Novel visible-light-driven Fe₂O₃/Ag₃VO₄ composite with enhanced photocatalytic activity toward organic pollutants degradation

Abstract

In this study, a new type of high-performance visible light photocatalyst Fe₂O₃/Ag₃VO₄ was prepared by a two-step method. Fe₂O₃ was prepared by a solvothermal method first and then the Fe₂O₃/Ag₃VO₄ photocatalyst was synthesized with different mass ratios by a simple chemical precipitation method. Powder X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray energy dispersive spectrometry (EDS), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), UV-vis absorption spectroscopy (in DRS mode), and electrochemical impedance spectroscopy (EIS) were applied to characterize the as-prepared samples. The results showed that the as-prepared photocatalyst was uniform in the shape of particles. Photocatalytic properties of the as-prepared samples were evaluated by degrading RhB under visible light. The results showed that 1% Fe₂O₃/Ag₃VO₄ composite presented the highest photocatalytic activity. After 60 min, 96.1% of RhB was degraded by 1% Fe₂O₃/Ag₃VO₄ composite. Finally, trapping experiments confirmed that the hole (h⁺) and superoxide free radical (˙O₂⁻) were the main active species in degrading RhB.

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1. Introduction

Over the past few years, semiconductor nanocrystals have been considered applicable for solving the problems of water pollution and energy crisis.^1–3 Anatase-type TiO₂ has outstanding photocatalytic activity in degrading organic pollutants. However, the photocatalytic activity of TiO₂ is limited by wide band gap energy and fast recombination of photogenerated carriers.^4,5 Therefore, numerous visible-light-active photocatalysts have been developed to overcome these limitations. Recently, Ag-based semiconductor photocatalysts, such as Ag₂CO₃, Ag₂WO₄, Ag₃PO₄ and Ag₃VO₄, have been studied in the application of environmental treatment and water purification.^6–9 Among them, the monoclinic scheelite Ag₃VO₄ has a narrower band gap (∼2.2 eV), which endows it with a greater spectral response for visible light. It has a special band structure: the Ag 5s orbitals hybrid with V 3d orbitals, constituting the conduction band of Ag₃VO₄. The valence band of Ag₃VO₄ is composed of O 2p⁶ orbitals hybridized with the entirely filled Ag 3d¹⁰ orbitals, which could form a valence band at a more positive energy level than that of O 2p⁶ and enhance the mobility of photoexcited holes for the oxidation of organic pollutants.^10–12 However, the photocatalytic activity of pure Ag₃VO₄ is limited due to its high electron–hole recombination rate, and only a small portion of the photogenerated electrons are transferred to the surface of the catalyst to drive the photocatalytic activity. Therefore, the importance of effective electron transfer has been widely studied.

To improve these shortcomings, researchers have carried out modifications on pure Ag₃VO₄. As far as we know, composites of Ag₃VO₄ and other oxide catalysts have been reported with enhanced photocatalytic activity, such as Gd₂O₃/Ag₃VO₄, NiO/Ag₃VO₄, Co₃O₄/Ag₃VO₄. These composites could increase the separation efficiency of electron–hole and thus enhance the photocatalytic activity.^11,13,14 In addition, some oxide catalysts, such as MgFe₂O₄, ZnFe₂O₄, NiTiO₃ and CoTiO₃, have been reported to improve the photocatalytic activity of Ag₃VO₄.^15–18 The results showed that Ag₃VO₄ can be modified by oxide, improving the photocatalytic performance. The method is promising and can be continued.

