Enhanced Photocatalytic Degradation Activity of BiFeO₃ Microspheres by Decoration with g-C₃N₄ Nanoparticles

Abstract

In this work, the g-C₃N₄ nanoparticles decorated BiFeO₃ microspheres composites (g-C₃N₄/BiFeO₃) were successfully synthesized by hydrothermal treatment of g-C₃N₄ nanoparticles together with BiFeO₃ microspheres. The SEM and HRTEM observation indicate that the C₃N₄ nanoparticles with size of 30-50 nm are well decorated on the surface of BiFeO₃ microspheres. The photocatalytic activities of the samples are investigated by the degradation of methylene blue (MB) under the irradiation of simulated sunlight. The as-prepared g-C₃N₄/BiFeO₃ composites exhibit remarkable enhanced photocatalytic activity compared with bare BiFeO₃. More importantly, the photocataltic performance of the composites is further confirmed by the degradation of colorless phenol. Furthermore, the favorable catalytic stability of composites is demonstrated through the recycling photocatalytic experiment. The enhanced photocatalytic activity of g-C₃N₄/BiFeO₃ composites is mainly attributed to the separation of the photogenerated electron-hole pairs, resulting from the migration of the photoinduced charge between g-C₃N₄ nanoparticles and BiFeO₃. A possible photocatalytic mechanism for dye degradation over g-C₃N₄/BiFeO₃ composite is proposed based on the active species trapping experiment, revealing that the photogenerated hole (h⁺) and hydrogen peroxide (H₂O₂) are regarded as the major active species for the decomposition of dye, while hydroxyl radicals (•OH) plays a minor role in the photocatalytic reaction.

Keywords:
BiFeO₃; g-C₃N₄ nanoparticles; composite; photocatalysis

1. Introduction

Semiconductor based photocatalysis has attracted considerable attentions owing to its promising application in solar energy conversion and environmental purification^1-6. Perovskite oxide has been proven to be an important class of photocatalysts⁷. As one of the perovskite photocatalysts, BiFeO₃ exhibits visible-light-driven photocatalytic activity for the degradation of dyes and water splitting due to its narrow bandgap and chemical stability^8-11. In addition to its photocatalytic property, BiFeO₃ has been also known to be a famous room-temperature multiferroic material¹². However, the photocatalytic performance of bare BiFeO₃ is not satisfactory owing to high recombination rate of photogenerated electron-hole pairs, and this shortage limits its practical applications in the field of photocatalysis. Therefore, many strategies have been employed to suppress the recombination of photoinduced charges^13-19. Among these methods, coupling BiFeO₃ with narrow-bandgap semiconductor of suitable band potential including BiVO₄, Ag/AgCl, Fe₂O₃ and Bi₂Fe₄O₉ is demonstrated to be an efficient way to promote photogenerated charges migration and separation, and then improve its photocatalytic activity^16-19.

Recently, A metal-free organic semiconductor photocatalysts, graphite-like carbon nitride (g-C₃N₄) with small bandgap of 2.7 eV, has been found to be a promising candidate for photocatalytic applications because of its narrow bandgap, non-toxicity, low cost and excellent chemical stability^20,21. Particularly, it can be easily obtained by the heat treatment of nitrogen-rich precursors at relatively low temperature^20,21. More importantly, g-C₃N₄ has been frequently employed as an ideal narrow-bandgap cocatalyst to combine with appropriate photocatalysts to form heterostructured composites with enhanced photocatalytic performance^22,23. It is worth noting that energy-band potentials of g-C₃N₄ and BiFeO₃ are well-matched^24,25. This suggests that the composites constructed by the two photocatalysts are expected to exhibit favorable photocatalytic performance. Wang et al. firstly reported that the bulk g-C₃N₄/BiFeO₃ composite can achieve the efficient separation of photoinduced charges²⁴. It is generally accepted that the photocatalytic performance of g-C₃N₄/metal oxides composites strongly related to the morphology and size of g-C₃N₄²³. Especially for the nanoscale g-C₃N₄ (such as nanosheets and nanoparticles), these materials possess large specific surface area, and thus provide more available surface active sites for the photocatalytic reaction and larger contact area for photogenerated charges transformation^23,26. Until now, g-C₃N₄ nanosheets have been successfully combined with BiFeO₃ to form g-C₃N₄ nanosheets/BiFeO₃ composites, and the composites exhibit enhanced photocatalytic activity towards the dye degradation²⁵. To the best of our knowledge, however, the enhanced photocatalytic activity of BiFeO₃ by decorated with g-C₃N₄ nanoparticles is rarely reported.

