English

• 0.   Introduction

  Water is one of the important resources that human beings depend on for survival and development. However, nearly a third of the population worldwide is estimated to lack access to safely managed drinking water services^[1-2]. In the last years, water pollution is becoming a major concern due to novel and dangerous anthropogenic pollutants. Reducing the release of wastewater into the environment and degrading the contaminants from wastewater are important strategies for water environment purification^[3-5].

  Many water treatment ways have been employed to degrade organic pollutants, such as physical absorption, biological purification, advanced oxidation processes (AOPs), electrochemical processes, and photocatalytic degradation. Among them, photocatalytic degradation is a kind of AOPs. Compared to traditional AOPs, the active-oxidizing species HO· or ·O₂^- is in-situ produced by semiconductor photocatalysis^[6-14]. Semiconductor photocatalysts can be excited by light with energy higher than their band gap values, and then generate electron-hole pairs. Hole-electron pairs separate and transfer to the photocatalyst surface, produce HO· or ·O₂^- , and then lead to the oxidation of organic pollutions. To obtain high photocatalytic efficiency, a semiconductor should have a small band gap enabling the utilization of a wide range of solar light^[15-20]. Fe₂O₃ with a narrow band gap of ca. 2.0 eV, can absorb a large amount of visible sunlight. Besides, Fe₂O₃ has many other advantages, such as being lowcost, and non-toxic, making it a promising photocatalyst material. However, Fe₂O₃ exhibits low conductivity and over-positive conduction band position, which are adverse to its photocatalytic efficiency.

  Many strategies were employed to overcome these problems and improve photocatalytic efficiency on Fe₂O₃. Nano-engineering, intentional n-type doping, and electrocatalyst loading have been often used to improve charge separation efficiency or the surface oxidation rate^[21-24]. Besides, constructing heterojunction with another semiconductor material is an effective way to improve the separation of electronhole pairs by the built-in electric field. This strategy has been successfully applied to many semiconductors^[21-26], including BiVO₄, WO₃, TiO₂, and so on.

  Constructing heterojunction between Fe₂O₃ particles and another semiconductor with a suitable band position benefits the separation of photo-generated carriers. Fe₂TiO₅ is such a semiconductor with a band gap of ca. 2.0 eV and similar to Fe₂O₃ while showing higher conduction and valence band levels, which can form staggered band positions with Fe₂O₃, therefore, effective step-scheme (S-scheme) heterojunction can be developed between Fe₂O₃ and Fe₂TiO₅^[27-29]. Moreover, Fe₂TiO₅ exhibits a high conduction band level located at ca.-0.2 eV vs reversible hydrogen electrode, making the composite materials propose the capacity to reduce O₂ to ·O₂^- and further improve the photocatalytic properties. In previous reports, Fe₂O₃/Fe₂TiO₅ composites were commonly applied in oxygen evolution^[30-32], and rarely seen in pollution degradation^[33]. The preparing Fe₂O₃/Fe₂TiO₅ composite was mainly by an ion-exchange method, i.e. employing Fe₂O₃ or TiO₂ as the substrate to inter-react with Ti or Fe precursors at high temperature^{[30, 32-33]}. Yu and Waqas fabricated Fe₂O₃/Fe₂TiO₅ composite utilizing sol-gel and calcination method^{[31, 34]}.

  In this work, for the first time, Fe₂O₃/Fe₂TiO₅ composite materials were prepared by a one-pot solvothermal method. Compared to the pure Fe₂O₃ and pure Fe₂TiO₅, the photocatalytic properties toward removing methylene blue (MB) were significantly improved, which is mainly due to the promoted charge separation efficiency and the preserved higher-energy electrons from Fe₂TiO₅ caused by the S-scheme heterojunction.