Hematite (Fe₂O₃) is one of the most important visible-light-responsive semiconductor materials under ambient conditions. It has the advantages of low cost, abundance, high chemical stability and environmental friendly features.^19,20 However, it also suffers from high electron resistance and high charge recombination rate.^21,22 Thus, great efforts have been made to synthesize Fe₂O₃–Ag-based nanocomposites to increase the photocatalytic ability of Ag-based photocatalysts. Zhang et al.²³ synthesized AgCl/iron oxide composites with enhanced photocatalytic activity towards rhodamine B (RhB) under visible light irradiation, which is due to the coupling action between the narrow band gap of hematite (Fe₂O₃) and the electron acceptor of AgCl. Fe₂O₃–AgBr nonwoven cloth was fabricated and its photocatalytic degradation abilities for rhodamine B (RhB) dyes and parachlorophenol (4-CP) were significantly enhanced, which resulted from the synergistic effects between Fe₂O₃ and AgBr.²⁴ Some studies have also reported that Fe₂O₃ has been used as a shell in the hybrid. Yang et al.²⁵ prepared a TiO₂ core followed by a hydrothermal reaction for the α-Fe₂O₃ shell. The enhanced photoactivity was attributed to the matching energy levels of the two materials. Huang et al.²⁶ synthesized core–shell structured Ag/AgBr@Fe₂O₃ with enhanced photoactivity for degrading methyl orange and bisphenol A, as well as for killing Escherichia coli. Zhang et al.²⁷ prepared Co₃O₄ as the core and Fe₂O₃ as the shell. The hybrid showed enhanced gas sensing performance. All these studies demonstrated that Fe₂O₃ can be used as a good material to modify other photocatalysts and the hybrid showed enhanced performance.

Based on the abovementioned analysis, it is feasible to apply the Fe₂O₃/Ag₃VO₄ compound in the field of photocatalysis. In addition, the cost of Fe₂O₃ is very less, and it has the potential to be widely used to modify other materials. To the best of our knowledge, there is no report on the fabrication of coupled Fe₂O₃/Ag₃VO₄. In this paper, we report a low-temperature method to synthesize Fe₂O₃/Ag₃VO₄ composite by a chemical precipitation method. The as-prepared photocatalysts were studied for the degradation of potential toxic and carcinogenic RhB dye under visible light irradiation.²⁸ The results demonstrated that the introduction of Fe₂O₃ can improve the photocatalytic activity of Ag₃VO₄. 1% Fe₂O₃/Ag₃VO₄ exhibits the highest photocatalytic performance, and the degradation efficiency could reach 96.1% in 60 minutes. According to the results of trapping experiments, the hole (h⁺) and superoxide free radical (˙O₂⁻) are the main active species in the process of photocatalytic degradation of RhB. Finally, a possible photocatalytic mechanism was proposed.

2. Experiment

2.1 Materials

All the starting materials in the experiment were purchased commercially and used without further purification.

The synthesis of Fe₂O₃ is similar to our previous report.²⁶ Typically, 8.08 g Fe(NO₃)₃·9H₂O was dissolved in 80 mL ethylene glycol (EG) with magnetic stirring. Then, 7.20 g CH₃COONa and 2.00 g polyethylene glycol-600 (PEG-600) were added into the solution and the stirring was continued for 1.5 h to obtain a homogeneous suspension, which was then transferred into several 25 mL Teflon-lined autoclaves, followed by treatment at 200 °C for 22 h. After allowing them to cool naturally, the products were washed 10 times thoroughly with pure water and ethanol. They were then dried at 60 °C overnight. Second, the as-prepared Fe₂O₃ precursor was calcined in air atmosphere at a temperature of 500 °C for 2 h.

2.2 Synthesis of Fe₂O₃/Ag₃VO₄

A certain amount of Fe₂O₃ (Fe₂O₃ : AgNO₃ = 0.5%, 1%, 2%, 3%, 5%, 10% in mass ratio) was added to distilled water by ultrasonication for 30 min. Then, 0.24 g AgNO₃ was added into the aforementioned solution with magnetic stirring at 0 °C for 30 min. 0.1884 g Na₃VO₄·12H₂O was dissolved in 40 mL H₂O at 0 °C. Subsequently, the prepared Na₃VO₄·12H₂O aqueous solution was mixed into the abovementioned solution to obtain a suspension by mechanical stirring at 0 °C for 60 min. The obtained products were centrifuged and washed three times with deionized water and ethanol, and dried at 60 °C overnight. According to this method, the samples of Fe₂O₃/Ag₃VO₄ with different mass ratios were recorded as Fe₂O₃/Ag₃VO₄ (0.5%), Fe₂O₃/Ag₃VO₄ (1%), Fe₂O₃/Ag₃VO₄ (2%), Fe₂O₃/Ag₃VO₄ (3%), Fe₂O₃/Ag₃VO₄ (5%) and Fe₂O₃/Ag₃VO₄ (10%). Ag₃VO₄ was obtained in the absence of Fe₂O₃ by the same method.