In this work, the g-C₃N₄ nanoparticles decorated BiFeO₃ microspheres (g-C₃N₄/BiFeO₃) composites with enhanced photocatalytic activity were prepared. The methylene blue (MB) and colorless phenol were selected as model pollutant to evaluate the photocatalyic properties of the composites under simulated sunlight irradiation. The promotion mechanism of g-C₃N₄ nanoparticles on the photocatalytic performance of BiFeO₃ microspheres was investigated in detail.

2. Experimental Section

2.1 Chemicals

All raw materials were purchased from Sinopharm Chemical Reagent Co., Ltd, which were analytical reagents and used without any purification.

2.2 Synthesis of g-C₃N₄ nanoparticles

The bulk g-C₃N₄ was prepared by directly heating melamine in a semiclosed system²⁷. Briefly, about 5 g of melamine was put into a corundum boat with a cover and calcinated at 520 ºC for 4 h. After the reaction, the product was ground and collected for further use.

The g-C₃N₄ nanoparticles were synthesized via the treatment of bulk g-C₃N₄ in H₂SO₄. 1 g of bulk g-C₃N₄ was added into 50 mL of H₂SO₄ (98 wt %) and magnetically stirred for 10 h. Subsequently, the mixture was ultrasonicated for 10 h. During the ultrasonic process, the color of the suspension was changed from yellow to slight yellow. After that, the as-prepared suspension was placed for 12 h to obtain the flocculent precipitate, and the absorbance was removed. The obtained precipitate was washed several times with distilled water to near-neutral pH, and then dried at 60ºC for 12 h to obtain final g-C₃N₄ nanoparticles.

2.3 Synthesis of g-C₃N₄ nanoparticles/BiFeO₃ composites

The BiFeO₃ microspheres were fabricated according to the hydrothermal route described in the literature²⁸. Then the g-C₃N₄/BiFeO₃ composites were prepared as follows: a certain amount of g-C₃N₄ nanoparticles were added into 40 ml of distilled water and then ultrasound for 2 h. Subsequently, BiFeO₃ microspheres were dispersed into the above suspension under magnetically stirring. After the suspension was uniform, this mixture was transferred into a 50 ml Te flon-lined stainless steel autoclave and heated at 130ºC for 2 h. Finally, the obtained precipitate were collected by centrifugation and dried at 70ºC for 10 h to get the composites. To study the effect of the g-C₃N₄ nanoparticles content on the photocatalytic activity of the composites, a series of sample were prepared with different g-C₃N₄ mass ratios of 4 %, 8 %, 16 % and 24 % (m_g-C3N4/(m_g-C3N4+m_BiFeO3)) and the corresponding samples were named as 4CN/BFO, 8CN/BFO, 16CN/BFO and 24CN/BFO, respectively.

For comparison, sample 16CN/BFO was synthesized under the same condition mentioned above except that the g-C₃N₄ nanoparticles were replaced by bulk g-C₃N₄, and this sample is termed as 16bulkCN/BFO. On the other hand, the sample denoted M16CN/BFO was fabricated by direct mixing of BiFeO₃ microspheres and g-C₃N₄ nanoparticles (m_g-C3N4/(m_g-C3N4+m_BiFeO3) =16 %).

2.4 Characterization

The phase purity of the samples was detected by X-ray diffractometer (Bruker D8 Advance XRD). The field mission scanning electron microscope (JEOL JSM-6701F FE-SEM) and transmission electron microscope (JEOL JEM-2010 TEM) was employed to observe the morphology and structure of the products. Fourier transform infrared spectroscopy (FTIR) analysis was performed on a Bruker IFS 66v/S spectrometer. A UV-visible spectrophotometer with an integrating sphere attachment (PERSEE TU 1901) was used to measure the diffuse reflectance spectra of the samples.