  1.   Experimental

  1.1   Preparation of Fe₂O₃/Fe₂TiO₅ heterojunction particles

  Fe₂O₃/Fe₂TiO₅ heterojunction particles were fabricated by a solvothermal method. Firstly, 2.51 mmol Fe(NO₃)₃·9H₂O was added to 50 mL isopropanol. Under stirring, 0.625 mmol of titanium isopropoxide was immediately added to the above solution. The precursor solution, after further being stirred for another 1 h, was transferred into a 100 mL Teflon-line stainless steel autoclave. Then the autoclave was sealed and heated in an oven at 150 ℃ for 12 h. After cooling down naturally, the prepared precipitates were washed with deionized water four times. As-prepared precipitates were dried at 80 ℃ overnight, then dried precipi-tates were annealed in air at 550 ℃ for 2 h and then 700 ℃ for 10 min. Then the Fe₂O₃/Fe₂TiO₅ heterojunction particles were obtained.

  Fe₂O₃ was prepared by the same steps employed for Fe₂O₃/Fe₂TiO₅ fabrication, except that only Fe(NO₃)₃·9H₂O was added to the precursor solution without titanium isopropoxide. Fe₂TiO₅ was also prepared by this solvothermal method^[31]. In the precursor solution, 2.51 mmol Fe(NO₃)₃·9H₂O and 1.25 mmol of titanium isopropoxide were added in sequence. Other steps were the same as that of Fe₂O₃/Fe₂TiO₅ fabrication. The photoelectrodes based on the prepared Fe₂O₃, Fe₂TiO₅, and Fe₂O₃/Fe₂TiO₅ were also fabricated with the same process. Firstly, 10 mg of each sample was dispersed in 1 mL glycol. 20 μL solution was spin-coating on F-doped SnO₂ coated glass (FTO), then another 20 μL solution was immediately dropped on the above FTO. After drying at 150 ℃ for 30 min, the film was calcined at 600 ℃ for 1 h.

  1.2   Characterization

  The crystal structures of the prepared samples were probed by a powder X-ray diffraction (XRD) on a Bruker diffractometer system, using Cu Kα radiation (λ =0.154 18 nm) with a working voltage and current of 40 kV and 40 mA, respectively. The scan rate was 0.04 (°)·s^-1 in a 2θ range of 5°-70°. The morphology test of the samples was carried out on a field emission scanning electron microscope (SEM; JEOL, JSM-6700F with an accelerating voltage of 5 kV). The working voltage for SEM-EDS (EDS=energy dispersive X-ray spectroscopy) mapping was 20 kV. Transmission electron microscope (TEM) images were recorded on a transmission electron microscope (HT7700). Highresolution TEM (HRTEM) was conducted at 200 kV. The optical absorption spectra of the samples were performed on a UV-visible (UV-Vis) spectrophotometer (Shimadzu, UV-Vis 2550). Electrochemical impedance spectra (EIS) of the three photoelectrodes were measured at 0.9 V (vs RHE) using an electrochemical workstation (Shanghai Chenhua, 660E) with a 10 mV amplitude perturbation and frequencies between 0.1 Hz and 1 MHz.

  1.3   Photocatalytic property measurements

  20 mg Fe₂O₃/Fe₂TiO₅ was added into a 100 mL water solution with an MB concentration of 10 mg·L^-1. After 40 min absorption, 3 mL solution was filtrated and taken for the test. Then the remained solution was stirred and irradiated under light with a power of 100 mW·cm^-2. TThe area of the beaker exposed to the light was ca. 20 cm². The light source used in this work was a 100 W LED lamp. The reaction solution was cooled by running water during the whole irradiating process to exclude the thermal effect. 3 mL solution was taken every 30 min. Current-potential curves were tested on an electrochemical workstation using a three-electrode system.

