English

• 0.   Introduction

  Fenton reaction as an efficient exhaustive oxidation process for advanced water treatment has drawn more due to the formation of non-selective hydroxyl radicals (·OH)^[1-2]. However, traditional homogeneous Fenton systems (Fe²⁺/H₂O₂) suffer severely from limited pH range, large iron sludge, and damage to the catalyst^[3]. Therefore, the science and technology behind the enhanced catalytic performance of homogeneous Fenton systems, especially based on homogeneous Fenton transition metal ions, have attracted increasing attention over the past several decades^[4-6]. for example, by adding various materials, or photo-, sono-, electro-assisting and using organic matter as the co-catalysts to enhance the generation of ·OH^[7-9]. Unfortunately, the strict acidic condition to prevent iron precipitation and the poor stability during redox cycling still remains the main challenges for iron-based homogeneous Fenton systems^[10]. On the other hand, organic co-catalysts can easily cause secondary pollution, making it very difficult to achieve complete mineralization of the organic molecule. Furthermore, maintaining stable activity of advanced oxidation processes with organic co-catalysts because of the self-degradation induced by the oxidation of the co-catalysts by ·OH is difficult^[11].

  To avoid the shortcomings of the traditional homogeneous Fenton reaction, heterogeneous Fenton systems, based on transitional metal or metal oxides in multiple oxidation states, are developed^[12-15]. However, many pure transitional metal or metal oxides, it has rarely been considered a direct catalyst in heterogeneous Fenton systems due to its intrinsic poor reactivity. For example, it is hard for the dissociation of H₂O₂ to occur on the defect-free Fe₂O₃ surface in heterogeneous Fenton systems, while modified materials, such as TiO₂/Au/Fe₂O₃, S doped Fe₂O₃, and Fe@Fe₂O₃ compo-sites, show high performance in the degradation of pollutants, making them complex and expensive^[16-18]. Currently, like many other transition metal oxides, it is still very challenging to apply a pure Fe₂O₃-based catalyst directly for heterogeneous Fenton systems.

  It has been reported that oxygen vacancy defects are able to modulate the surface-electron state and serve as active sites for redox reactions^{[19, 20]}. Further-more, surface oxygen vacancy defects can modulate both the coordination structures and electronic states of adsorbates via the surface dangling bonds and localized electrons, influencing the kinetics, energet-ics, and the mechanisms of catalytic processes^[21]. Recently, Mao et al. reported that oxygen vacancy concentration enhanced the selectivity in photocatalytic oxidation of amines to imines^[22]. They found that BiClO surface oxygen vacancy concentration could alter the molecular oxygen activation pathway as well as the mechanisms of catalytic processes. According to Li et al., based on BiClO surface oxygen vacancy of electron-donor nature for the heterolytic H₂O₂ dissociation, generated ·OH could effectively degrade organic pollutants in the pH range of 3~10, having a much wider pH working window than traditional Fenton systems^[23]. Xing et al. reported that unsaturated S atoms on the surface of metal sulfides expose metallic active sites with reductive properties, which can serve as excellent co-catalysts to greatly increase the efficiency of H₂O₂ decomposition^[24]. Sun et al. using density functional theory calculations, conducted an investigation into the adsorption characteristics of H₂O₂ on defect-free and oxygen vacancy defect of α-Fe₂O_3-x (001) surfaces. The results show that H₂O₂ adsorption on a defect-free surface was also in a molecular form. However, on defect-containing α-Fe₂O₃(001) surface oxygen vacancy remarkably enhanced the adsorption intensities of H₂O₂ and oxygen vacancy and Fe²⁺ as the "Fenton-catalytic" center to promote H₂O₂ decomposition on the catalyst surface^[25]. Consid-ering these oxygen vacancy and Fe²⁺ reaction sites for H₂O₂ activation and oxygen vacancy with the excellent adsorption property of vacancy oxygen α-Fe₂O_3-x these factors. Herein, we put a surface-electronic-state-modulation-based concept utilizing oxygen vacancy defect α-Fe₂O_3-x (001) surfaces with dual reaction sites for dissociation H₂O₂, thus expected to be effective in improving the Fenton-like catalysis.