2.5 Photocatalytic activity test

The photocatalytic activities of samples were evaluated by the degradation of MB and phenol under the simulated sunlight irradiation of a 300 W xenon l amp. In a typical experiment, the sample (0.1 g) was added into the solution of MB or phenol (200 mL, 5 mg/L). Before the photocatalytic reaction, the mixture was magnetically stirred in dark for 0.5 h to reach the adsorption-desorption equilibrium of organic molecule on the surface of catalysts. Under the simulated sunlight irradiation, a small amount of reaction solution was taken at a given time intervals and centrifuged at 6000 rpm/min for 10 min to separate the photocatalysts. During the photocatalytic reaction, the xenon lamp was cooled with water-cooling system to keep the solution at room temperature (~ 20ºC). The concentrations of MB and phenol were measured by detecting the absorbance of the supernatant at a given wavelength ((λ_AO7 = 665 nm and λ_phenol = 270 nm) on an UV-visible spectrophotometer. In order to evaluate the photocatalytic reusability of the samples, the recycling catalytic experiments toward the degradation of MB were carried out under the same conditions. After the first catalytic experiment, the photocatalyst in the reaction solution was recovered by centrifugation (6000 rpm/min, 10 min), washing with distilled water and drying in an oven (70ºC, 10 h). The collected photocatalyst was dispersed into the fresh MB solution for the next cycle of the catalytic experiment. In the 1st run, the photocatalysts was 100% used. The recovery percentage of photocatalysts for 2nd, 3rd, 4th and 5th run was ~ 97 %, ~ 93 %, ~ 87 % and ~ 82 %, respectively.

To detect the active species formed in this photocatalytic reaction, ammonium oxalate (AO, 2mM) and ethanol (10 % by volume) were introduced aiming to trap photogenerated hole (h⁺) and hydroxyl radicals (•OH), respectively^29,30. Furthermore, the N₂ purging experiment (0.1 L/min) was also performed to expel the effect superoxide radicals (^•O₂) and/or hydrogen peroxide (H₂O₂) on the photocatalytic degradation. The active species trapping experiments procedure were similar to the above photocatalytic experiment process.

3. Results and Discussion

3.1 Morphology and structure characterization

Fig.1 shows the XRD patterns of bulk g-C₃N₄, g-C₃N₄ nanoparticles, bare BiFeO₃ and g-C₃N₄/BiFeO₃ samples. For bulk g-C₃N₄, the peaks at 13.0° and 27.4° can be indexed to (100) and (002) crystal planes of the graphite-like carbon nitride, respectively. After the H₂SO₄ treatment, it is seen that the (100) peak of g-C₃N₄ nanoparticles almost disappears which is mainly attributed to the decreased planar size of the g-C₃N₄ layers. Similar phenomenon is also found in the g-C₃N₄ nanosheets prepared though the H₂SO₄ exfoliation of bulk g-C₃N₄²⁶. In the XRD pattern of BiFeO₃, all the diffraction peaks are consistent with the standard diffraction data of rhombohedral structured BiFeO₃ (PDF card 74-2016), indicating the preparation of high-quality BiFeO₃. After BiFeO₃ composing with g-C₃N₄, only the characteristic diffraction peaks of BiFeO₃ are observed in the XRD pattern of g-C₃N₄/BiFeO₃ samples, and no traces of g-C₃N₄ are visible. This is possibly due to the low weight loading of g-C₃N₄ and weak diffraction intensity of g-C₃N₄ nanoparticles. Moreover, the impurity peaks cannot be detected in the XRD pattern of composite, suggesting that no any byproducts are generated during the fabrication of the composites.

Figure 1
XRD patterns of bulk g-C₃N₄, g-C₃N₄ nanoparticles, BiFeO₃ and g-C₃N₄/BiFeO₃ composites.

The chemical functional groups and composition of bulk g-C₃N_4, g-C₃N₄ nanoparticles, bare BiFeO₃ and g-C₃N₄/BiFeO₃ composites were further investigated by FTIR spectrum, as shown in Fig. 2. Most of absorption peaks in the bulk g-C₃N₄ and g-C₃N₄ nanoparticles are the same, indicating that the two g-C₃N₄ samples have similar chemical structure. The sharp absorption peak at about 808 cm^-1 belongs to the bending vibration of C-N heterocycles. Several adsorption peaks at 1200-1700 cm^-1 are assigned to the stretching vibration of C-N. The typical adsorption peak for the C=N stretching vibration is detected at ~1644 cm^-1. As to the spectrum of bare BiFeO_3, the obvious absorption peaks located at around 440 and 560 cm^-1 are attributed to the Fe-O stretching and bending vibrations, respectively³¹. For the g-C₃N₄/BiFeO₃ composites, all the characteristic adsorption peaks of g-C₃N₄ nanoparticles and BiFeO₃ can be observed. This suggests that the as-prepared composites are composed of g-C₃N₄ nanoparticles and BiFeO₃. Further observation finds that compared with the absorption peak of C=N stretching vibration (1644 cm^-1) for g-C₃N₄ nanoparticles, a slight shift to higher wavenumber in g-C₃N₄/BiFeO₃ composites is detected , indicating the interaction between the two materials.

