Facile Synthesis and Enhanced Visible-Light Photocatalytic Activity of Novel p-Ag₃PO₄/n-BiFeO₃ Heterojunction Composites for Dye Degradation

Abstract

In this work, Ag₃PO₄ microparticles were decorated onto the surface of BiFeO₃ microcuboids through a precipitation method to obtain p-Ag₃PO₄/n-BiFeO₃ heterojunction composites. The composites were employed for the degradation of acid orange 7 (AO7) under visible-light irradiation. It is found that the composites exhibit much higher photocatalytic efficiency than bare BiFeO₃. Meanwhile, the intrinsical visible-light-driven photocatalytic activity of Ag₃PO₄/BiFeO₃ composites was further confirmed by the degradation of phenol. In addition, the photo-Fenton-like catalysis property of the composite was also evaluated. The photocurrent analysis indicates that the combination of BiFeO₃ with Ag₃PO₄ leads to the inhibition of recombination of photoinduced electrons and holes. The obvious enhancement in the photocatalytic activity of the composite is mainly ascribed to the efficient photogenerated charge separation and interfacial charge migration caused by the formation of Ag₃PO₄/BiFeO₃ p-n heterojunctions.

PACS

81.05.Hd82.65.+r82.50.–m

Background

Recently, the semiconductor photocatalysis has received considerable attention as a promising technology for energy conversion and pollution treatment [1–3]. As we know, the widely investigated photocatalyst TiO₂ is merely active under ultraviolet (UV) light irradiation which only constitutes about 5% of solar light and thus greatly limits its photocatalytic applications under sunlight. Consequently, it is necessary to develop the visible-light-driven photocatalysts [4–8].

Bismuth- or ferrum-based semiconductor oxides generally possess a moderate bandgap energy (~ 2.0 eV) and are regarded as an important class of visible-light-responsive photocatalysts [9–19]. Among them, BiFeO₃ with a perovskite-type structure is found to exhibit interesting photocatalytic performance for the dye degradation and water splitting under visible-light irradiation [20–25]. However, its photocatalytic activity is not impressive due to the high recombination rate of photogenerated electrons (e⁻) and holes (h⁺). Fortunately, it is demonstrated that coupling of BiFeO₃ with a narrow-bandgap semiconductor of matched band edge potentials to form a heterojunction is one of the most promising strategies to promote the separation of photogenerated charges, thus leading to improved photocatalytic activity of BiFeO₃ [26–31]. For example, Chaiwichian et al. reported that BiFeO₃-Bi₂WO₆ nanocomposites exhibited enhanced activity in dye degradation [29]. Wang et al. observed that AgCl/Ag/BiFeO₃ showed much higher visible-light photocatalytic activity than bare BiFeO₃ [30]. Fan and co-workers found that the combination of g-C₃N₄ with BiFeO₃ can obviously improve the catalytic activity compared with pure BiFeO₃ [31].

Silver orthophosphate (Ag₃PO₄), as an excellent visible-light-driven photocatalyst, has attracted considerable attention in the photocatalytic field [32–37]. It has been shown that Ag₃PO₄ can achieve extremely high quantum yield (~ 90%) for oxygen generation from water splitting [32, 33]. Furthermore, it possesses superior photooxidation capability for organic pollution degradation due to its highly positive valence band position [34]. In most cases, owing to its appropriate energy band position and narrow bandgap, Ag₃PO₄ is widely employed as the cocatalyst to combine with other photocatalysts to form composites, leading to an obvious improvement of photocatalytic behavior, such as Ag₃PO₄/Bi₂WO₆, Ag₃PO₄/BiPO₄, Ag₃PO₄/Bi₂O₂CO₃, Ag₃PO₄/g-C₃N₄, Ag₃PO₄/BiVO₄, Bi₄Ti₃O₁₂/Ag₃PO₄, Ag₃PO₄/ZnFe₂O₄, Ag₃PO₄/WO₃, Ag₃PO₄/ZnO, and Bi₂MoO₆/Ag₃PO₄ [38–47]. It is reported that BiFeO₃ is an n-type semiconductor and Ag₃PO₄ is known as a p-type semiconductor [43, 48]. The construction of Ag₃PO₄/BiFeO₃ p-n heterojunction composites may be a feasible method to obtain efficient photocatalyst. However, to the best of our knowledge, little work has been devoted to the investigation of photocatalytic performance of Ag₃PO₄/BiFeO₃ composites.