  We select α-Fe₂O₃, a typical transitional metal oxide with high earth abundance and low toxicity, as the model catalyst to check this possibility. In this study, we construct a surface Fenton system with defective α-Fe₂O_3-x (001) nanosheets, and H₂O₂ (α-Fe₂O_3-x (001)/H₂O₂) to degrade various organic pollutants, including p-nitrophenol, methylene blue, and rhoda-mine B. The presence of surface Fe²⁺ ion and OVs is confirmed by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The electron spin resonance (ESR), 1, 10-phenanthroline complexant experiment proved that Fe²⁺ and oxygen vacancy on defect-contain-ing α-Fe₂O_3-x surfaces as the "Fenton-catalytic" center for dissociate H₂O₂ to produce ·OH. Moreover, the stability and the reusability of defective α-Fe₂O_3-x (001) nanosheets are also investigated.

  1.   Experimental

  1.1   Chemicals

  FeCl₃·6H₂O, NaOH, HCl, H₂O₂ (30%, w/w), sodium acetate, ethyl alcohol rhodamine B (RhB), methylene blue (MB), p-nitrophenol (PNP), tert-butyl alcohol (TBA), hydroxylamine hydrochloride 1, 10-phenan-throline, and sodium acetate; all of analytical grade were used as received without any further purification and bought from Medicines Corporation Macklin. Milli-Q water (18 MΩ cm in resistivity, Millipore Corporation) was used throughout the experiments. All the concentra-tions of organic pollutant solutions were 20 mg·L^-1. The aqueous NaOH (0.1 mol·L^-1) and HCl (0.1 mol·L^-1) solutions were utilized to adjust the pH values of the degradation solutions. H₂O₂ solutions were freshly prepared before use and covered with foil paper to protect them from light.

  1.2   α-Fe₂O₃ nanosheets preparation

  The synthesis of α-Fe₂O₃ nanosheets, was based on previous reported methods^[26]. Briefly, FeCl₃·6H₂O (1 g) was dissolved under vigorous stirring in ethanol (40 mL) with Milli-Q water (2.8 mL). Afterward, sodium acetate (3.2 g) was added under stirring. Then, the mixture was sealed in a Teflon-lined stainless steel autoclave and heated at 180 ℃ for 12 h. Upon cooling to room temperature, the obtained products were washed with Milli-Q water and ethanol several times, respectively, and dried at 65 ℃ for 12 h. Then, the products were annealed in H₂ atmosphere (10% H₂+90% Ar, V/V)) at different temperatures (25, 300, 350) for 2 h at a ramp rate of 10 ℃·min^-1 (the samples were designated as T-1, α-Fe₂O₃-25, T-2, α-Fe₂O₃-300, T-3, and α-Fe₂O₃-330, respectively).

  1.3   Characterization

  Morphology and structure were characterized by high-resolution transmission electron microscopy (HR-TEM) measurements (Model No. JEM-2010, JEOL, Ltd., Tokyo, Japan). X-ray diffraction (XRD) patterns were recorded on a Philips X′Pert Pro X-ray diffracto-meter with Cu Kα radiation (0.154 18 nm, U=40 kV, I=40 mV, 2θ=20°~80°). XPS data were obtained using Perkin-Elmer PHI 5000C, and all binding energies were corrected for specimen charging by referencing the C1s peak to 284.8 eV. Raman measurements were carried out on a Lab RAM HR800 confocal micro-scope Raman system (Horiba Jobin Yvon, Inc., USA). The infrared spectra were recorded between 4 000 and 600 cm^-1 on a Fourier transform-infrared spectro-meter (FT-IR) (MagnaIR 750, Nicolet Instrument, Inc., USA). The surface charge was determined by a Zetasizer Nano ZSP. The scanning electron microscope (SEM) images were obtained with a SU8010 field-emission scanning electron microscope (Hitachi, Japan).

  1.4   Degradation procedures

  Batch trials were carried out in 50 mL conical flasks under shaking table concentrator at (25±2) ℃. Briefly, the reactions were triggered by adding 10 mg of catalyst (T-1, T-2, T-3) and 1 mL of 0.5 mol·L^-1 H₂O₂ solution into 20 mL of rhodamine B solution (20 mg·L^-1) in sequence. After that, 800 μL of the reaction solution was sampled at predetermined intervals. Followed by high-speed centrifugation and collection of the supernatant solution for the rhodamine B concentration measurement, 0.1 mol·L^-1 HCl and 0.1 mol·L^-1 NaOH were used to adjust the initial pH values of the reaction solutions. To assess the stability of T-3, the catalyst after reaction was separated from the solution, then washed thoroughly with deionized water and ethanol, and finally vacuum-dried for the reuse. Besides rhodamine B, degradation experiments on other organic pollutants, including methylene blue and p-nitrophenol were also conducted. All the other experimental procedures were the same as those of rhodamine B degradation.

