Low-temperature fabrication of Bi₂₅FeO₄₀/rGO nanocomposites with efficient photocatalytic performance under visible light irradiation

Abstract

Bismuth ferrite/reduced graphene oxide (Bi₂₅FeO₄₀/rGO) nanocomposites have been synthesized by a hydrothermal method, followed by a simple room temperature liquid phase process. Interestingly, X-ray diffraction and scanning electron microscopy analyses indicated that the presence of rGO triggered the transformation of perovskite phase BiFeO₃ to sillenite phase Bi₂₅FeO₄₀ at room temperature, while the spindle-like morphology was maintained. Under visible light irradiation, the obtained Bi₂₅FeO₄₀/rGO nanocomposites exhibit high photocatalytic performance for the degradation of methyl orange (MO), suggesting potential applications in photocatalytic and related areas. Furthermore, the function of rGO for the enhancement of photocatalytic activity and the probable mechanism were also discussed on the basis of the results.

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1. Introduction

Bismuth ferrites, a large family of transition-metal oxides, have recently attracted great interest due to their fascinating fundamental physics and extensive technical applications.^1–3 BiFeO₃ is known to be a typical perovskite-type bismuth ferrite with simultaneous ferroelectric and ferromagnetic ordering at room temperature.^4–6 With a relatively narrow band gap of ∼2.2 eV,^7,8 BiFeO₃ is also an important visible-light responsive photocatalyst for the degradation of organic pollutants and water splitting according to recent studies.^9–12 In our previous work, we successfully synthesized BiFeO₃ nanostuctures with enhanced photocatalytic activities via a facile one-pot hydrothermal approach.¹³ However, phase pure BiFeO₃ is very difficult to synthesize and a slight impurity phase Bi₂₅FeO₄₀ as a by-product is often present.^14,15 Shen et al. found that mixed phases of Bi₂₅FeO₄₀ and the perovskite type compound of BiFeO₃ were obtained in the hydrothermal process.¹⁶ Xu et al. have developed a facile hydrothermal method to prepare single-crystal BiFeO₃ microplates, where the Bi₂₅FeO₄₀ phases firstly formed at the initial stage. With the extension of reaction time, the original Bi₂₅FeO₄₀ phases can transform to rhombohedral phases gradually.¹⁷

Compared to perovskite-type BiFeO₃, there has seldom research focus on the optical properties of Bi₂₅FeO₄₀ in previous studies. Top seeded solution growth technique has been used to produce Bi₂₅FeO₄₀ single crystals for the first time and the optical absorption measurements were performed in a broad spectral (IR, VIS and UV).¹⁸ With a low band gap of 1.8 eV, Bi₂₅FeO₄₀ was thought to be a promising visible-light response material.¹⁹ As a representative sillenite-type compound, Bi₂₅FeO₄₀ also displayed photocatalytic activity for the degradation of organic dyes under visible-light irradiation.^20,21 It should be emphasized that Bi₂₅FeO₄₀ was found to be superparamagnetic,²² providing the photocatalyst with an extra advantage for feasible separation and recovery.

The combination of the photocatalyst with carbon material to design relevant nanocomposites catalyst can effectively enhance the photocatalytic activity. For example, Li et al. have successfully synthesized a novel g-C₃N₄/BiFeO₃ nanocomposite photocatalyst by a simple hydrothermal method.²³ The as-prepared 50% g-C₃N₄/BiFeO₃ composites exhibit high efficiency for the degradation of methyl orange (MO) under visible light irradiation. Besides, He et al. have successfully fabricated bismuth ferrite/graphene composites via a facile and effective two-step hydrothermal treatment which exhibit improved photo-degradation activity toward Congo red.²⁴

Graphene is the most important 2-dimensional carbon nanomaterial, which has attracted significant attention owing to the advantages of large surface area and excellent conductivity.^25–27 However, there exists relatively weak interaction and inadequate interfacial contact between the semiconductor and graphene. Notably, reduced graphene oxide (rGO) with oxygen-containing functional groups shows intimate interfacial contact and strong interaction with semiconductors. In this work, the novel Bi₂₅FeO₄₀/rGO photocatalysts were successfully synthesized via a two-step green liquid phase approach. The Bi₂₅FeO₄₀ nanoparticles with homogeneously morphology decorated on the surfaces of the rGO nanosheets. The spindle-like morphology of Bi₂₅FeO₄₀ was reported for the first time. Interestingly, at room temperature, a phase transition from rhombohedral to sillenite took place with adding rGO in the solution, while the spindle-like structures were maintained after the reaction. The phase transition at room temperature is significant for its potential applications.

2. Experimental section

2.1. Fabrication of Bi₂₅Fe₄₀/rGO nanocomposites

Bismuth nitrate pentahydrate (Bi(NO₃)₃·5H₂O) was purchased from Guangdong Xilong Chemical Co., Ltd., P. R. China. Ferric chloride hexahydrate (FeCl₃·6H₂O), sodium hydroxide (NaOH), methyl orange (MO) and polyvinylpyrrolidone K-30 (PVP) were obtained from Sinopharm Chemical Reagent Corp, P. R. China. All chemicals were used as received without further purification.