In this work, Ag₃PO₄/BiFeO₃ p-n heterojunction composites were facilely prepared via the precipitation of Ag₃PO₄ microparticles on the BiFeO₃ microcuboids. Acid orange 7 (AO7) and phenol were selected as the model pollutant to evaluate the photocatalytic activity of the composites under visible-light irradiation. Moreover, the photo-Fenton-like catalysis activity of the composite was also investigated. The underlying mechanism of the composites for the degradation of organic pollutants was discussed.

Methods

Preparation of Ag₃PO₄/BiFeO₃ Composites

BiFeO₃ microcuboids were synthesized via a hydrothermal route. 0.005 mol of Bi(NO₃)₃•5H₂O and 0.005 mol of Fe(NO₃)₃•9H₂O were dissolved in 20 mL of dilute nitric acid solution (5 mL HNO₃ + 15 mL deionized water). Sixty milliliters of KOH solution with concentration of 4.5 mol/L was added to the above solution drop by drop under magnetic stirring. After 8 min of ultrasonic treatment and another 30 min of vigorous magnetic stirring, the mixture solution was sealed in a Teflon-lined stainless steel autoclave of 100 mL capacity and submitted to hydrothermal reaction at 200 °C for 6 h. After the autoclave was cooled naturally to room temperature, the precipitate was collected by centrifugation, washed with deionized water (two times) and absolute ethanol (three times), and then dried at 80 °C for 12 h to obtain final BiFeO₃ product. Ag₃PO₄ microparticles were prepared by a precipitation method. Three millimoles of AgNO₃ was dissolved into 30 mL deionized water, and 1 mmol Na₃PO₄·12H₂O was added into 30 mL deionized water with the aid of magnetic stirring. After the solution was homogeneous, the latter solution was added dropwise into the former under vigorous magnetic stirring for 7 h. During the reaction, the color of the solution changed into yellow. Finally, the mixture was centrifuged to collect the precipitate. The obtained precipitate was washed several times with deionized water and then dried in a vacuum oven at 60 °C for 8 h.

Ag₃PO₄/BiFeO₃ composites were synthesized as follows: 0.1 g of BiFeO₃ microcuboids were dispersed in 30 mL deionized water and then ultrasonicated for 2 h. After that, a certain amount of AgNO₃ was dissolved into the above suspension. To this mixture was added drop by drop a certain concentration of Na₃PO₄ solution (30 mL) under vigorous magnetic stirring for 7 h. The as-obtained composites were separated by centrifugation, washed repeatedly with deionized water, and dried in a vacuum oven at 60 °C for 8 h. To investigate the effect of Ag₃PO₄ content on the photocatalytic property of obtained composites, a series of sample was fabricated with different Ag₃PO₄ mass ratios of 5%, 10%, 20%, and 40% and the corresponding samples were termed as 5wt%Ag₃PO₄/BiFeO₃, 10wt%Ag₃PO₄/BiFeO₃, 20wt%Ag₃PO₄/BiFeO₃, and 40wt%Ag₃PO₄/BiFeO₃, respectively. For comparison, the composite termed as 20wt%Ag₃PO₄/BiFeO₃-M was also prepared by direct mechanical mixing of BiFeO₃ microcuboids and Ag₃PO₄ microparticles, where Ag₃PO₄ occupies a mass fraction of 20% in the composite.

Photoelectrochemical Measurements

The photocurrent test was carried out on the electrochemical workstation (CST 350) with a three-electrode cell as described in the literature [49]. In this three-electrode system, a platinum foil and a standard calomel electrode were used as the counter electrode and reference electrode, respectively. The working electrode was fabricated as follows: 15 mg photocatalysts, 0.75 mg carbon black, and 0.75 mg polyvinylidene fluoride (PVDF) were added into 1-methyl-2-pyrrolidione (NMP) to produce slurry, which was then uniformly coated on a 1.0 × 1.0 cm² fluoride-doped tin oxide glass electrode. After that, the electrode was dried at 60 °C for 5 h. A 300-W Xe lamp with a 420-nm cut-off filter was employed as the visible light source. The photoelectrochemical measurement was performed in the 0.1-M Na₂SO₄ electrolyte solution, and its pH value was measured to be ~ 5.3.The photocurrent-time (I-t) curves were measured at a fixed bias potential of 0.2 V. The electrochemical impedance spectroscopy (EIS) test was performed by using the sinusoidal voltage pulse with amplitude of 5 mV and in the frequency range from 10⁻² to 10⁵ Hz.