  1.5   Analytical methods

  The solution (methylene blue, rhodamine B, p-nitrophenol) was directly added to a 1 cm×1 cm glass cell before being measured at a variable wavelength mode, using a UV-Vis spectrophotometer (Phenix Optical Co. Ltd). The absorbance at 660, 554, and 317 nm, respectively, was used for the quantification. The pollutant removal efficiency was obtained by using the equation (1):

  $ \eta = \frac{{{C_0} - C}}{{{C_0}}} $

  (1)

  η is the removal efficiency (%), C₀, and C are the initial concentrations and moment concentration at time t of pollutants, respectively. The concentrations of dissolved iron ions were detected with inductively coupled plasma optical emission spectrometer (ICP-OES, Agilent, Varian 700) in some cases. The ·OH radicals were detected via ESR using 5, 5-dimethyl-1-pyrroline-N-oxide as the radical spin-trapped reagent (Bruker A200, Germany).

  2.   Results and discussion

  2.1   Morphology and structure of Fe₂O₃ nanosheets

  The structure and oxidation state of the as-prepared α-Fe₂O₃ architectures were characterized using XRD, Raman spectra, infrared spectrum, and high-resolution XPS. The morphology and crystallinity of as-prepared α-Fe₂O₃ architectures were analyzed using SEM, high-resolution HR-TEM. T-1 (Fig. 1a), T-2 (Fig. 2a), and T-3 (Fig. 2b) exhibit a similar morphology. Based on SEM and TEM analysis (Fig.S1), the width and thickness of the plates is determined to be 161.17 nm and 27.93 nm, respectively. A representative HR-TEM image shows the (001) crystal facet with a lattice fringe of 0.25 nm in Fig. 1(b). The (102) crystal facet, in the lower part, shows a lattice spacing of 0.46 nm Fig. 1(d) suggesting that the bottom/top surfaces are enclosed by the (001) facet^[26]. As shown in Fig. 2(c) and (d), disordered lattice fringes induced by defects for T-2 and T-3 are shown. Moreover, T-1, T-2, and T-3 exhibit a similar morphology, indicating that the high-temperature conditions do not cause further changes in the morphology. The XRD patterns in Fig. 3(a) show that the as-prepared T-1, T-2, and T-3 are monoclinic phase, and the diffraction peaks match well with T-1, T-2 (Hematite, PDF No.33-0664). However, T-3 generates a new peak at 30.04° which matches with Fe₃O₄ (PDF No.19-0629), and a peak disappears at 24.8°, with the results showing that the crystal structure has changed^[27]. However, from XRD it is hard to discriminate that the reduction of α-Fe₂O₃ may produce some Fe₃O₄ on the surface. Therefore, Raman spectra and infrared spectrum were recorded for these samples. However, in Fig. 3(c), the Raman spectra of the T-3 contain strong peaks at around 224 (A_g), 287 (E_g), and 402 cm^-1 (E_g), indicating that are mainly composed of hematite α-Fe₂O₃^[28], and no peaks referring to Fe₃O₄ are found at 665 cm^{-1 [29]}. Furthermore, in the infrared spectrum (Fig. 3(b)), there are no peaks referr-ing to Fe₃O₄ found at 568 cm^-1 on the T-3 surface^[30]. This result shows that T-3 is comp-rised of an intermed-iate state of α-Fe₂O₃ to Fe₃O₄ and excludes the formation of Fe₃O₄ on the T-3 surface, which leads it to show the magnetic properties of the T-3 catalyst^[31-32].

  Figure 1

  

  Figure 1.  Morphology and structure of the α-Fe₂O_3-x nanosheets

  (a) Overview TEM images; (c) Thickness TEM images of α-Fe₂O_3-x; (b) and (d) The (001) and (102) facets are marked

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  Figure 2

  

  Figure 2.  (a) and (b) Overview TEM images of T-2, T-3; (c) and (d) Disordered lattice structures of T-2 and T-3 are highlighted

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  Figure 3

  

  Figure 3.  Structure and surface-electronic-state characterizations

  (a) XRD patterns; (b) FT-IR spectra; (c) Raman spectra; (d) XPS of the α-Fe₂O_3-x nanosheets; (e) Raman spectra; (f) Solid ultraviolet visible spectrum of the α-Fe₂O_3-x nanosheets