Graphene oxide (GO) was synthesized according to the method reported by Hummers and Offeman.²⁸ Precursors of BiFeO₃ were then synthesized by a modified co-precipitation method according to our previous reports.²³

The typical preparation of Bi₂₅FeO₄₀/rGO nanocomposite photocatalysts was as follows: an appropriate amount of BiFeO₃ and rGO were completely dispersed in methanol assisted by ultrasonication for 3 h, respectively. The as-prepared BiFeO₃ solution and rGO solution were mixed together and stirred in a fume hood for 24 h. After volatilization of the methanol, an opaque powder was obtained after drying at 80 °C in air. The Bi₂₅FeO₄₀/rGO photocatalysts with varied BiFeO₃ contents (10, 20, 30, 50 wt%) were synthesized, which were denoted as BG1, BG2, BG3 and BG5, respectively.

2.2. Characterization

The morphologies of Bi₂₅FeO₄₀/rGO nanocomposites were observed via scanning electron microscopy (SEM, S-4800, Hitachi; accelerating voltage = 8 kV). The obtained samples were characterized by XRD on a Bruker D8 Advance X-ray powder diffractometer with Cu Kα radiation (λ = 1.5418 Å) in the 2θ range of 20° to 60° with a step size of 0.002° and a scan speed of 0.5 second per step. The accelerating voltage and the applied current were 40 kV and 40 mA, respectively. UV-vis absorption spectra were obtained using a PerkinElmer Lambda 35 spectrophotometer. The XPS spectrum was recorded on the Thermo Scientific ESCALAB 250Xi X-ray photoelectron spectrometer. The specific surface area was determined by V-Sorb 2800P (Gold APP Instruments Corporation, China) through the BET method.

2.3. Photocatalytic tests

The photocatalytic activities of Bi₂₅FeO₄₀/rGO nanocomposites were evaluated by degradation of MO aqueous solution under visible light irradiation. A 500 W Xe lamp was used as the light source, and visible-light irradiation was realized by attaching a 420 nm cutoff filter to completely remove any radiation below 420 nm. In a typical photocatalytic test, 50 mg catalyst was dispersed in 50 mL MO aqueous solution with a concentration of 5 mg L⁻¹. Prior to irradiation, the suspensions were magnetically stirred in the dark for 1 h to ensure the establishment of an adsorption–desorption equilibrium. Then, at selected time intervals of 30 min, samples were collected and filtered to remove the photocatalyst particles by centrifugation. After that, the solution was analyzed using an ultraviolet-visible light spectrophotometer. A blank test was also carried out on an aqueous MO solution without photocatalyst under the same condition.

3. Results and discussion

3.1. Characterization of samples

The morphology of BiFeO₃ nanostructures were assessed by SEM, as depicted in Fig. 1. From Fig. 1a, it can be easily found that BiFeO₃ is comprised of many spindle-like particles with relatively uniform size distribution. The high magnification SEM images displayed in Fig. 1b showed that the BiFeO₃ spindles are consist of large amounts of small particles and the surfaces are quite rough. The width and length of those spindle-like particles are ca. 200 nm and ca. 400 nm, respectively.

Fig. 1 SEM images with different magnifications of the BiFeO₃ nanostructures.

The crystal phase of the photocatalysts is determined by XRD, as shown in Fig. 2. For BiFeO₃ without rGO added, the diffraction peaks can be indexed on the basis of a BiFeO₃ rhombohedral phase with the space group R3c (JCPDS card no. 86-1518), a slight impurity phases can be observed. Interestingly, the presence of rGO in Bi₂₅FeO₄₀/rGO nanocomposites preferentially induces the formation of the sillenite-type phase of Bi₂₅FeO₄₀ (JCPDS card no. 46-0416). Bi₂₅FeO₄₀ is an important member of the ternary metal oxides with a body-centered cubic crystal structure (space group I23 with cell parameter of about 1.018 nm).²⁹ Previous report has proved the existence of dipole moment in the Bi₂₅FeO₄₀ crystal structure, which may enhance the electron–hole separation.³⁰ Thanks to high content of acidic oxygen-containing functionality, the presence of rGO could resulted in affected acidity of the aqueous solution, which eventually suppressed the generation of iron oxide.³¹ Jiang et al. have found that sillenite-type phase Bi₂₅FeO₄₀ with a relatively low Fe content was obtained. Hence, sillenite-type phase Bi₂₅FeO₄₀ was preferentially formed.

Fig. 2 XRD patterns of BiFeO₃ and various Bi₂₅FeO₄₀/rGO nanocomposites.