Photocatalytic Activity Test

The photocatalytic activity of samples was evaluated toward the degradation of AO7 and phenol under visible-light irradiation. Typically, the initial AO7 or phenol concentration was 5 mg/L with a catalyst loading of 0.5 g/L. The pH values of AO7 and phenol solution were measured to be ~ 6.8 and ~ 6.2, respectively. Prior to illumination, the mixture was stirred in the dark for 0.5 h to achieve the adsorption-desorption equilibrium of organic molecule on the surface of catalysts. This reaction solution was then exposed to a 300-W xenon lamp with a 420-nm cut-off filter, and the corresponding light intensity was measured to be ~ 50 mW cm⁻². During the photocatalytic experiment, a small amount of reaction solution was collected at the given time intervals and then centrifuged to separate catalysts. The concentration of AO7 or phenol was determined by detecting the absorbance of the supernatant at a given wavelength (λ _AO7 = 484 nm and λ _phenol = 270 nm) using a UV-visible spectrophotometer. To evaluate the photocatalytic reusability of the photocatalysts, the recycling experiment for the degradation of AO7 was performed. After the first photocatalytic test was completed, the photocatalysts were collected by centrifugation, washed with distilled water, and dried. The collected photocatalysts were added into the fresh dye solution for the next cycle of the photocatalytic experiment. To investigate the photo-Fenton-like catalysis ability of the photocatalysts, H₂O₂ (5 mmol/L) was added into the reaction solution. The photo-Fenton-like experiment procedure was similar to the above photocatalytic process.

Characterization

The phase purity of the samples was investigated by X-ray diffractometer (XRD, Bruker D8 Advanced) using Cu Kα radiation. The morphology of the samples was observed by a field-emission scanning electron microscope (SEM, JEOL JSM-6701F) and field-emission transmission electron microscope (TEM, JEOL JEM-2010). The composition of the samples was measured by energy dispersive X-ray spectroscopy. The chemical state of the element was tested using X-ray photoelectron spectroscopy (XPS, PHI-5702), where the binding energy scale of the XPS data was calibrated against the adventitious C 1s peak at the binding energy of 284.8 eV. The ultraviolet-visible (UV-vis) diffuse reflectance spectra of the products were obtained using a UV-vis spectrophotometer (PERSEE TU-1901) with BaSO₄ as a reference. The PL spectra of the samples were recorded on a fluorescence spectrophotometer (SHIMADZU RF-6000) with the excitation wavelength of ~ 350 nm.

Results and Discussion

XRD Analysis

Figure 1 presents the XRD patterns of BiFeO₃, Ag₃PO₄, and Ag₃PO₄/BiFeO₃ composites with different Ag₃PO₄ contents. For bare BiFeO₃ sample, all the diffraction peaks match well with the rhombohedral structure of BiFeO₃ (PDF card no. 74-2016), and for bare Ag₃PO₄ sample, the diffraction peaks can be perfectly indexed to cubic Ag₃PO₄ phase (PDF card no. 06-0505); this indicates that high-purity BiFeO₃ and Ag₃PO₄ have been successfully prepared. In the case of the composites, the XRD patterns can be assigned to the characteristic diffraction peaks of BiFeO₃ and Ag₃PO₄, and no diffraction peaks of impurity appear in the patterns. Moreover, it is seen that by increasing the content of Ag₃PO₄, the intensity of the characteristic peaks of Ag₃PO₄ increases gradually. The results suggest that the composites consist of rhombohedral BiFeO₃ and cubic Ag₃PO₄, and no other phase is generated during the preparation of the composites.

Fig. 1.