  2.   Results and discussion

  2.1   Characterization of Fe₂O₃, Fe₂TiO₅, and Fe₂O₃/Fe₂TiO₅

  To investigate the crystallinity and phase of asprepared samples, XRD was carried out (Fig. 1). All the peaks of the black curve at 24.2°, 33.3°, 35.7°, 40.9°, 49.5°, 54.1°, 57.6°, 62.4°, and 64.1° can be assigned to Fe₂O₃ (hematite, PDF No.33-0664), while all peaks of the red curve belong to the pseudobrookite Fe₂TiO₅ (PDF No.41-1432). The results indicate that Fe₂O₃ and Fe₂TiO₅ have been successfully prepared. Both XRD peaks of Fe₂O₃ and Fe₂TiO₅ were observed in Fe₂O₃/ Fe₂ TiO₅, demonstrating that Fe₂O₃/Fe₂TiO₅ was obtained by the one-pot solvothermal method. Moreover, the high peak intensity of the three samples indicates their well crystalline nature. Note that the prepared Fe₂O₃/ Fe₂TiO₅ composite showed only two relatively low peaks at 18.1° and 25.6°. To evaluate the contents of Fe₂O₃ and Fe₂TiO₅ in the composite, EDS has been performed. Elemental Ti was not observed in the pure Fe₂O₃, while in Fe₂TiO₅, both Fe and Ti were detected with an atomic ratio of 1.93 which is close to the Fe/Ti stoichiometric proportion in Fe₂TiO₅. In terms of Fe₂O₃/ Fe₂TiO₅ composite, the atomic ratio of Fe/Ti was 3.99, which is the same as the feed proportion, indicating the content of Fe₂O₃ and Fe₂TiO₅ was equal in molar quantity. EDS mapping on the three samples has also been tested and the results are shown in Fig. 2. For Fe₂O₃/ Fe₂ TiO₅, Fe, O, and Ti were uniformly distributed in the sample, demonstrating that Fe₂O₃ or Fe₂TiO₅ can contact each other well.

  Figure 1

  

  Figure 1.  XRD patterns of Fe₂O₃, Fe₂TiO₅, and Fe₂O₃/Fe₂TiO₅

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  Figure 2

  

  Figure 2.  SEM-EDS element mapping images of (a) Fe₂O₃, (b) Fe₂TiO₅, and (c) Fe₂O₃/Fe₂TiO₅

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  TEM was performed to further investigate the crystallinity and phase of the Fe₂O₃/Fe₂TiO₅ composite. As shown in Fig. 3b-3d, elemental Fe was evenly distributed in the particles, while elemental Ti is mainly distributed on the particle surface and interface. HRTEM has also been measured and the results are shown in Fig. 3e and 3f. Fringe spacing of 0.251 and 0.247 nm can be indexed to the (110) plane of Fe₂O₃ and (301) plane of Fe₂TiO₅, especially, demonstrating the formation of heterojunction between Fe₂O₃ and Fe₂TiO₅. In addition, Fe₂O₃ is well crystalline in the whole Fe₂O₃ sample, while the crystalline region in Fe₂TiO₅ is relatively small and enshrouded with an amorphous phase. This result indicates that Fe₂TiO₅ spreads over the surface of Fe₂O₃, and the crystallinity of Fe₂TiO₅ is restricted to some extent. Moreover, the TEM-EDS results suggest that the atomic ratio of Fe and Ti was ca. 5.6, which is larger than the feed proportion and SEM-EDS values. It is understandable considering that part of small Fe₂TiO₅ particles falls away from Fe₂O₃ during the ultrasonic process. Overall, the results of XRD, SEM-EDS, and TEM demonstrate that Fe₂O₃/Fe₂TiO₅ heterojunction composites have been successfully prepared.

  Figure 3

  

  Figure 3.  TEM-EDS element mapping images (a-d), and HRTEM images (e, f) for Fe₂O₃/Fe₂TiO₅