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  However, on the T-3 surface, there is no formation of Fe₃O₄, as verified by Raman spectra and infrared spectrum analysis. It is noted that the red-brown Fe₂O₃ changes to brown-black (Fig.S2(a)), indicating that the surface of Fe₂O₃ particles is partly reduced during the reaction. The Fe oxidation state was probed by XPS, and Raman spectra, respectively. The surface iron species were investigated by XPS analysis, using the Fe2p_3/2 spectral region. After the hydrogen treatment, the T-2 and T-3 samples, an evident shoulder appeared in the negative shift of the Fe2p_3/2 spectrum (Fig. 3(d)), which suggests that Fe(Ⅱ) was generated on the α-Fe₂O₃ surface^[33]. This clearly shows that the surface of Fe₂O₃ is partly reduced in the hydrogenation reaction. Peak positions are shifted toward lower wave numbers and their full width at half-maximum values are increased in the Raman spectra from T-2 and T-3, compared with those of T-1 (Fig. 3(e)). These results suggest that more oxygen vacancies T-2, T-3 are introduced into oxygen-deficient α-Fe₂O_3-x^[34]. To describe the presence of oxygen vacancies qualitatively in the oxygen-deficient T-2 and T-3 surfaces, the UV-Vis absorption spectra are acquired. As shown in Fig. 3(f), the wavelength absorp-tion ranges of the T-2 and T-3 gradually shift more toward the visible light region. Such phenomena can be attributed to the increased oxygen vacancies on the T-2 and T-3 surfaces^[35]. Therefore, based on the results of these Raman, XPS analyses, UV-Vis absorption spectra, and FT-IR spectrum, we demonstrate that oxygen vacancies and Fe²⁺ are created in both T-2 and T-3 surfaces.

  2.2   Catalytic activity

  The catalytic activity of the as-prepared catalyst was evaluated by degrading rhodamine B model pollutants. As shown in Fig. 4 (a), in the presence of H₂O₂, about 78.8% and 91.6% of rhodamine B was degraded in the T-2 and T-3 systems within 4 h, respectively. However, rhodamine B removal efficiency was only 9.1% over T-1 /H₂O₂ systems under the same conditions, much lower than that (>78%) in the T-2 and T-3 systems with pH=7. To give a clear comparison of the catalytic performance, the RhB removal kinetics was fitted by the first-order reaction (Fig.S3(a)). As shown in Fig. 4(b), the reaction constants for T-1/H₂O₂ systems, T-2/H₂O₂ systems, and T-3/H₂O₂ systems were 0.019 3, 0.425, and 0.534 h^-1, respectively. The reaction rate constant (K_a) values for the T-2 and T-3 systems were 22.02 and 27.66 times that of the T-1 systems, respectively. Since the T-1, T-2, and T-3 surfaces exhibit similar morphology and phase, the discrepancies in the catalytic activity can be ascribed to the surface defects.

  Figure 4

  

  Figure 4.  (a, b) Time profiles of the RhB remove efficiency and kinetic rate constant in different systems; (c) and (d) Time profiles of RhB degradation and kinetic rate constant in the T-3/H₂O₂ system at different pH conditions

  Initial RHB and H₂O₂ concentration were 20 mg·L^-1, 50 mmol·L^-1; the dosage of T-3 was 0.5 g·L^-1; the initial pH was 7.0

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  Since heterogeneous Fenton process are always considered to be complex systems and many factors can affect their performance, we used the T-3 systems as a model reaction and investigated a number of factors that may have an influence on heterogeneous Fenton process performance, such as the pH value, H₂O₂, and T-3 catalyst dosage. The pH of the bulk solution is known to play a governing role in the Fenton and Fenton-like systems. Many of the transitional metal heterogeneous Fenton catalysts were restricted to acidic conditions, and a significant decrease in catalytic activity was observed in neutral or even alkaline condition. However, in this study, as shown in Fig. 4(c), T-3/H₂O₂ systems always exhibited an efficient catalytic activity over a wide pH range from 2 to 10, and rhodamine B was degraded by 77.3%~100%, and the rate constants were 0.363 4~1.007 6 h^-1 (Fig. 4(d)), validating its efficient application for real wastewater. We then monitored the surface charge of T-3 via zeta potential measurement and found that surface of T-3 was negatively charged, with the pH increase as the surface negative charge decreased (Fig.S3(a)). The latter observation might be linked to the pH increase and the gradual increase of OH^- concentration. OH^- were supposed to be preferen-tially adsorbed on surface Fe atoms of T-3 that might compete with the RhB adsorption of T-3 (Fig.S3(b)). Therefore, the decreased adsorption of RhB over T-3 accounted for its lower reactivity under higher pH. Furthermore, as shown in Fig.S3(b) and Fig. 4(d), the RhB adsorption efficiency was found to be correlated well to the Fenton-like performance, and a higher adsorption efficiency would always lead to a higher value of k in the RhB removal. Since 0.1 mol·L^-1 HCl and 0.1 mol·L^-1 NaOH were used for the adjustment of the system pH in the experiments, the influence of NaCl on the catalytic performance of T-3/H₂O₂ system was studied. The results in Fig.S1(b) showed that NaCl had no obvious influence on the catalytic activity. As shown in Fig. 5(a), the H₂O₂ dosage was the important factor in rhodamine B degradation efficiency. It can be seen that only 35.2% of rhoda-mine B was removed with the H₂O₂ concentration lower than 2.5 mmol·L^-1, but its removal efficiency increases dramatically from 0 to 90.4% with a concentration ranging from 2.5 mmol·L^-1 to 100 mmol·L^-1. Furthermore, appropriate increasing of the dosage of H₂O₂ can improve reaction rates of advanced oxidation processes.