XRD patterns of BiFeO₃, Ag₃PO₄, and Ag₃PO₄/BiFeO₃ composites

Morphology Observation

The morphology of the samples was observed by SEM and TEM. Figure 2a, b shows the SEM image and TEM image of bare BiFeO₃, revealing that the prepared BiFeO₃ particles exhibit cuboid-like shape with 200–500 nm in size and have a smooth surface. The inset of Fig. 2a displays the length-to-width ratio distribution of BiFeO₃ particles, which reveals that the length-to-width ratio ranges from 1.1/1 to 2.5/1. As can be seen from the TEM image in Fig. 2c, bare Ag₃PO₄ consists of irregular sphere-like particles. The size distribution of Ag₃PO₄ particles is shown in the inset of Fig. 2c, indicating a wide distribution of particle size ranging from 110 to 180 nm. From the TEM image of the 20wt%Ag₃PO₄/BiFeO₃ composite (Fig. 2d), one can see that the irregular microspheres are attached to the cuboid-shaped particle. The high-resolution TEM (HRTEM) images obtained from the different particles indicate two distinct sets of lattice fringes (insets in Fig. 2d). The interplanar spacing of ~ 0.288 nm matches the BiFeO₃ (110) planes, whereas the interplanar distance of ~ 0.267 nm corresponds to the Ag₃PO₄ (210) planes. In addition, the EDX analysis suggests that the composite includes all the elements of Ag₃PO₄ and BiFeO₃ phases (Fig. 2e). The observed C and Cu signals in the EDX spectrum of Fig. 2e could arise from the microgrid used for supporting the sample [50]. These results reveal that Ag₃PO₄ particles are decorated on the surface of BiFeO₃ microcuboids, resulting in the formation of Ag₃PO₄/BiFeO₃ p-n heterostructures.

Fig. 2.

a SEM (the inset is the length-to-width ratio distribution of BiFeO₃ microcuboids) and b TEM image of BiFeO₃ microcuboids. TEM image of c Ag₃PO₄ microparticles (the inset is the size distribution of Ag₃PO₄ microparticles) and d 20wt%Ag₃PO₄/BiFeO₃ sample; inset shows its HRTEM image. e EDX spectrum of 20wt%Ag₃PO₄/BiFeO₃ sample

XPS Analysis

The XPS analysis was performed to reveal the chemical states of BiFeO₃ and 20wt%Ag₃PO₄/BiFeO₃, as shown in Fig. 3. Figure 3a shows the high-resolution XPS spectrum of Ag 3d in the composite. The two obvious peaks at 373.8 and 367.7 eV are attributed to the Ag 3d_3/2 and Ag 3d_5/2 binding energies of Ag⁺. Figure 3b presents the P 2p high-resolution XPS spectrum of the composite. The peak at around 133.2 eV corresponds to the characteristic binding energy of P⁵⁺ oxidation state in Ag₃PO₄ [51]. Figure 3c, d, shows the Bi 4f and Fe 2p high-resolution XPS spectra, respectively. For bare BiFeO₃, the Bi 4f spectrum shows two strong peaks at binding energies of 164.1 eV and 158.8 eV, belonging to the Bi 4f_5/2 and Bi 4f_7/2, respectively, which indicates that Bi ion possesses the oxidation state of + 3. For Fe 2p spectrum, the peak located at 723.7 eV is assigned to the Fe 2p_1/2 of Fe³⁺. Another strong XPS signal at ~ 711.6 eV can be fitted into two peaks at 711.7 and 709.9 eV. The peak at 709.9 eV corresponds to the binding energy of Fe 2p_3/2 of Fe²⁺. The binding energy at 711.7 eV belongs to the Fe 2p_3/2 of Fe³⁺. In addition, a satellite peak is found at around 718.2 eV, which is attributed to the mixed oxidation states of Fe. From the XPS analysis of the Fe element, it can be seen that Fe exists in the form of Fe³⁺ and Fe²⁺ in bare BiFeO₃. It is worth noting that the Bi 4f and Fe 2p binding energies in 20wt%Ag₃PO₄/BiFeO₃ exhibit a slight shift in comparison to bare BiFeO₃, which is mainly attributed to the interaction between BiFeO₃ and Ag₃PO₄. Figure 3e displays the O 1s high-resolution XPS spectra of BiFeO₃ and 20wt%Ag₃PO₄/BiFeO₃. For bare BiFeO₃, the O 1s signal can be divided into two peaks at 529.8 and 531.0 eV. The binding energy of 529.8 eV corresponds to the lattice oxygen while the small peak at higher binding energy of 531.0 eV is caused by surface defects and chemisorbed oxygen species. Compared with bare BiFeO₃, the O 1s peak in the composite experiences a shift, which is also due to the interaction between Ag₃PO₄ and BiFeO₃.