2.3 Characterization

XRD patterns of the samples were obtained on an X-ray diffractometer (Bruker D8) with Cu Kα radiation (λ = 1.5418 Å) in the range of 2θ = 10–80°. Scanning electron microscopy (SEM) analysis was carried out on a tungsten filament SEM (Hitachi S-3400N) equipped with an energy-dispersive X-ray spectroscope (EDS) running at an acceleration voltage of 10 kV. Transmission electron microscopy (TEM) was carried out with a JEOL-JEM-2010 (JEOL, Japan) operated at 200 kV. Infrared (IR) spectra of all the catalysts (KBr pellets) were recorded on the Nicolet Model Nexus 470 IR equipment at room temperature. The surface compositions and chemical state of the composites were detected by X-ray photoelectron spectroscopy (XPS) analysis (Thermo ESCALab MKII X-ray photoelectron spectrometer using Mg Kα radiation). The UV-vis absorption spectra (in DRS mode) of the samples were obtained on a UV-vis spectrophotometer (UV-2450, Shimadzu Corporation, Japan) using BaSO₄ as the reference. The electrochemical impedance spectroscopy (EIS) was carried out on an electrochemical work-station (CHI 660B Chenhua Instrument Company).

2.4 Photocatalytic activity measurement

The photocatalytic performance of the as-prepared catalysts was evaluated by the degradation of rhodamine B (RhB) under visible light irradiation using a 300 W xenon lamp with a 420 nm cutoff filter as the light source. 0.05 g of photocatalyst was placed into 100 mL (10 mg mL⁻¹) aqueous solution of RhB in a Pyrex reaction vessel. Prior to irradiation, the suspensions were magnetically stirred for 30 min to ensure adsorption–desorption equilibrium between photocatalysts and RhB solution in the dark. During the photodegradation process, about 4 mL of suspensions were collected from each sample at given time intervals of visible-light irradiation, followed by centrifugation for 3 min. Then, the concentration was tested by a Shimadzu UV-2450 spectrophotometer at 553 nm, which is the maximal absorption band of RhB.

2.5 Photoelectrochemical measurements

EIS was investigated with an electrochemical analyzer (CHI660B, Chen Hua Instruments, Shanghai, China) in a standard three-electrode system, which used a platinum wire as the counter electrode, a saturated calomel electrode (SCE) as the reference electrode and an ITO as the working electrode. Typically, 5 mg of sample powder was dispersed ultrasonically in 1 mL ethylene glycol, and 20 μL of the resulting colloidal dispersion was drop-cast onto a piece of ITO slice with a fixed area of 0.5 cm² and dried at 80 °C. A 500 W Xe arc lamp was utilized as the light source. Electrochemical impedance spectra (EIS) were obtained in a frequency range from 0.01 Hz to 100 kHz at 0.24 V, and the amplitude of the applied sine wave potential in each case was 5 mV. A 0.1 M Na₂SO₄ aqueous solution was used as the impedance liquid.

3. Result and discussion

3.1 XRD analysis

Fig. 1 shows the XRD patterns of Fe₂O₃, Ag₃VO₄ and a series of Fe₂O₃/Ag₃VO₄ composites. As can be seen in Fig. 1a, the diffraction peaks of Fe₂O₃ at 24.16°, 33.16°, 35.62°, 40.88°, 49.54°, 54.06°, 57.60°, 63.86°, and 63.98° can be indexed to (012), (104), (110), (113), (024), (116), (018), (214), and (300) planes, which correspond to α-Fe₂O₃ (JCPDS no. 33-0664).²⁹ The characteristic diffraction peaks (Fig. 1b) for Ag₃VO₄ were detected at 19.46°, 31.00°, 31.08°, 35.18°, 36.02°, 39.10°, 41.26°, 48.60°, 51.54° and 54.14°, which were matching well with the (011), (−121), (121), (301), (202), (022), (400), (−322), (132), and (331) planes of Ag₃VO₄ (JCPDS no. 43-0542).¹⁷ As shown in Fig. 1c–f, for the XRD of the Fe₂O₃/Ag₃VO₄ composites, the diffraction peaks of Ag₃VO₄ appeared and did not shift, indicating that the combination of Fe₂O₃ and Ag₃VO₄ was not in the lattice, but only on the surface. In addition, the diffraction peaks of Fe₂O₃ in the complexes are not obvious with Fe₂O₃ content less than 10%. Therefore, the existence of Fe₂O₃ was confirmed by EDS, TEM, IR and XPS analyses.