  Figure 5

  

  Figure 5.  (a) Time profiles of RhB degradation and kinetic rate constant in the T-3/H₂O₂ system at different initial H₂O₂ concentration, (b) Reusability of T-3 for the degradation of RhB in the T-3/H₂O₂ system, (c) and (d) PNP/MB/RhB degradation kinetic rate constant and remove efficiency in T-3/H₂O₂ systems

  Initial RhB and H₂O₂ concentration were 20 mg·L^-1, 50 mmol·L^-1; the dosage of T-3 was 0.5 g·L^-1; the initial pH value was 7.0

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  To highlight the potential of defects in α-Fe₂O₃ heterogeneous catalysts for advanced water treatment for industrial applications, the catalytic stability of the constructed T-3/H₂O₂ systems was estimated by recycling the used T-3 for cyclic rhodamine B degradation in the presence of H₂O₂ as a common oxidant. It can be seen from Fig. 5(b) that when pH is 7, after recycling three times, the recycled oxidative removal of RhB T-3/H₂O₂ systems gradually loses about 44% of their initial reactivity. After the reactivation of T-3 via the same thermal treatment, however, reusability of catalyst for organic pollutants oxidation is suggested at pH=7. The after-reactivation of T-3 was further validated by Raman, XRD, SEM analysis. As shown in Fig.S4, the morphology and structure of the T-3 catalyst had no obvious change after recycling five times, indicating its good stability. Furthermore, as shown in Fig.S5, the used catalyst can be easily recovered by magnetic separation. Moreover, rhodamine B (20 mg·L^-1), methylene blue (20 mg·L^-1), and p-nitrophenol (20 mg·L^-1) can be efficiently degraded (reaction rate constant K_a values of rhodamine B (20 mg·L^-1); methylene blue (20 mg·L^-1) and p-nitrophenol (20 mg·L^-1) were 0.575, 0.895 and 0.515 h^-1, respectively (Fig. 5(c) and (d)). Subsequently, we scaled up our testing system from 20 mL to 1.5 L (Fig.S6(a)). As shown in Fig.S6(b), the T-3 systems still displayed excellent catalytic performance, further suggesting the potential of defect-containing α-Fe₂O_3-x heterogeneous catalyst in advanced water treatment for industrial applications.

  We observed another interesting phenomenon. Oxygen vacancy remarkably enhanced the adsorption intensities of RhB on the catalyst surfaces. As shown in Fig. 6(a) and (b), a much faster adsorption equilibrium of within 10 min, with RhB adsorption efficiency as high as 26.4%, could be achieved on T-3. This result may like ultrathin vacancy oxygen BiOCl nanosheets defects with negative charges which favors the adsorption of positively charged RhB molecules^[36]. The RhB adsorption efficiency was found to be correlated well to the Fenton-like performance, and a higher adsorption efficiency would always lead to a higher value of k in RhB removal. These results suggest that the excellent Fenton-like performance of T-3 may result from the balanced adsorption of RhB and activation of H₂O₂.