Figure 2
FTIR spectra of bulk g-C₃N₄, g-C₃N₄ nanoparticles, BiFeO₃ and g-C₃N₄/BiFeO₃ composites.

Fig.3 shows the SEM and HRTEM images of bulk g-C₃N_4, g-C₃N₄ nanoparticles, bare BiFeO₃ and 16CN/BFO samples. It can be seen that bulk g-C₃N₄ exhibits an aggregated layered structure about several micrometers in size (Fig. 3(a)). After H₂SO₄ treatment, the bulk g-C₃N₄ becomes irregular nanoparticles with 30-50 nm in particle size (Fig. 3 (b)). As shown in Fig. 3(c) and (d), the bare BiFeO₃ exhibits regular sphere-like shape with diameter about 30-40 micrometers. The magnified SEM image (Fig. 3(e)) taken from a part of BiFeO₃ microsphere (Fig. 3(d)) shows that the single microsphere is composed of numerous particles with a size of several hundred nanometers and possesses a smooth and clean surface. The SEM image of 16CN/BFO sample obviously displays that the BiFeO₃ microsphere undergo no morphology change in the composite (Fig. 3(f)). The detail view of the sample shows that the g-C₃N₄ nanoparticles are uniformly attached on the surface of BiFeO₃ microspheres (Fig. 3(g)). From the HRTEM image of 16CN/BFO sample (Fig. 3(h)), it is seen that the interplanar spacing of ~0.287 nm corresponds to the BiFeO₃ (110) planes and the g-C₃N₄ with low crystallinity is on the edge of BiFeO₃, indicating that the intimate contact between the two materials is formed.

Figure 3
SEM images of bulk g-C₃N₄ (a), g-C₃N₄ nanoparticles (b), bare BiFeO₃ (c, d and e) and 16CN/BFO sample (f and g); (h) HRTEM image of 16CN/BFO sample.

3.2 Optical absorption properties

To study the optical absorption properties of samples, the UV-visible diffuse reflectance spectra of samples are recorded (Fig.4 (a)). Moreover, the corresponding first derivative spectra of the reflectance (R) with respect to wavelength (λ) are also obtained (Fig.4 (b)), in which the peak wavelength are characterized to be the absorption edge of sample. The bandgap energy (E _g) of sample is evaluated by the formula: E _g = 1240/λ_g, where λ_g is the absorption edge of sample. For the bare g-C₃N₄ samples, one can see from the Fig.4 (b) that the light absorption edge of sample presents a blue shift from ~ 455 nm for the bulk to ~ 430 nm for the nanoparticles. According to these absorption edges, the bandgap energies of bulk g-C₃N₄ and g-C₃N₄ nanoparticles are calculated to be ~ 2.72 and ~ 2.88 eV, respectively. Compared with bulk g-C₃N₄, the enlarged bandgap of g-C₃N₄ nanoparticles is mainly attributed to the quantum confinement effect³². As seen from the Fig.4 (b), the absorption edge of BiFeO₃ is located at ~ 563 nm, and the corresponding bandgap energy is estimated to be ~ 2.2 eV. It is worth noting that the g-C₃N₄/BiFeO₃ composites exhibit almost the same absorption edge as BiFeO₃ and g-C₃N₄ nanoparticles, suggesting that BiFeO₃ and g-C₃N₄ nanoparticles undergo no obvious bandgap change in the composites. On the other hand, according to the bandgap energies of BiFeO₃ and g-C₃N₄ nanoparticles, the valence band (VB) and conduction band (CB) potentials of the two materials can be estimated using the following relation (Eq (1) and (2))³³:

(1) ${\mathit{E}}_{\mathit{VB}}=\mathit{X}-{\mathit{E}}^{\mathit{e}}+0.5{\mathit{E}}_{\mathit{g}}$

(2) ${\mathit{E}}_{\mathit{CB}}=\mathit{X}-{\mathit{E}}^{\mathit{e}}-0.5{\mathit{E}}_{\mathit{g}}$

Figure 4
(a) UV-visible diffuse re flectance spectra of bulk g-C₃N₄, g-C₃N₄ nanoparticles, BiFeO₃ and g-C₃N₄/BiFeO₃ composites. (b) the corresponding first derivative of the diffuse reflectance spectra.