Fig. 1 XRD spectra of Fe₂O₃ (a), Ag₃VO₄ (b), 0.5% Fe₂O₃/Ag₃VO₄ (c), 1% Fe₂O₃/Ag₃VO₄ (d), 5% Fe₂O₃/Ag₃VO₄ (e), and 10% Fe₂O₃/Ag₃VO₄ (f).

3.2 SEM-EDS and TEM analysis

The morphology and composition of the samples were determined by SEM, TEM and EDS. In Fig. 2A and B, it can be observed that Fe₂O₃ and Ag₃VO₄ appear as nanoparticles with a size of less than 100 nm. The 1% Fe₂O₃/Ag₃VO₄ composite (Fig. 2C) displays a morphology similar to them. It is difficult to distinguish Fe₂O₃ from the Ag₃VO₄ particle from the SEM images. Thus, the EDS of the 1% Fe₂O₃/Ag₃VO₄ composites was carried out; the results (Fig. 2D) displayed that the Fe element was detected in addition to the original Ag, V and O elements in Ag₃VO₄. It revealed that Fe₂O₃ existed in the 1% Fe₂O₃/Ag₃VO₄ sample. Furthermore, TEM was used to distinguish the introduced Fe₂O₃. The TEM image in Fig. 2E shows that the Ag₃VO₄ exists as nanoparticles with diameters about 5–50 nm, and they tend to reaggregate together. The granular morphologies of Fe₂O₃ (Fig. 2F) are regular, angular and even a few hexagonal nanoparticles appear. As shown in Fig. 2G, it is a little difficult to distinguish the whole Fe₂O₃ particles from the Ag₃VO₄ particles.

Fig. 2 SEM images of pure Fe₂O₃ (A), Ag₃VO₄ (B), 1% Fe₂O₃/Ag₃VO₄ (C) and EDS spectrum of 1% Fe₂O₃/Ag₃VO₄ (D) and TEM images of Ag₃VO₄ (E), pure Fe₂O₃ (F) and 1% Fe₂O₃/Ag₃VO₄ (G).

To investigate the distribution of Fe₂O₃, EDS elemental mapping of 1% Fe₂O₃/Ag₃VO₄ was carried out and the results are shown in Fig. 3. The image demonstrates the X-ray signal coming from Fe, Ag, O and V elements, which are represented by yellow, green, red and blue color, respectively. It is clear that the Fe element dispersed well. Considering that the Fe is present in the form of Fe₂O₃, it can be confirmed that the Fe₂O₃ is well-dispersed in the hybrid.

Fig. 3 The SEM image of 1% Fe₂O₃/Ag₃VO₄ (A) and the corresponding EDS elemental mapping of Fe (B), Ag (C), O (D) and V (E) elements.

3.3 IR analysis

Fig. 4 shows the IR spectra of pure Fe₂O₃, Ag₃VO₄, and a series of Fe₂O₃/Ag₃VO₄ composites. For Fig. 4a, the absorption peaks at 531 and 465 cm⁻¹ in the low frequency region are the Fe–O–Fe stretching vibration band, suggesting that the synthesized sample is α-Fe₂O₃.^30–33 In the spectrum of pure Ag₃VO₄ (Fig. 4b), three main absorption peaks at 918, 844, and 696 cm⁻¹ can be clearly observed. The absorption belt at 918 cm⁻¹ is because the tetrahedral VO₄³⁻ condensation formed symmetric and asymmetric stretching vibration of the coke vanadium ion VO₃ group, and the absorption peak at 696 cm⁻¹ is the asymmetric stretching vibration of V₂O₇⁴⁻.^34–36 It can be seen that the absorption peaks of Ag₃VO₄ appear in Fig. 4c–f, and there is no shift. When the content of Fe₂O₃ is less than 5%, the characteristic absorption peaks of Fe₂O₃ in Fig. 4c, 3d and e are not observed, which may be due to the low content of Fe₂O₃. When the mass fraction of Fe₂O₃ reached 10%, the absorption peaks of Fe₂O₃ appeared at 525 and 455 cm⁻¹. In addition, the absorption peaks showed a little blue shift when compared to the pure Fe₂O₃. The results may be due to the interaction between Fe₂O₃ and Ag₃VO₄.