Fig. 3.

High-resolution XPS spectra of BiFeO₃ and 20wt%Ag₃PO₄/BiFeO₃ sample. a Ag 3d and b P 2p of 20wt%Ag₃PO₄/BiFeO₃ sample. c Bi 4f, d Fe 2p, and e O1s of BiFeO₃ and 20wt%Ag₃PO₄/BiFeO₃ sample

Optical Absorption Property

The optical absorption behavior of the samples was investigated by measuring their UV-vis diffuse reflectance spectra, as presented in Fig. 4a. The corresponding absorption spectra transformed from the diffuse reflectance spectra according to the Kubelka-Munk (K-M) theory is shown in Fig. 4b [52]. It is seen that all the samples exhibit an important light absorption at λ < 600 nm. In order to obtain the absorption edge of the samples, the first derivative of the reflectance (R) with respect to wavelength λ (i.e., dR/dλ) was carried out, as shown in Fig. 4c. The absorption edge can be determined from the peak wavelength in the derivative spectra [53]. It can be seen that the light absorption edge of bare Ag₃PO₄ is located at ~ 527 nm, corresponding to the bandgap energy (E_g) of ~ 2.35 eV. Bare BiFeO₃ exhibits an absorption edge at around 567 nm, corresponding to the E_g of ~ 2.18 eV. In addition to the absorption edge, a weak peak at ~ 700 nm is observed, which is probably attributed to the existence of surface states in the middle of the bandgap of BiFeO₃. When coupled with Ag₃PO₄, the absorption edge of BiFeO₃ does not undergo obvious change, which indicates that the introduction of Ag₃PO₄ has no apparent effect on the bandgap structure of BiFeO₃.

Fig. 4.

a UV-visible diffuse reflectance spectra of BiFeO₃, Ag₃PO₄, and Ag₃PO₄/BiFeO₃ composites. b The corresponding absorption spectra and c the corresponding first derivative of the diffuse reflectance spectra

Photocatalytic Activity Measurement

AO7 was selected as a target pollutant for evaluating the photocatalytic performance of the samples. The photocatalytic degradation of AO7 was investigated under visible-light irradiation, and the result is shown in Fig. 5. Prior to photocatalytic reaction, blank and absorption experiments were performed. It is seen that no obvious degradation of dye is detected under irradiation without the catalysts or in the presence of catalysts without irradiation, suggesting that self-degradation and absorption of AO7 during the photocatalytic process are negligible. Bare BiFeO₃ has weak photocatalytic activity, and only ~ 27% of AO7 is degraded with 120 min of irradiation. When BiFeO₃ microcuboids are combined with Ag₃PO₄ microparticles, the formed Ag₃PO₄/BiFeO₃ composites exhibit superior photocatalytic activity to bare BiFeO₃. After 120 min of exposure, the degradation percentage of AO7 over the samples is in the order 40wt%Ag₃PO₄/BiFeO₃ (~ 91%) > 20wt%Ag₃PO₄/BiFeO₃ (~ 87%) > 10wt%Ag₃PO₄/BiFeO₃ (~ 69%) > 5wt%Ag₃PO₄/BiFeO₃ (~ 46%) > BiFeO₃ (~ 27%). It is found that the photocatalytic performance of the composites exhibit an increasing trend with the increase of Ag₃PO₄ content. Among these composites, the photocatalytic efficiency of 40wt%Ag₃PO₄/BiFeO₃ is very close to that of 20wt%Ag₃PO₄/BiFeO₃. Thus, in the present study, the most appropriate mass ratio of Ag₃PO₄ can be considered as 20% in the composites. Moreover, it is worth noting that the mechanical mixture sample 20wt%Ag₃PO₄/BiFeO₃-M exhibits much lower photocatalytic activity than 20wt%Ag₃PO₄/BiFeO₃. This reveals that the construction of heterojunction between BiFeO₃ and Ag₃PO₄ is necessary for the enhancement of photocatalytic activity. Moreover, compared with BiFeO₃/a-Fe₂O₃ and BiFeO₃-Bi₂WO₆ composites [26, 29], the Ag₃PO₄/BiFeO₃ heterojunction composites prepared in the present study manifest a higher photocatalytic activity toward the dye degradation.