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  Light absorption properties show a great effect on the final photocatalytic degradation performance. Therefore, UV-Vis diffuse reflectance spectra (DRS) of the prepared samples were measured to evaluate their absorption properties. The UV-Vis DRS results have been converted to absorption form using the Kubelka-Munk function as shown in Fig. 4a. Besides, the curves have been normalized. As shown in Fig. 4a, all samples exhibited absorption regions from UV to visible wavelengths. Fe₂O₃ exhibited the widest light absorption, while the absorption edge of Fe₂TiO₅ blue shift compared to Fe₂O₃ and proposed the narrowest light absorption region. While, from 500 to 600 nm, the absorption of Fe₂O₃/Fe₂TiO₅ was higher than that of Fe₂TiO₅ and smaller than the absorption of Fe₂O₃. The band gaps of Fe₂O₃, Fe₂TiO₅, and Fe₂O₃ /Fe₂ TiO₅ were estimated by drawing Tauc plots, which are shown in Fig. 4b. In the ordinate of the curves, n=2 represents the direct band gap, while n=1/2 represents the indirect band gap^{[13, 35]}. As Fe₂O₃ and Fe₂TiO₅ are indirect band gap semiconductors, n=1/2 was employed here to calculate their band gaps. The band gaps of the three samples were similar with the values of 2.05-2.08 eV, which match the previously reported values^[31].

  Figure 4

  

  Figure 4.  (a) Absorption spectra and (b) Tauc plots of Fe₂O₃, Fe₂TiO₅, and Fe₂O₃/Fe₂TiO₅

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  Morphologies of the prepared samples were analyzed by SEM. As shown in Fig. 5, all the samples show an ellipsoidal shape with uniform distribution. The particle size of the nanoparticles was less than 50 nm. The small particle size enables a large semiconductor/ solution interface, facilitating the injecting of photoexcited charges into the solution. In consideration of the similar shape and size of Fe₂O₃, Fe₂ TiO₅, and Fe₂O₃/ Fe₂TiO₅, their MB absorption amount should be a little different, which is following the MB absorption experiment.

  Figure 5

  

  Figure 5.  SEM images of (a) Fe₂O₃, (b) Fe₂TiO₅, and (c) Fe₂O₃/Fe₂TiO₅

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  2.2   Photocatalytic performance

  To evaluate the performance of Fe₂O₃, Fe₂TiO₅, and Fe₂O₃/Fe₂TiO₅, the photocatalytic degradation of MB organic pollutants experiments was performed. 10 mg·L^-1 MB aqueous solution was employed in this experiment, the concentration of the three samples was 20 mg per 100 mL MB aqueous solution. The filter for filtering Fe₂O₃, Fe₂TiO₅, and Fe₂O₃/Fe₂TiO₅ was saturated by MB first. After 40 min of adsorption-desorption equilibrium, the photocatalytic degradation experiment was carried out by exposing the illumination solution. 2 mL solution was taken out from the MB solution every 30 min using a disposable syringe with the filter. The MB degradation rates on Fe₂O₃, Fe₂TiO₅, and Fe₂O₃/ Fe₂ TiO₅ are shown in Fig. 6a. Fe₂O₃/Fe₂TiO₅ exhibited the highest degradation rate. The degradation efficiencies of the three samples were also calculated according to the formula of (c₀-c_t)/c₀, where c₀ is the initial concentration after adsorption-desorption equilibrium, c_t is the concentration at t time. After 150 min irradiation, MB degradation efficiency of Fe₂O₃/Fe₂TiO₅ reached 98.4%, while the efficiencies of Fe₂O₃ and Fe₂TiO₅ were just 50.9% and 62.9%, respectively. As discussed above, though the light absorption property of Fe₂O₃ was better than that of Fe₂TiO₅, their photocatalytic activity was similar. This is because the conduction band minimum (CBM) of Fe₂TiO₅ was higher and can reduce O₂ to ·O₂^- , which provides another pathway for degradation, except for oxidizing MB by the hole in valence band maximum (VBM). Among the three samples, Fe₂O₃/Fe₂TiO₅ presented the highest degradation rate and highest degradation efficiency. To further understand the photocatalytic degradation process, the data were fitted by a first-order kinetic equation, ln(c₀ / c_t)=kt, which is commonly used as a mode to analyze organic pollutant degradation^[35]. The results are shown in Fig. 6b, and the degradation rate constant k was fitted from the slope of the line. The k value of Fe₂O₃/Fe₂TiO₅ was 2.787×10-2 min-1, which is significantly higher than that of Fe₂O₃ (5.06×10^-3 min^-1) and Fe₂TiO₅ (7.47×10^-3 min^-1).