  Figure 6

  

  Figure 6.  (a) RhB adsorption efficiency of different catalysts; (b) Rate constant catalysts of different catalytic systems

  Reaction condition: c_RhB=20 mg·L^-1, c_catalyst=0.5 g·L^-1, and initial solution pH=7.0

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  2.3   Possible catalytic mechanism

  Sun first used the density functional theory calculations were conducted by investigating the adsorption and reaction characteristics of H₂O₂ on perfect and oxygen defect α-Fe₂O_3-x (001) surfaces^[25]. The results indicate that, on the defect-free α-Fe₂O₃ (001) surfaces, the H₂O₂ was parallel to the α-Fe₂O_3-x (001) surface and bonded with an Fe atom of the surface. The bond length of Fe-O reached 0.214 4 nm. Hydrogen bonds were also formed between two H atoms of H₂O₂ and two surface oxygen atoms with representative bond lengths of 0.147 4 and 0.148 0 nm, which are slightly longer than the calculated value for the free H₂O₂ (0.147 4 nm). According to the adsorption energy and the representative lengths of O-O and Fe-O, dissociation of H₂O₂ did not occur on the defect-free α-Fe₂O₃ (001) surface. However, on the oxygen defect α-Fe₂O_3-x (001) surface, the best stable chemisorption is the H₂O₂ molecule presumably dissociates into two surface hydroxyls and adsorption energy of reached to -456.66 kJ·mol^-1. One of the hydroxyls occupied the oxygen vacancy site ((001)-OV+H₂O₂→(001)-OH^-+·OH) and bonded with two surface Fe atoms (≡Fe(Ⅱ)+H₂O₂→≡Fe(Ⅲ)-OH+·OH) with bond lengths of 0.205 0 and 0.203 0 nm, which donated a short of ·OH (0.186 nm) to nearby hydroxyl groups, indicating its radical nature^{[25, 37]}. Analyses of Mulliken population, electron density difference, and partial density of states showed that H₂O₂ decomposition on the oxygen defect α-Fe₂O_3-x (001) surface followed the Haber-Weiss mechanism, in which the electron transfers from the surface Fe atom to H₂O₂ molecule and creates hydroxyl radicals^[38-39].

  The above report suggests that Fe atoms close to the oxygen vacancy and localized electron donation from oxygen vacancy have stronger reducibility to adsorbed H₂O₂ could directly facilitate the H₂O₂ disso-ciation in a Fenton reaction manner^[25]. Subsequently, we monitored the change of the chemical environment of oxygen element by X-ray photoelectron spectroscopy (XPS) analysis on the oxygen defect α-Fe₂O_3-x (001) surface before and after the H₂O₂ dissociation. As depicted in Fig. 7(a) and (b), the O1s spectrum of oxygen defect α-Fe₂O_3-x (001) could be divided into four peaks. The O1s peaks at 529.7 and 530.5 eV were attributed to the lattice oxygen (Fe-O) and lattice oxygen (O^2-) of oxygen defect α-Fe₂O_3-x (001), respectively^[39-40]. The binding energy of 531.7 eV which was ascribed to the -O₂^2- groups was adsorbed on α-Fe₂O_3-x surfaces and 533.2 eV for α-Fe₂O_3-x are assigned to hydroxyl groups (Fe-OH adsorbed) and water present on the α-Fe₂O_3-x as a result of the adsorption and dissociation of water^[40-42]. As expected, We found at 533.3 eV increased obviously for oxygen defect α-Fe₂O_3-x in the α-Fe₂O_3-x(001)/H₂O₂ system (Fig. 7b). These phenomena similar to the generation of surfacebound ·OH via the H₂O₂ decomposition in α-FeOOH-HA/H₂O₂ and BOCl-010-OV/H₂O₂^{[23, 43]}.

  Figure 7

  

  Figure 7.  (a) O1s XPS spectra of the T-3; (b) O1s XPS spectra of the T-3/H₂O₂; (c) 1, 10-phenanthroline as the Fe²⁺ complexant; (d) Solid ultraviolet visible spectrum of T-3 before and after the RhB degradation

  Initial RhB, H₂O₂, 1, 10-phenanthroline concentration were 20 mg·L^-1, 50 mmol·L^-1, 0.01 mol·L^-1; the dosage of T-3 was 0.5 g·L^-1; the initial pH value was 7.0