In the above equation, the X and E ^e are the absolute electronegativity of materials (defined as the arithmetic mean of the electron affinity and the first ionization of the constituent atoms) and energy of free electrons on the hydrogen scale (~ 4.5 eV), respectively. The X value of BiFeO₃ is calculated to be 5.93 eV based on the data reported in literatures^34,35, and the X value for g-C₃N₄ nanoparticles is reported to be 4.72 eV³⁶. The VB potentials of BiFeO₃ and g-C₃N₄ nanoparticles are calculated to be 2.53 and 1.66 V vs. NHE, and the CB potentials of BiFeO₃ and g-C₃N₄ nanoparticles are estimated to be 0.33 and -1.22 V vs. NHE, respectively.

3.3 Photocatalytic properties

Fig.5 (a) presents the photocatalytic performance for the degradation of MB over g-C₃N₄ nanoparticles, bare BiFeO₃ and g-C₃N₄/BiFeO₃ composites under simulated sunlight irradiation. To clarify the effect of adsorption and direct photolysis of dye on the photocatalytic result, the photolysis and adsorption experimental were performed. It can be seen from Fig.5 (a) that only ~ 17 % of MB is degraded after 9 h simulated sunlight irradiation without catalyst, and the dye is slightly absorbed on the photocatalyst. When BiFeO₃ microspheres are employed as photocatalyst, about 35 % of MB is degraded with 9 h irradiation, indicating its weak photocatalystic activity. After combination with g-C₃N₄ nanoparticles, the g-C₃N₄/BiFeO₃ composites exhibit enhanced phtotcatalytic activity compared with bare BiFeO₃ under the same conditions. In the case of composites, the photocatalytic efficiency gradually increases with the increase of the content of g-C₃N₄ nanoparticles, achieving the highest degradation precentage of ~ 60 % at the g-C₃N₄ nanoparticles content of 16 %. However, further increasing the content of g-C₃N₄ nanoparticles leads to the decrease of photocatalytic efficiency. This is mainly attributed to the fact that the excessive g-C₃N₄ nanoparticles may cover the surface of BiFeO₃ and decrease the photon absorption by the BiFeO₃ microspheres, and moreover, the large amount of g-C₃N₄ nanoparticles tend to self-agglomerate, consequently making it difficult to form heterojunction in the composites. As a consequence, there exists an appropriate ratio of g-C₃N₄ nanoparticles to achieve optimal photocatalytic activity. The experimental data in Fig 5 (a) were fitted by first-order kinetic equation ln(C ₀/C _t) = k _app t, where k _app is the apparent first-order reaction rate constant (Fig 5 (b)). The reaction rate constant of samples are shown in Fig 5 (b) . It can be seen that the 16CN/BFO sample exhibits the highest reaction constant, which is almost 2.1 times as that of bare BiFeO₃.

Figure 5
(a) Photocatalytic degradation of MB as a function of irradiation time without catalyst and in the presence of g-C₃N₄ nanoparticles, BiFeO₃ and g-C₃N₄/BiFeO₃ samples. (b) Kinetic plots of the photocatalytic degradation of MB over g-C₃N₄ nanoparticles, BiFeO₃ and g-C₃N₄/BiFeO₃ samples. (c) Degradation percentage and adsorption percentage of MB using 16CN/BFO, M16CN/BFO and 16bulkCN/BFO as photocatalyst at an irradiation time of 9 h. Initial conditions: C(MB) = 5 mg/L, C(catalysts) = 5 mg/L, T = ~ 20ºC.

For comparison, the photocatalytic performances of the physical mixture sample M16CN/BFO and the bulk g-C₃N₄/BiFeO₃ composite (16bulkCN/BFO) toward the degradation of MB were also investigated, as shown in Fig.5 (c). It is seen that the photocatalytic activities of the two samples are much lower than that of 16CN/BFO sample. Compared with physical mixture, the hydrothermal treatment in this work is more likely to promote the formation of the heterojunction between g-C₃N₄ and BiFeO₃. This is beneficial to the migration of the photogenerated charge between the two materials. On the other hand, it is generally accepted that the g-C₃N₄ nanoparticles exhibit much higher surface area than bulk g-C₃N₄, thus offering more available surface active sites for the photocatalytic reaction. The above results illustrate that the nanoscaled g-C₃N₄ in the composite and strong heterojunction between g-C₃N₄ and BiFeO₃ are necessary for the efficient improvement of photocatalytic activity.

