50] Second, H2O2 and O2 can capture the photogenerated electrons to form ·OH radicals and ·O2– radicals.[9] Simultaneously, the reduction of Fe3+ to Fe2+ can happen with the photogenerated electrons. More ·OH radicals can be produced by the reaction between formed Fe2+ and H2O2.[51] Likewise, the h+ will react with H2O molecules and be converted into ·OH radicals. Finally, the generated radicals of ·OH and ·O2– can oxidize the organic pollutants of TC to form some intermediates (2,3-dioxosuccinic acid, carbamate, 2-oxomalonic acid, and other small organic compounds), even mineralizing them into harmless products, CO2, H2O, and NH4+.

Scheme 1

Proposed Photo-Fenton Mechanism of TC Degradation for the Co(5%)-Fe2O3 Photocatalyst under Visible Light Irradiation

Similar with Fe2+, Co2+ ions in Co(5%)-Fe2O3 spindles are also capable of reacting with H2O2 to generate ·OH. Also, the reduction of Co3+ with the photogenerated electrons can produce Co2+.

Furthermore, on the one hand, Co-doped Fe2O3 spindles not only formed the Fe vacancies to reduce the band gap but also could build up an internal electric field, which inhibits electron and hole pair recombination and facilitates the transfer of photoexcited charge carriers as evident from the photoluminescence spectroscopy study (Figure S10).[52] On the other hand, combined with radical detection, the accelerated electron transfer is beneficial to improve the interface between Fe2+ and H2O2, which results in more ·OH radicals from an efficient degradation.[53] Additionally, the intrinsic Co2+/Co3+ redox cycling can accelerate the circulation between Fe2+ and Fe3+ in Co(5%)-Fe2O3 spindles due to the inherent activity of cobalt for water oxidation. In addition, the crystalline structure and morphology of Co(5%)-Fe2O3 spindles have no changes after photo-Fenton recycling experiments as indicated by the XRD spectrum and the SEM image in Figure a,b, respectively. Fe and Co chemical states in Co(5%)-Fe2O3 spindles before and after TC degradation were also characterized by XPS, and the results are shown in Figure c,d, respectively. These results further indicate better chemical stability. This work provides a promising strategy to fabricate high-efficiency metal-doped Fe2O3 for photo-Fenton degradation of dyes from wastewater.

Figure 10

(a) XRD, (b) SEM image, and XPS spectra for elements (c) Fe and (d) Co of Co(5%)-Fe2O3 spindles before and after TC degradation in the photo-Fenton system.

Conclusions

In summary, we have successfully fabricated Co-doped Fe2O3 spindles with different Co contents via a facile hydrothermal method. Co doping has almost no influence on the crystal structural and morphology of Fe2O3 spindles. However, Co2+ incorporated into the Fe2O3 lattice reduces the band gap. The obtained Co(5%)-Fe2O3 spindle catalyst exhibited superior photo-Fenton degradation activity of TC and good stability. The improvement of the photo-Fenton performance might be attributed to the moderate Co2+ incorporated into the Fe2O3 lattice; the intrinsic Co2+/Co3+ redox cycling under visible light irradiation can accelerate the circulation between Fe2+ and Fe3+ in Co(5%)-Fe2O3 spindles to facilitate H2O2 consumption and provide more active sites (·OH radical) for TC degradation.

Experimental Section

Preparation of Co-Doped Fe2O3 Spindles

All the reagents were used as received without further purification. The detailed preparation was carried out according to the previous literature.[54] A certain amount of FeCl3 and CoCl2 at room temperature was dissolved in 60 mL of deionized water, followed by the addition of NaH2PO4 under constant stirring for 1.5 h. The mixture solution was transferred to a 100 mL Teflon-lined stainless-steel autoclave, which was heated at 110 °C for 48 h. Then, the autoclave was cooled spontaneously to room temperature after the reaction, and the as-obtained products were washed with deionized water for several times, separated, and collected by centrifugation. Finally, the samples were dried at 50 °C for 12 h. The final samples were identified as Co(x%)-Fe2O3 spindles according to the molar ratio of Co/Fe. The molar ratios of Co/Fe (0, 2.5, 5, 7.5, and 10%) correspond to Co(x%)-Fe2O3 spindles named as Fe2O3 spindles, Co(2.5%)-Fe2O3 spindles, Co(5%)-Fe2O3 spindles, Co(7.5%)-Fe2O3 spindles, and Co(10%)-Fe2O3 spindles, respectively.