  Figure 6

  

  Figure 6.  (a) Degradation performance of MB over the different samples; (b) Plots of ln(c₀/c_t) vs illuminated time; (c) Stability of Fe₂O₃/Fe₂TiO₅ on MB degradation

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  To evaluate the stability of Fe₂O₃/Fe₂TiO₅, cycle tests for the photocatalytic degradation of MB were tested. The solid particles should be relatively evenly dispersed in the solution after ultrasonic dispersion before the reaction and agitation during the reaction. Moreover, the intermediate solution was taken each time. Therefore, after the first cycle, since the 10 mL solution has been taken out, ca. 1/10 of Fe₂O₃/Fe₂TiO₅ has also been taken out along with the solution. Therefore, we just added MB into the remained 90 mL solution to keep MB and the catalyst concentrations still at 10 and 0.2 mg·mL^-1, respectively. As shown in Fig. 6c, compared with the 1st cycle, although the degradation rates of the 2nd and 3rd cycles slightly decreased, degradation rates were still high. This result indicates the high stability of as-synthesized Fe₂O₃/Fe₂TiO₅ composite material.

  2.3   Mechanism for the improvement

  Photocatalytic performance highly depends on light absorption of the semiconductor and charge separation in the bulk semiconductor. As discussed above, the light absorption property of Fe₂O₃/Fe₂TiO₅ was not the best, the highest photocatalytic property of Fe₂O₃/ Fe₂TiO₅ must result from the significantly improved charge separation efficiency. Moreover, in Fe₂O₃/ Fe₂TiO₅, besides the preserved higher-energy holes from Fe₂O₃ which oxidize MB, the preserved higher-energy electrons from Fe₂TiO₅ which provide another pathway for MB degradation.

  To verify the mechanism proposed above, photo-electrodes based on Fe₂O₃, Fe₂TiO₅, and Fe₂O₃/Fe₂TiO₅ were prepared to evaluate their separation efficiency of the photogenerated charge carriers. The photoelectrodes were tested in a three-electrode system, where photoelectrode was used as a working electrode, Ag/ AgCl was used as a reference electrode, and Pt was used as the counter electrode. The scan rate was 30 mV·s^-1. The electrolyte was 1 mol·L^-1 NaOH. As shown in Fig. 7, the Fe₂O₃/Fe₂TiO₅ photoelectrode presented significantly higher photocurrents than that of Fe₂O₃ and Fe₂TiO₅. The significantly increased photocurrent density of Fe₂O₃/Fe₂TiO₅ can be attributed to the higher charge separation efficiency due to the formed heterojunction between Fe₂O₃ and Fe₂TiO₅^[27].

  Figure 7

  

  Figure 7.  Chopped current vs potential curves on Fe₂O₃, Fe₂TiO₅, and Fe₂O₃/Fe₂TiO₅ photoelectrodes

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  EIS was also conducted to confirm the charge transport properties of the three samples. As shown in Fig. 8, the Fe₂O₃/Fe₂TiO₅ photoelectrode exhibited the smallest diameter, indicating the faster charge transfer kinetics in the film. This phenomenon can be attributed to the build-in field induced by the heterojunction between Fe₂O₃ and Fe₂TiO₅ since the built-in field can facilitate the separation of electron-hole pairs. It should be pointed out that the charge transfer resistance of Fe₂O₃ is smaller than that of Fe₂TiO₅, which is consistent with the reported result^[36].