Fig. 5.

Photocatalytic activities of BiFeO₃ and Ag₃PO₄/BiFeO₃ composites toward the degradation of AO7 under visible-light irradiation, along with the blank and absorption experiment results

To further confirm the photocatalytic property of the composites, the photocatalytic degradation of colorless phenol over 20wt%Ag₃PO₄/BiFeO₃ and BiFeO₃ under visible-light irradiation was also investigated. As shown in Fig. 6, the self-degradation and absorption of phenol can be neglected based on the results of blank and absorption experiments. It can be seen that just ~ 9% of phenol is degraded catalyzed by BiFeO₃ after 120 min of exposure. Whereas, when 20wt%Ag₃PO₄/BiFeO₃ is used as the photocatalyst, the degradation percentage of phenol can be obviously enhanced under the same conditions. The result suggests that the degradation of the dye on the visible-light-irradiated Ag₃PO₄/BiFeO₃ composites is attributed to their intrinsical photocatalytic activity instead of dye sensitization.

Fig. 6.

Photocatalytic degradation of phenol over BiFeO₃ and 20wt%Ag₃PO₄/BiFeO₃ sample under visible-light irradiation, along with the blank and absorption experiment result

To evaluate the reusability of the photocatalysts, the recycling photocatalytic degradation experiments of AO7 over 20wt%Ag₃PO₄/BiFeO₃ and Ag₃PO₄ were carried out under the same photocatalytic conditions. As shown in Fig. 7, after three successive recycling runs, the composite still exhibits relatively high photocatalytic activity, while the degradation efficiency over Ag₃PO₄ undergoes an obvious decrease. Figure 8a, b shows the TEM image and XRD pattern of the composite after cycling experiment, respectively. It is clear that Ag₃PO₄ microparticles are still assembled on the surface of BiFeO₃ microcuboids without destruction of the heterostructures, and no obvious crystal structure change is observed. This suggests that Ag₃PO₄/BiFeO₃ p-n heterojunction composites possess good photocatalytic reusability.

Fig. 7.

Photocatalytic degradation of AO7 over Ag₃PO₄ and 20wt%Ag₃PO₄/BiFeO₃ sample during three cycles

Fig. 8.

a TEM image and b XRD pattern of 20wt%Ag₃PO₄/BiFeO₃ sample after cycling photocatalytic experiments

Photo-Fenton-like Catalytic Activity

Apart from its photocatalytic property, BiFeO₃ also exhibits prominent photo-Fenton-like catalysis ability [54–56]. Figure 9 shows the photo-Fenton-like degradation of AO7 over 20wt%Ag₃PO₄/BiFeO₃ and BiFeO₃ in the presence of H₂O₂. Compared with the reaction systems without H₂O₂, the introduction of H₂O₂ remarkably enhances the degradation percentage of the dye. This result is mainly due to the photo-Fenton-like reaction mechanism. In the presence of visible-light irradiation and H₂O₂, Fe³⁺ on the surface of BiFeO₃ can be converted to Fe²⁺ with the generation of hydroxyl (•OH) radicals (Eq. 1). Consequently, Fe²⁺ can react with H₂O₂ to produce Fe³⁺ and •OH (Eq. 2). During the above cycle reaction, more •OH is produced, which is generally considered to be a primary active species for the dye degradation (as evidenced by active species trapping experiment given in Fig. 11). In the case of bare BiFeO₃, the high recombination rate of the photogenerated charges limits the yield of photogenerated electrons, which tends to suppress the reduction of Fe³⁺ into Fe²⁺(Eq. 3). This leads to the limited enhancement of degradation percentage. For Ag₃PO₄/BiFeO₃ composites, photogenerated electrons and holes can be efficiently separated, and thus, more photogenerated electrons are available for promoting the quick conversion from Fe³⁺ into Fe²⁺ (Eq. 3) [57]. Benefitting from this electron reduction, the photo-Fenton process for the composites is more efficient than that for bare BiFeO₃. As a result, Ag₃PO₄/BiFeO₃ p-n heterojunction composites manifest much enhanced photo-Fenton performance.

$${\mathrm{Fe}}^{3+}+{\mathrm{H}}_2\mathrm{O}+\mathrm{h\nu }\to {\mathrm{Fe}}^{2+}+•\mathrm{OH}+{\mathrm{H}}^{+}$$ 1

$${\mathrm{Fe}}^{2+}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{Fe}}^{3+}+•\mathrm{OH}+{\mathrm{OH}}^{-}$$ 2

$${\mathrm{Fe}}^{3+}+{\mathrm{e}}^{-}\to {\mathrm{Fe}}^{2+}$$ 3

Fig. 9.