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  In this study, radical quenching experiments were conducted to identify the active radical species generated during H₂O₂ activation. As shown in Fig. 8(a), if TBA, a radical scavenger for ·OH, was added to the T-3/H₂O₂ system, the RhB degradation efficiency decreased slightly, suggesting that ·OH was the major reactive species in the T-3/H₂O₂ system. To test these theoretical results in the Fenton-like reaction, N-oxide (DMPO) was used as a spin trap in electron paramagnetic resonance (EPR) tests to detect the spin reactive ·OH adsorbed on α-Fe₂O₃ (001) surfaces or dissolved in water. No obvious EPR signal was observed over α-Fe₂O₃ (001) in the presence of H₂O₂, confirming that defect-free α-Fe₂O₃ (001) surfaces were not able to dissociate H₂O₂ for ·OH generation. As expected, strong four-line EPR spectra, with relative intensities of 1:2:2:1 corresponding to DMPO- ·OH adduct were observed in defect-containing α-Fe₂O₃ (001) reacting with H₂O₂. Based on the Haber-Weiss mechanism, H₂O₂ decomposition induced by dissolved metal ions could be ruled out by non-leaching of Fe ions into the solution. We subsequently investigated the T-3 reduction process in the presence of H₂O₂ by ICP-OES (Fig. 8b). The results show that no Fe ions were released from T-3 in the H₂O₂ solution. Therefore, we concluded that the H₂O₂ decomposition should mainly occur on the α-Fe₂O₃ (001) surfaces, as the theoretical calculations predicted. To further confirm the existence of surface bound ·OH generated on T-3, we compared the amounts of ·OH generated in the supernatants of the T-3/H₂O₂ and T-3/H₂O₂/F^- systems, with a DMPO trapped ESR technique (Fig. 8d), taking into consideration that F^- in the solution could desorb ·OH bound on defect-containing α-Fe₂O₃ surfaces by forming strong hydrogen bonds (surface bound ·OH…F-Fe₂O_3-x)^[44]. As expected, the addition of F^- improved the intensity of the DMPO trapped ·OH ESR signal by 26.8% in the T-3/H₂O₂ system after 120 s of reaction, fully validating that the surface bound ·OH was generated by the surface on T-3 in the T-3/H₂O₂ system.

  Figure 8

  

  Figure 8.  (a) Degradation of RhB in the absence and presence of tert-butyl alcohol (TBA) as the ·OH scavenger, (b) DMPO trapped ESR spectra in the T-1/H₂O₂, T-3/H₂O₂ systems, (c) Changes of the total dissolved Fe concentration released from α-Fe₂O_3-x in the T-3/H₂O₂ systems at pH 7.0, (d) DMPO trapped ESR spectra in the T-3/H₂O₂/F^- systems

  Initial RhB, H₂O₂, NaF, and TBA concentration were 20 mg·L^-1, 50, 2, 0.74 mmol·L^-1; the dosage of T-3 was 0.5 g·L^-1; the initial pH value was 7.0

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  Previous studies have suggested that the oxygen vacancies and Fe²⁺ were like "catalytic active centers" promoting the decomposed H₂O₂ to produce hydroxyl and hydroxyl radical on the defect-containing α-Fe₂O₃(001) catalyst surface^[25]. To identify the chemical states of ·OH generated on the T-3 surfaces, we checked the contribution of surface Fe(Ⅱ), bound on T-3 to the rhodamine B degradation, by chelating the surface Fe(Ⅱ) with 1, 10-phenanthroline during the rhodamine B degradation. Interestingly, the surface Fe(Ⅱ) chelation with 1, 10-phenanthroline significantly inhibited the rhodamine B degradation in the T-3/H₂O₂ system, confirming the important role of surface Fe(Ⅱ) in the rhodamine B removal within the T-3/H₂O₂ systems. However, as shown in Fig. 7(c), the degradation of rhodamine B was not completely inhibited. The results suggest that the oxygen vacancy was involved in the H₂O₂ decomposition. In previous studies, it had been suggested that the oxygen vacancy concentration will be decreased in the H₂O₂ reactivation^[23]. Therefore, we use UV-Vis absorption spectra to qualitatively describe the presence of defects concentration in T-3 before and after reaction. As shown in Fig. 7(d), according to the absorptive capacity change of UV-Vis absorption spectra signal of T-3 (before and after), a distinct quenching of defect concentration is suggested, as the Sun theoretical calculations predicted^[25]. This is attributed to the hydroxyls occupying the oxygen vacancy active site on defective α-Fe₂O_3-x surfaces for H₂O₂ activation and leading to defective α-Fe₂O_3-x surfaces oxygen vacancy concentration decrease, which is similar to oxygen vacancies in previous reports on defect-containing BiOCl systems.