Fig. 4 IR spectra of Fe₂O₃ (a), Ag₃VO₄ (b), 0.5% Fe₂O₃/Ag₃VO₄ (c), 1% Fe₂O₃/Ag₃VO₄ (d), 5% Fe₂O₃/Ag₃VO₄ (e), 10% Fe₂O₃/Ag₃VO₄ (f).

3.4 XPS analysis

XPS analysis was used to investigate the element compositions and the valencies of the surface elements of Ag₃VO₄ and Fe₂O₃/Ag₃VO₄. Fig. 5A shows the survey spectrum of Fe₂O₃/Ag₃VO₄ composite (10%). It is obvious that the sample consists of Ag, V, Fe, O and C. The C element is ascribed to the XPS instrument itself. Fig. 5B–E show the high resolution XPS spectra of Ag 3d, V 2p, O 1s, Fe 2p and Ag 3s of the samples. Fig. 5B displays the high-resolution spectra for Ag 3d; the center of the two peaks at 368.16 and 374.24 eV can be attributed to the Ag 3d_5/2 and Ag 3d_3/2 in Ag₃VO₄.^10,37 When Fe₂O₃ was introduced, the center of the Ag 3d_5/2 peak did not shift obviously, while the peak at 374.24 eV shifted to 374.15 eV. As shown in Fig. 5C, two typical peaks for V 2p were 516.91 and 524.55 eV, which correspond to V 2p_3/2 and V 2p_1/2. This indicated that in both samples V exists as V⁵⁺. The peak at 516.91 eV shifts to 516.80 eV.^10,13,37,38 Furthermore, O 1s spectra exhibit the peak at 530.18 eV, which shifted to 530.08 eV, as shown in Fig. 5D.³⁷ According to the XPS results of Ag 3d, V 2p and O 1s, it can be inferred that there is a weak interaction between Fe₂O₃ and Ag₃VO₄ in Fe₂O₃/Ag₃VO₄ composite synthesized by chemical adsorption. Fig. 5E shows the high resolution spectrum of Fe 2p and Ag 3s. The four peaks around 711.3 eV, 719.0 eV, 724.2 eV and 734.6 eV are ascribed to the Fe 2p_3/2, Fe 2p_3/2 satellite, Fe 2p_1/2 and Fe 2p_1/2 satellite, respectively.³⁹ The shakeup satellite peak at around 719.0 eV is a characteristic of the Fe³⁺ species.⁴⁰ The typical peak at 717.3 eV can be assigned to the binding energies of Ag 3s.^13,41

Fig. 5 The survey spectra of Fe₂O₃/Ag₃VO₄ (A) and the high resolution XPS spectra of Ag 3d (B), V 2p (C), O 1s (D), Fe 2p and Ag 1s (E).

3.5 UV-vis analysis

The UV-vis absorption spectra (in DRS mode) of Fe₂O₃, Ag₃VO₄ and Fe₂O₃/Ag₃VO₄ composites with different Fe₂O₃ contents are shown in Fig. 6A. It can be seen that Fe₂O₃ has good absorption ability in the region of 200–800 nm, especially in the region of 200–600 nm. Similar results have been reported in other studies.^42,43 All the composites show strong absorbance in the region of 400–600 nm and the absorbing boundary of the composite and Ag₃VO₄ is about 470 nm in Fig. 6A. The absorption intensity of the composite is higher than that of the pure Ag₃VO₄ and increases with increasing Fe₂O₃ content in the composite in the region of 400–550 nm (the energy used from the visible light in this region is very important). This means that the composite can absorb more light energy than the pure Ag₃VO₄, which is beneficial to enhance the photoactivity of the composite. To obtain the estimated value of the band gap energies, the absorption onsets of the samples are acquired by linear extrapolation from the plots of (Ahν)² versus the photon energy (hν) in Fig. 6B. It can be seen that the tangent extrapolation estimate values are 2.01 and 2.30, which correspond to the band gap energies of Fe₂O₃ and Ag₃VO₄, respectively. This showed that Ag₃VO₄ was a direct transition and its band gap energy was around 2.30 eV, which is close to a reported study.¹⁸

Fig. 6 (A) UV-vis diffuse reflectance spectra of Ag₃VO₄ and Fe₂O₃/Ag₃VO₄ composites with various Fe₂O₃ contents, (B) (Ahν)² versus hν curves of Ag₃VO₄ and 1% Fe₂O₃/Ag₃VO₄.