Characterization

The crystallographic phases of the as-prepared samples were investigated by an X-ray diffractometer (XRD, X’Pert-PRO, PANalytical, Holland) with Cu Kα radiation at an accelerating voltage of 40 kV and a current of 40 mA (X-ray, λ = 0.15418 nm). Field-emission scanning electron microscopy (FESEM) images were conducted on a Hitachi S-4800 microscope with a Horiba EX250 X-ray energy-dispersive spectrometer (EDX). High-resolution transmission electron microscopy (HRTEM) was carried out using a JEM-2100F microscope operated at 200 kV. X-ray photoelectron spectroscopy (XPS) was performed on an AXIS Ultra DLD (Kratos, Britain). The Raman spectra and UV–Vis absorption spectra were performed by a LabRAM HR800 (Horiba Jobin Yvon, France) and a Shimadzu U3010 UV–Vis spectrophotometer at room temperature, respectively. The photoluminescence (PL) spectra of the samples were recorded using a PerkinElmer LS 550 spectrofluorometer with a 150 W xenon lamp as the excitation source. The total organic carbon (TOC) was measured using a TOC analyzer (Elementar, Vario TOC, Germany) to determine the extent of mineralization. Electron spin resonance (ESR) was performed on a model ESR JES-FA300 spectrometer. ·O2– and ·OH detections were performed with 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as a spin-trapping agent in a methanol and water system, respectively. The intermediate products formed by the degradation of TC were detected using high-performance liquid chromatography–mass spectrometry (HPLC–MS) (Agilent 1260-6420, USA).

Photoelectrochemical Measurements

A 300 W Xe lamp was used as the light source. The light intensity was 100 mW/cm2 and the area of the sample exposed to light was 1 cm2. All the photoelectrochemical measurements under visible light irradiation were carried out in a three-electrode photoelectrochemical cell with a quartz window at room temperature. A Pt counter electrode and a saturated calomel reference electrode (saturated with KCl) were adopted as the counter electrode and the reference electrode, respectively. The as-prepared Fe2O3 and Co(m%)-Fe2O3 (m = 2.5, 5, 7.5, and 10) photoelectrodes were individually prepared and tested as the working electrode. The transient photocurrent and impedance spectra were measured by a CS350H electrochemical station. All photoelectrochemical experiments were conducted in an aqueous solution containing 0.1 M NaOH.

Photocatalytic Activity

The photocatalytic and photo-Fenton activities of TC degradation were evaluated using a self-made reactor. A high-pressure sodium lamp (125 W) was used as the light source with main emission in the range of 400–800 nm, and the irradiation intensity was calibrated to 80 mW/cm2. In each experiment, 30 mg of the photocatalyst was dispersed in 600 mL of tetracycline aqueous solution with an initial concentration of 40 mg/L to form the reaction suspension. Before the irradiation, the suspension underwent ultrasonic dispersion for 20 min and stirred for 1 h in the dark to reach an adsorption–desorption equilibrium. Cooling water was recycled throughout the reaction period to maintain the room temperature of the suspension. A total of 0.5 mL of H2O2 aqueous solution (30%) was added to the above solution before the irradiation. The absorbance peak of tetracycline aqueous solution was recorded every 10 min to enable the calculation of the degradation rate. A UV–Vis (U-3010, Hitachi) spectrophotometer was used to determine the concentration of the tetracycline solution during the photocatalytic degradation reaction at the characteristic absorption wavelength of ∼356 nm. The reusability and stability of the catalysts were investigated by the recycling tests. After each run, the photocatalysts were collected by centrifugation, washed for four times, and finally dried at a temperature of 60 °C in an oven. The relative roles of reactive species in the photo-Fenton system were validated by quenching experiments. Additionally, 10 mmol/L tert-butyl alcohol (TBA), 5 mmol/L ammonium oxalate (AO), and 1 mmol/L benzoquinone (BQ) were added to the degradation system as the scavengers of ·OH, h+, and ·O2–, respectively.