  Figure 8

  

  Figure 8.  Nyquist plots of the Fe₂O₃, Fe₂TiO₅, and Fe₂O₃/Fe₂TiO₅ photoelectrodes

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  A possible mechanism for photocatalytic degradation of MB by Fe₂O₃/Fe₂TiO₅ is illuminated in Fig. 9. Fe₂TiO₅ has higher CBM and VBM levels than that Fe₂O₃. Under irradiation, Fe₂O₃ and Fe₂TiO₅ absorb light and generate electron-hole pairs, respectively. An S-scheme heterojunction^[37] is formed between Fe₂O₃ and Fe₂TiO₅. In this S-scheme heterojunction, electrons in CBM of Fe₂O₃ and holes in VBM of Fe₂TiO₅ can recombination with each other, while holes in VB of Fe₂O₃ and electrons in CBM of Fe₂TiO₅ separate and transfer to Fe₂O₃/solution and Fe₂TiO₅/solution surface, respectively. Due to the recombination electrons presenting relatively lower energy, the preserved electrons with higher energy in Fe₂TiO₅ can reduce O₂ to ·O₂^- , and then produce ·OH, which can degrade MB effec-tively. While the reserved holes in Fe₂O₃ can degrade MB directly. Therefore, the Fe₂O₃/Fe₂TiO₅ heterojunction can remove MB more effectively.

  Figure 9

  

  Figure 9.  Schematic diagram of charge carrier transfer process and possible photocatalytic mechanism of Fe₂O₃/Fe₂TiO₅

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  3.   Conclusions

  In summary, Fe₂O₃, Fe₂TiO₅, and Fe₂O₃/Fe₂TiO₅ have been prepared by a facile one-pot solvothermal method. In Fe₂O₃/Fe₂TiO₅, Fe₂O₃ and Fe₂TiO₅ can form S-scheme heterojunction, and thus promotes the electrons in CB of Fe₂TiO₅ and holes in VB of Fe₂O₃ transfer to the surface. In this way, carriers with higher ener-gy were preserved. Compared to Fe₂O₃ and Fe₂TiO₅, the photocatalytic degradation rate and efficiency of Fe₂O₃/ Fe₂TiO₅ were significantly improved. This approach provides a facile way to achieve Fe₂O₃/Fe₂TiO₅ S-scheme heterojunction materials and can offer a reference to construct heterojunction on other materials.

  
  Acknowledgments: The National Nonprofit Institute Research Grants of TIWTE (Grant No.TKS190408), Science and Technology Development Fund of Tianjin Waterway Engineering Research Institute, Ministry of Transport (Grant No. KJFZJJ190201), and Scientific Research Program of Shanghai Science and Technology Commission (Grant No.19DZ1204303).
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  Figure 1  XRD patterns of Fe₂O₃, Fe₂TiO₅, and Fe₂O₃/Fe₂TiO₅

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  Figure 2  SEM-EDS element mapping images of (a) Fe₂O₃, (b) Fe₂TiO₅, and (c) Fe₂O₃/Fe₂TiO₅

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  Figure 3  TEM-EDS element mapping images (a-d), and HRTEM images (e, f) for Fe₂O₃/Fe₂TiO₅

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  Figure 4  (a) Absorption spectra and (b) Tauc plots of Fe₂O₃, Fe₂TiO₅, and Fe₂O₃/Fe₂TiO₅

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  Figure 5  SEM images of (a) Fe₂O₃, (b) Fe₂TiO₅, and (c) Fe₂O₃/Fe₂TiO₅

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  Figure 6  (a) Degradation performance of MB over the different samples; (b) Plots of ln(c₀/c_t) vs illuminated time; (c) Stability of Fe₂O₃/Fe₂TiO₅ on MB degradation

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  Figure 7  Chopped current vs potential curves on Fe₂O₃, Fe₂TiO₅, and Fe₂O₃/Fe₂TiO₅ photoelectrodes

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  Figure 8  Nyquist plots of the Fe₂O₃, Fe₂TiO₅, and Fe₂O₃/Fe₂TiO₅ photoelectrodes

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  Figure 9  Schematic diagram of charge carrier transfer process and possible photocatalytic mechanism of Fe₂O₃/Fe₂TiO₅

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