3.6 Photocatalytic decomposition of RhB

The performance of adsorption-photocatalysis of the sample is tested by measuring the degradation rate of RhB in a wastewater model. The adsorption–desorption balance was achieved between the catalyst and RhB for 30 min adsorption process in darkness. Fig. 7 exhibits the photocatalytic performance of Fe₂O₃/Ag₃VO₄ composites with different Fe₂O₃ contents as well as pure Ag₃VO₄ under visible light irradiation. It can be seen that the RhB is only slightly degraded without any photocatalyst under light irradiation for 120 min, which demonstrates that self-decomposition of RhB can be negligible. Moreover, by increasing the mass fraction of Fe₂O₃, the photocatalytic activity of the composites increases. 0.5% Fe₂O₃/Ag₃VO₄ can degrade 89.1% at 60 min, which is much higher than that degraded by pure Ag₃VO₄ (which can degrade 65.2% at 60 min). When the Fe₂O₃ content was increased to 1%, the 1% Fe₂O₃/Ag₃VO₄ composite shows the highest activity, which can degrade about 96.1% RhB in 60 min. However, the photoactivity of the composite decreased gradually with further increasing the Fe₂O₃ content. 2% Fe₂O₃/Ag₃VO₄ can degrade 93.6% of RhB, 3% Fe₂O₃/Ag₃VO₄ can degrade 84.6% of RhB, 5% Fe₂O₃/Ag₃VO₄ can degrade 79.0% of RhB and 10% Fe₂O₃/Ag₃VO₄ can degrade only 65.8% at 60 min.

Fig. 7 Photocatalytic degradation of RhB in the presence of pure Ag₃VO₄ and Fe₂O₃/Ag₃VO₄ composites, and photolysis of RhB under visible light irradiation. Results are expressed as means ± the standard error from three independent experiments.

3.7 EIS analysis

The electrochemical impedance spectroscopy of pure Ag₃VO₄ and Fe₂O₃/Ag₃VO₄ composites was carried out to investigate the process of electron transfer. Fig. 8 shows that the EIS data fits well with the R_s(Q(R_ctZ_w)) equivalent circuit. The diameter of the semicircular part at higher frequencies is equal to the electrochemical reaction impedance, R_ct, which reflects the charge transfer rate occurring at the contact interface between the working electrode and electrolyte solution.⁴⁴ The smaller radius of the Nyquist circle represents the lower charge-transfer resistance.⁴⁵ Fig. 8 shows that the arc radius on the EIS Nyquist plot of 1% Fe₂O₃/Ag₃VO₄ composite is smaller than that of pure Ag₃VO₄. This means that a faster interfacial charge transfer to electron acceptor occurs, which resulted from effective separation of electron–hole pairs by the introduction of Fe₂O₃.^46,47

Fig. 8 Electrochemical impedance spectroscopy of pure Ag₃VO₄ and 1% Fe₂O₃/Ag₃VO₄ composite. The inset shows the equivalent circuit: R_s(Q(R_ctZ_w)); R_s: ohmic resistance of the solution, Q: electric double-layer capacitance, R_ct: electrochemical reaction impedance, and Z_w: solid phase diffusion of optical electron.

3.8 Mechanism of pollutant photodegradation

The photocatalytic stability of Fe₂O₃/Ag₃VO₄ was evaluated by the RhB cycle degradation experiments under visible light. As shown in Fig. 9, the photoactivity of Fe₂O₃/Ag₃VO₄ can remain very high after three consecutive cycles. Therefore, Fe₂O₃/Ag₃VO₄ composite can be used as an effective, stable photocatalyst for the degradation of organic pollutants.

Fig. 9 Cycling runs of 1% Fe₂O₃/Ag₃VO₄ photocatalyst for RhB degradation.