Photocatalytic activities of BiFeO₃ and 20wt%Ag₃PO₄/BiFeO₃ sample toward the degradation of AO7 under visible-light irradiation in the presence of H₂O₂

Fig. 11.

Effects of ethanol, EDTA, and BQ on the degradation percentage of AO7 over 20wt%Ag₃PO₄/BiFeO₃ composite

Photogenerated Charge Performance

To evaluate the separation behavior of photogenerated charges of the samples, transient photocurrent responses, ESI spectra, and PL spectra of BiFeO₃ and 20wt%Ag₃PO₄/BiFeO₃ were measured. Figure 10a shows the photocurrent-time (I-t) curves of the photocatalysts under intermittent visible-light irradiation with several on-off cycles. It can be seen that the photocurrent value of the composite is much higher than that of bare BiFeO₃, indicating that the construction of Ag₃PO₄/BiFeO₃ p-n heterojunctions is beneficial to inhibit the recombination of photogenerated electrons and holes. Figure 10b presents the ESI spectra of the samples. One can see that the composite exhibits smaller impedance arc radii compared with BiFeO₃, which suggests the lower charge transfer resistance of the composite. These results reveal that the separation and migration of the photogenerated charges can be improved in the composite, thus providing more photoinduced holes and electrons for the photocatalysis. Figure 10c shows the Mott-Schottky plot at frequency of 3000 Hz for Ag₃PO₄. The negative slope of the plot indicates that Ag₃PO₄ is a p-type semiconductor, which is consistent with the report [43]. The PL spectra of BiFeO₃ and 20wt%Ag₃PO₄/BiFeO₃ are shown in Fig. 10d. The two samples exhibit obvious emission peaks at ~ 522 nm, which are mainly attributed to the recombination of the photogenerated electron/hole pairs. It is worth noting that the PL intensity of the composite is much smaller than that of bare BiFeO₃. This further confirms that the construction of Ag₃PO₄/BiFeO₃ heterojunction promotes the separation of photoinduced charges.

Fig. 10.

a Transient photocurrent response and b EIS spectra of BiFeO₃ and 20wt%Ag₃PO₄/BiFeO₃ sample. c Mott-Schottky plot of Ag₃PO₄. d PL spectra of BiFeO₃ and 20wt%Ag₃PO₄/BiFeO₃ samples

Active Species Trapping

It is well known that photogenerated hole (h⁺), hydroxyl (•OH), and superoxide (•O²⁻) are considered to the main active species responsible for the photocatalytic degradation of dye. In order to clarify the role of the active species in the present photocatalytic system, the active species trapping experiments were carried out, as shown in Fig. 11. It can be seen that the degradation percentage of AO7 undergoes an obvious decrease after the introduction of ethanol (scavenger of •OH, 10% by volume) or ethylene diamine tetraacetic acid (EDTA, scavenger of h⁺, 2 mM). This indicates that •OH and h⁺ are the major active species involved in the photocatalytic reaction. After the addition of benzoquinone (BQ, scavenger of •O²⁻, 1 mM), a slight decrease of degradation percentage is detected, suggesting that •O²⁻ plays a relatively minor role in the dye degradation.