To exclude the dye sensitization effect during the photocatalytic process, phenol is used as another colorless target pollutant to test the photocatalytic activity of BiFeO₃ microspheres and 16CN/BFO sample under simulated sunlight irradiation, as shown in Fig.6 (a). The blank experiment indicates that the direct photolysis of phenol is about 5 % for 9 h without photocatalyst. Bare BiFeO₃ can only degrade ~ 13 % of phenol after 9 h irradiation. The 16CN/BFO sample exhibits improved photocatalytic activity for the degradation of phenol compared with bare BiFeO₃ and about 25 % of phenol is degraded after 9 h exposure. The photocatalytic degradation kinetics of phenol is also investigated (Fig.6 (b)), indicating that the reaction rate constant of 16CN/BFO (0.0202 h^-1) is higher than that of bare BiFeO₃ (0.0111 h^-1). The result demonstrated that the degradation of dye over g-C₃N₄/BiFeO₃ composites is caused by its intrinsical photocatalytic property rather than dye sensitization.

Figure 6
(a) Photocatalytic degradation of phenol versus irradiation time over BiFeO₃ and 16CN/BFO sample, along with blank experiment. (b) Kinetic plots of the photocatalytic degradation of phenol over BiFeO₃ and 16CN/BFO sample. Initial conditions: C(phenol) = 5 mg/L, C(catalysts) = 0.5 g/L, T = ~ 20ºC.

In addition to the catalytic activity, the recyclability of photocatalysts is another important parameter for their photocatalytic application. The recycling runs for the photocatalytic degradation of MB over 16CN/BFO sample under simulated sunlight illumination were carried out, as presented in Fig.7. It can be seen that the degradation percentage of dye still reaches ~ 53 % after five successive runs. This indicates that g-C₃N₄/BiFeO₃ composites exhibit good photocatalytic stability for the dye degradation.

Figure 7
Photocatalytic recyclability of 16CN/BFO sample for the degradation of MB under simulated sunlight irradiation. Initial conditions: C(MB) = 5 mg/L, T = ~ 20ºC.

3.4 Photocatalytic mechanism

In order to clarify the photocatalytic degradation mechanism, the active species trapping experiments were carried out. As shown in Fig. 8, the degradation percentage of MB decreases obviously by the addition of AO (a scavenger of h⁺), indicating that the photogenerated h⁺ plays an important role in this photocatalytic reaction. Nevertheless, the photocatalytic degradation of dye is slightly inhibited with the introduction of ethanol (a scavenger of •OH), implying relatively minor role of •OH responsible for the dye degradation. Besides h⁺ and •OH, ^•O₂ and H₂O₂ generated from the reaction between photogenerated electrons and O₂ are considered to be another active species for dye degradation. It is generally accepted that the O₂ molecules dissolved in reaction solution can be expelled by the N₂ purging, and then inhibits the generation of ^•O₂ and H₂O₂. Upon bubbling with N₂, a drastic decrease of the degradation percentage is observed, suggesting that ^•O₂ and/or H₂O₂ is the major active species in the photocatalysis.

Figure 8
Effect of N₂, ammonium oxalate (AO) and ethanol on the photocatalytic degradation of MB over 16CN/BFO sample. Initial conditions: C(MB) = 5 mg/L, C(catalysts) = 0.5 g/L, C(AO) =2×10^-3 mol/L, C(ethanol) =10 % by volume, N₂ flow rate = 0.1 L/min, T = ~ 20ºC.