To explore the photocatalytic mechanism of RhB, a series of active species trapping experiments was carried out. The results of adding different radical scavengers over the Fe₂O₃/Ag₃VO₄ photocatalysts under visible-light irradiation are described in Fig. 10. When triethanolamine (a quencher of h⁺) was added, the photocatalytic degradation of RhB greatly declined compared to the reaction without radical scavengers, which indicates that h⁺ is the main active species of Fe₂O₃/Ag₃VO₄ for the degradation of RhB. Another quencher of h⁺ (EDTA-2Na) was used to verify this result. The photoactivity is remarkably decreased, which is consistent with the result of triethanolamine addition. Furthermore, when ascorbic acid as a scavenger for ˙O₂⁻ is added, the photodegradation process can be inhibited effectively. This suggests that ˙O₂⁻ also plays the critical role in the photocatalysis process. Finally, RhB photodegradation is not obviously affected when isopropanol is applied as ˙OH scavenger. Therefore, the ˙OH is not the main active species in the reaction of the RhB degradation. The free radicals trapping experiments indicate that h⁺ and ˙O₂⁻ are the main active species.

Fig. 10 Trapping experiment of active species during the photocatalytic degradation of RhB over 1% Fe₂O₃/Ag₃VO₄ composites under visible light irradiation.

Based on the results mentioned above, a possible mechanism for the photocatalytic degradation of organic compounds by Fe₂O₃/Ag₃VO₄ is proposed. As shown in Fig. 11, both Ag₃VO₄ and Fe₂O₃ can be excited to generate electrons and holes under visible light irradiation. E_CB and E_VB can be calculated according to the formulas: E_CB = χ − E_C − 0.5E_g and E_VB = E_CB + E_g. E_g is the band gap of the semiconductor, E_CB is the conduction band potential, E_C is the energy of free electrons on the hydrogen scale (4.5 eV), E_VB is the valence band potential and χ is the absolute electronegativity of the semiconductor. According to the UV-vis analysis and potential calculation, the E_CB of Ag₃VO₄ was located at −0.01 eV, which was more negative than the CB potential of Fe₂O₃ (0.37 eV). In addition, the E_VB of Ag₃VO₄ was located at 2.29 eV, which was more negative than the VB potential of Fe₂O₃ (2.38 eV). Therefore, the photogenerated electrons in the Ag₃VO₄ particles could be quickly transferred to Fe₂O₃. The holes generated in the VB of Fe₂O₃ could easily move to Ag₃VO₄, which would promote the effective separation of photoexcited electron–hole pairs and decrease the recombination of electron and hole. This is consistent with the result of the EIS analysis. The electrons on the Fe₂O₃ could combine with oxygen in the solution to form ˙O₂⁻. The generated ˙O₂⁻ on the CB of Fe₂O₃ and the hole on the VB of Ag₃VO₄ would play important roles in the photocatalytic reaction process, leading to enhanced photocatalytic performance.

Fig. 11 Schematic of the separation and transfer of photogenerated charges in the Fe₂O₃/Ag₃VO₄ composites combined with the possible reaction mechanism of the photocatalytic procedure.

4. Conclusions

In this study, Fe₂O₃ modified Ag₃VO₄ photocatalysts with various Fe₂O₃ contents were successfully prepared by a simple precipitation technique. They were used for the first time as a photocatalyst for the degradation of RhB under visible light irradiation. The photocatalytic degradation activity of Ag₃VO₄ was greatly improved by Fe₂O₃ modification. 1% Fe₂O₃/Ag₃VO₄ composite showed much higher photoactivity than Ag₃VO₄ and can degrade 96.1% of RhB in 60 min. According to the results of EIS and the trapping experiments, the introduced Fe₂O₃ effectively accelerates the separation of photogenerated electrons and holes, leading to enhanced photocatalytic activity. As a result, the Fe₂O₃/Ag₃VO₄ photocatalyst is a potential catalyst for the treatment of organic pollutants.

Acknowledgements

This study is financially supported by the National Natural Science Foundation of China for Youths (No. 21407065 and 21507046), the Natural Science Foundation of Jiangsu Province for Youths (BK20140533), China Postdoctoral Science Foundation (No. 2014M551520 and 2015T80514), Jiangsu University Scientific Research Funding (No. 14JDG052).

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