Proposed Photocatalytic Mechanism

It is well known that the redox ability and migration of photogenerated charges are highly related to the energy-band potentials of photocatalysts. The valence band (VB) and conduction band (CB) of BiFeO₃ and Ag₃PO₄ can be obtained using the following equation [58, 59]:

$$E_{\mathrm{VB}}=X-E^{\mathrm{e}}+0.5E_{\mathrm{g}}$$ 4

$$E_{\mathrm{CB}}=X-E^{\mathrm{e}}-0.5E_{\mathrm{g}}$$ 5

X is the absolute electronegativity of semiconductor (calculated as the arithmetic mean of the electron affinity and the first ionization of the constituent atoms). E^e is the energy of free electrons on the hydrogen scale (~ 4.5 eV). The X values of BiFeO₃ and Ag₃PO₄ are estimated to be 5.93 and 5.98 eV, respectively [43, 60]. Based on Eqs. (4) and (5), the CB/VB potentials of BiFeO₃ and Ag₃PO₄ are calculated to be 0.34/2.52 V and 0.31/2.66 V vs. NHE, respectively. The energy-band potential diagram of the two photocatalysts is shown in Fig. 12a. It is reported that BiFeO₃ is an n-type semiconductor and its Fermi level lies close to the CB [48]. Ag₃PO₄ is demonstrated to be a p-type semiconductor (see Fig. 10c), whose Fermi energy level is close to the VB [43].When BiFeO₃ is combined with Ag₃PO₄ to form p-n heterojunction (see Fig. 12b), the diffusion of electrons and holes between the two photocatalysts will build an internal electric field at the interface region of the p-n heterojunction with direction from BiFeO₃ to Ag₃PO₄. Simultaneously, the energy-band potential of BiFeO₃ tends to move down along with its Fermi level whereas that of Ag₃PO₄ tends to raise up accompanied by its Fermi level until an equilibrium state of Fermi level of the two photocatalysts is achieved. Upon visible-light irradiation, both BiFeO₃ and Ag₃PO₄ can be excited to generate photoinduced electron and hole pairs. Under the promotion of the internal electric field, the photogenerated electrons in the CB of Ag₃PO₄ will migrate to the CB of BiFeO₃, while the photogenerated holes will transfer from the VB of BiFeO₃ to that of Ag₃PO₄. As a result, the recombination of photogenerated charges can be effectively inhibited, as evidenced by the photocurrent and PL analysis (see Fig. 10a, d). Thus, more photogenerated electrons and holes can participate in the photocatalytic redox reaction, leading to the enhancement of the photocatalytic activity for the Ag₃PO₄/BiFeO₃ p-n heterojunction composites.

Fig. 12.

Schematic illustration of proposed photocatalytic mechanism for Ag₃PO₄/BiFeO₃ composite. a Before contact. b After contact

Conclusions

Ag₃PO₄/BiFeO₃ p-n heterojunction composites were synthesized through the decoration of Ag₃PO₄ spherical-like microparticles on the surface of BiFeO₃ microcuboids. Compared with bare BiFeO₃, the as-obtained composites exhibit enhanced visible-light photocatalytic activity for the degradation of AO7 and phenol. Moreover, the composites are demonstrated to be excellent photo-Fenton-like catalysts. The improved photocatalytic activity of the composites is mainly attributed to the efficient separation of photogenerated electrons and holes owing to the formation of the p-n heterojunction between BiFeO₃ and Ag₃PO_4.

Abbreviations

AO7

Acid orange 7

CB

Conduction band

DRS

UV-vis diffuse reflectance spectra

EDX

Energy dispersive X-ray

E_g

Bandgap energy

I-t

Photocurrent-time

NMP

1-Methyl-2-pyrrolidione

PVDF

Polyvinylidene fluoride

R

Reflectance

SEM

Scanning electron microscope

TEM

Transmission electron microscope

VB

Valence band

XPS

X-ray photoelectron spectroscopy

XRD

X-ray diffractometer

Authors’ Contributions

HY and LD conceived the idea of experiments. LD and TX carried out the experiments. LD, HY, TX, and XC participated in the discussion and analysis of the experimental result. LD wrote the manuscript. HY and XC improved the manuscript. All authors read and approved the final manuscript.

Authors’ Information

HY is a professor and a Ph.D. degree holder specializing in the investigation of photocatalytic and nanometer materials. XC is a professor and a Ph.D. degree holder specializing in the investigation of functional materials. LD is a doctoral candidate major in the preparation of photocatalytic materials. TX is a Ph.D. degree holder major in the investigation of photocatalytic materials.

Competing Interests

The authors declare that they have no competing interests and the mentioned received funding in our manuscript does not lead to any conflict of interests regarding the publication of this work.

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