Based on the above experimental results, a possible photocatalytic mechanism of g-C₃N₄/BiFeO₃ composite for the degradation of MB is proposed, as displayed in Fig. 9. When the composite is irradiated by simulated sunlight, the g-C₃N₄ nanoparticles and BiFeO₃ are excited and generated photoinduced electron-hole pairs (Eq (3) and (4)). As mentioned above, the CB (-1.22 V) and VB (1.66 V) potentials of g-C₃N₄ are negative to the CB (0.33 V) and VB (2.53 V) potentials of BiFeO₃, respectively, suggesting that the two photocatalysts exhibit matchable overlapping band-structures. Therefore, the photoinduced electrons in the CB of g-C₃N₄ can facilely migrate to the CB of BiFeO₃, meanwhile the photogeneated holes are prone to transfer from the VB of BiFeO₃ to that of g-C₃N₄. Due to the above charge transfer process, the recombination of the photogenerated electrons-holes pairs is inhibited to a large extent, leaving more photogenerated charges to form active species, which is beneficial to improving the photocatalytic activity. It is well known that the photogenerated holes in the VB of g-C₃N₄ possess strong ability to oxidize dyes directly. However, because the VB potential of g-C₃N₄ is negative to the redox potentials of H₂O/•OH (+2.72 V vs. NHE) and OH^-/•OH (+1.89 V vs. NHE) ³⁷, photogenerated holes cannot oxidize OH^- or H₂O to form •OH radicals. On the other hand, the CB potential of BiFeO₃ is more positive than the redox potentials of O₂/^•O₂ (-0.13 V vs. NHE) but more negative than that of O₂/H₂O₂ (0.68 V vs. NHE) ³⁸, indicating that the photogenerated electrons in the CB of BiFeO₃ cannot reduce O₂ to generate ^•O₂ but can react with O₂ and H⁺ to produce H₂O₂ (Eq (5)). The analysis result confirms the role of H₂O₂ for the dye degradation. This is why N₂ purging exhibit obvious inhibiting effect on the photocatalytic efficiency (Fig.8). Furthermore, the H₂O₂ can also take part in a series of reactions to generate •OH radicals (Eq (6)), which is regarded to be the major route for the production of •OH radicals in this photocatalytic process. As a consequence, the photocatalytic degradation of MB over simulated sunlight irradiated g-C₃N₄/BiFeO₃ composite is mainly caused by the attack of h⁺, •OH and H₂O₂ (Eq (7)). This is consistent with the results of active species trapping experiment.

(3) ${\mathit{BiFeO}}_3+\mathit{hv}\to {\mathit{BiFeO}}_3\left(e^{-}+h^{+}\right)$

(4) ${\mathit{C}}_3{\mathit{N}}_4+\mathit{hv}\to {\mathit{C}}_3{\mathit{N}}_4\left({\mathit{e}}^{-}+h^{+}\right)$

(5) ${\mathit{O}}_2+2{\mathit{H}}^{+}+2e^{-}\to {\mathit{H}}_2{\mathit{O}}_2$

(6) ${\mathit{H}}_2{\mathit{O}}_2+\mathit{hv}\to 2•\mathit{OH}$

(7) $h^{+},H_{\mathit{2}}{O}_{\mathit{2}}\mathrm{or}•\mathrm{OH}+\mathrm{dye}\to \mathrm{deg}\mathrm{radation}\mathrm{products}$

Figure 9
A proposed photocatalytic mechanism of g-C₃N₄/BiFeO₃ composite for the degradation of MB under simulated sunlight irradiation.

4. Conclusion

The g-C₃N₄ nanoparticles were decorated on the BiFeO₃ microspheres through hydrothermal method to obtain g-C₃N₄ nanoparticles/BiFeO₃ composites. The as-prepared g-C₃N₄/BiFeO₃ composites exhibit much higher photocatalytic activity towards the degradation of MB under simulated sunlight irradiation than bare BiFeO₃. Furthermore, the intrinsical photocatalytic property of composite is demonstrated by the degradation of colorless phenol under the same condition. In addition, the composite possess good photocatalytic stability. The enhancement of the photocatalytic activity for the g-C₃N₄/BiFeO₃ composite is mainly ascribe to the photoinduced electron-hole pairs separation derived from the charge migration between g-C₃N₄ and BiFeO₃. The investigation of phtotcatalytic mechanism of g-C₃N₄/BiFeO₃ composite suggests that the main active species responsible for dye degradation are the h⁺ and H₂O₂, while •OH plays a minor role in the photocatalytic reaction.

5. Acknowledgments

This work was supported by the National Natural Science Foundation of China (Grant No. 51662027, 51602170), the Natural Science Foundation of Qinghai, China (Grant No.2016-ZJ-954Q), “ChunHui” Program of Ministry of Education of China (Grant No. Z2016074, Z2016075) and the Youth Science Foundation of Qinghai Normal University (15ZR07)

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Publication Dates

• Publication in this collection
  26 July 2018
• Date of issue
  2018

History

• Received
  02 Feb 2018
• Reviewed
  18 May 2018
• Accepted
  04 July 2018