  3.   Conclusions

  In summary, we present a surface-electronic-state-modulation-based concept on pure metal oxides (α-Fe₂O₃ (001) nanocrystal) for efficient degradation of persistent organic compounds in the presence of H₂O₂. The oxygen-deficient α-Fe₂O_3-x catalysts prepared in this work, The dual reaction-site mechanism as displayed in Fig. 9 shows, oxygen-deficient α-Fe₂O_3-x catalysts not only provided dual reaction sites (Fe²⁺ and oxygen vacancy) for H₂O₂ activation promoting H₂O₂ to decompose the in situ-generated ·OH. but also afforded rich oxygen vacancy sites for organic pollutant adsorption and the generated ·OH to rapid react with adjacent adsorbed pollutant, thus showing excellent Fenton-like catalytic performance. Our findings show that the surface electronic-state modulation could obviate the need for modification of many transition metal oxides with other materials, addressing poor reactivity issues.

  Figure 9

  

  Figure 9.  Proposed Fenton-like reaction mechanism on oxygen-deficient α-Fe₂O_3-x catalyst

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  Acknowledgements

  The authors are thankful for the financial support from the National Natural Science Foundation of China (Grants No.21273050, 21573048, 21873022, 51574092).

  Notes: No conflict of interest exits in the submission of this manuscript, and the manuscript is approved by all authors for publication. I would like to declare on behalf of my co-authors that the work described was original research and has not been under consideration for publication elsewhere, in whole or in part. All the authors listed have approved the manuscript that is enclosed.

  Supporting information is available at http://www.wjhxxb.cn

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• 

  Figure 1  Morphology and structure of the α-Fe₂O_3-x nanosheets

  (a) Overview TEM images; (c) Thickness TEM images of α-Fe₂O_3-x; (b) and (d) The (001) and (102) facets are marked

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  Figure 2  (a) and (b) Overview TEM images of T-2, T-3; (c) and (d) Disordered lattice structures of T-2 and T-3 are highlighted

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  Figure 3  Structure and surface-electronic-state characterizations

  (a) XRD patterns; (b) FT-IR spectra; (c) Raman spectra; (d) XPS of the α-Fe₂O_3-x nanosheets; (e) Raman spectra; (f) Solid ultraviolet visible spectrum of the α-Fe₂O_3-x nanosheets

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  Figure 4  (a, b) Time profiles of the RhB remove efficiency and kinetic rate constant in different systems; (c) and (d) Time profiles of RhB degradation and kinetic rate constant in the T-3/H₂O₂ system at different pH conditions

  Initial RHB and H₂O₂ concentration were 20 mg·L^-1, 50 mmol·L^-1; the dosage of T-3 was 0.5 g·L^-1; the initial pH was 7.0

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  Figure 5  (a) Time profiles of RhB degradation and kinetic rate constant in the T-3/H₂O₂ system at different initial H₂O₂ concentration, (b) Reusability of T-3 for the degradation of RhB in the T-3/H₂O₂ system, (c) and (d) PNP/MB/RhB degradation kinetic rate constant and remove efficiency in T-3/H₂O₂ systems

  Initial RhB and H₂O₂ concentration were 20 mg·L^-1, 50 mmol·L^-1; the dosage of T-3 was 0.5 g·L^-1; the initial pH value was 7.0

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  Figure 6  (a) RhB adsorption efficiency of different catalysts; (b) Rate constant catalysts of different catalytic systems

  Reaction condition: c_RhB=20 mg·L^-1, c_catalyst=0.5 g·L^-1, and initial solution pH=7.0

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  Figure 7  (a) O1s XPS spectra of the T-3; (b) O1s XPS spectra of the T-3/H₂O₂; (c) 1, 10-phenanthroline as the Fe²⁺ complexant; (d) Solid ultraviolet visible spectrum of T-3 before and after the RhB degradation

  Initial RhB, H₂O₂, 1, 10-phenanthroline concentration were 20 mg·L^-1, 50 mmol·L^-1, 0.01 mol·L^-1; the dosage of T-3 was 0.5 g·L^-1; the initial pH value was 7.0

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  Figure 8  (a) Degradation of RhB in the absence and presence of tert-butyl alcohol (TBA) as the ·OH scavenger, (b) DMPO trapped ESR spectra in the T-1/H₂O₂, T-3/H₂O₂ systems, (c) Changes of the total dissolved Fe concentration released from α-Fe₂O_3-x in the T-3/H₂O₂ systems at pH 7.0, (d) DMPO trapped ESR spectra in the T-3/H₂O₂/F^- systems

  Initial RhB, H₂O₂, NaF, and TBA concentration were 20 mg·L^-1, 50, 2, 0.74 mmol·L^-1; the dosage of T-3 was 0.5 g·L^-1; the initial pH value was 7.0

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  Figure 9  Proposed Fenton-like reaction mechanism on oxygen-deficient α-Fe₂O_3-x catalyst

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•