After hybridization of rGO, the rhombohedral BiFeO₃ phase disappeared and transformed to the sillenite Bi₂₅FeO₄₀ phase. Interestingly, the morphology of Bi₂₅FeO₄₀ kept well. The SEM images of Bi₂₅FeO₄₀/rGO nanocomposites are displayed in Fig. 3. The spindle-like Bi₂₅FeO₄₀ nanoparticles were randomly attached to the surface of rGO nanosheets which resemble crumpled silk veil waves. When adding rGO, the agglomeration of Bi₂₅FeO₄₀ can be effectively avoided. The images also imply that more and more spindles located at the nanosheets surface while continuously increasing the amount of BiFeO₃ powders in the synthesis process. The Bi₂₅FeO₄₀ spindles maintained their highly irregular and rough surfaces, which can magnify the surface area of the catalyst resulting in higher photocatalytic activity.

Fig. 3 SEM images of the Bi₂₅FeO₄₀/rGO nanocomposites. (a) BG1, (b) BG2, (c) BG3 and (d) BG5.

Similar results are elucidated from TEM analysis, as shown in Fig. 4. The Bi₂₅FeO₄₀ spindles dispersed well in the rGO matrix which consist of corrugated and transparent sheets. The rGO nanosheets, where spindles are wrapped in, preferred to wrinkle up. It clearly showed that the Bi₂₅FeO₄₀ spindles featured a size of 300–400 nm anchored uniformly on both sides of the rGO nanosheets. Some Bi₂₅FeO₄₀ spindles were encapsulated within the rGO sheets, which can efficiently prevent the aggregation of particles. The results also revealed rGO as a rather thin microstructure, which enhanced the specific surface ratio.³² The rGO nanosheets interconnected with each other to form an open pore system, which suppressed the dissolution and agglomeration of particles, thereby promoting the stability of the composites.^33,34

Fig. 4 TEM images of the Bi₂₅FeO₄₀/rGO nanocomposites. (a) BG1, (b) BG2, (c) BG3 and (d) BG5.

In order to confirm the valence of some elements in the products, XPS analyses were carried out. Fig. 5a shows a typical wide-scan XPS spectrum, in which all peaks can be assigned to Bi, Fe, O and C elements. Fig. 5b depicts the fine XPS spectrum of Bi 4f, the peaks at 159.3 eV and 164.5 eV can be attributed to the binding energies of Bi 4f_7/2 and Bi 4f_5/2 in Bi₂₅FeO₄₀, which reveal that Bi is in the Bi³⁺ oxidation state. As shown in Fig. 5c, the Fe 2p spectra doublet consist of two peaks of Fe 2p_3/2 (710.7 eV) and Fe 2p_1/2 (724.7 eV), which corresponding to the characteristics of Fe³⁺ ions. In Fig. 5d, the peak of O 1s at 530.1 eV and 532.5 eV should be assigned to the binding energies of O in the Fe–O and Bi–O chemical bonds, respectively.

Fig. 5 The XPS spectra of (a) full survey of Bi₂₅FeO₄₀/rGO, (b) Bi 4f core levels of Bi₂₅FeO₄₀/rGO, (c) Fe 2p core levels of Bi₂₅FeO₄₀/rGO and (d) O 1s core levels of Bi₂₅FeO₄₀/rGO.

3.2. Optical and photocatalytic performance

The optical properties of the as-prepared Bi₂₅FeO₄₀/rGO nanocomposites were measured by UV-vis diffuse reflectance spectroscopy (DRS). As can be seen from Fig. 6, all the samples showed strong and broad absorption in the range of 350–600 nm, which indicated that Bi₂₅FeO₄₀/rGO nanocomposites could response to visible light. It can be seen that the absorbance peak in the visible region of BG3 samples is stronger than that of the other nanocomposites. Furthermore, the absorption spectrum of the BG3 sample shows an obvious red-shift trend compared with that of other samples.

Fig. 6 UV-vis absorption spectra of Bi₂₅FeO₄₀/rGO nanocomposites.

The photocatalytic degradation of methyl orange (MO) aqueous solution under visible light irradiation was detected to evaluate the photocatalytic performance of Bi₂₅FeO₄₀/rGO nanocomposites and the results are shown in Fig. 7. The suspension was placed in the dark for 1 h with magnetic stirring before irradiation to reach the adsorption/desorption equilibrium. It is observed that MO degradation is negligible in the absence of the photocatalyst. The photocatalytic degradation efficiencies are calculated to be 48% for BiFeO₃ after three hours of irradiation. By comparison, the degradation efficiency of Bi₂₅FeO₄₀/rGO nanocomposites is increased to 74% for BG1 and reaches the maximum value of 88% for BG3 under the same experimental conditions, which are much higher than the pure BiFeO₃ photocatalyst. The enhancement of the photocatalytic performance should be mainly ascribed to the increase of the light absorption in the Bi₂₅FeO₄₀/rGO nanocomposites for the interfacial structure,^35–37 which can reduce the recombination rate of photogenerated electrons and holes.

Fig. 7 Photocatalytic activity of different catalysts for the degradation of MO solution at room temperature.

In addition to the photocatalytic efficiency, the stability and recyclability of a photocatalyst was also very important from the point of view of its practical application. The sample of BG3 was selected to evaluate the reusability under the same reaction conditions. From Fig. 8, the BG3 photocatalyst shows a good catalytic stability after four recycles. The slight decrease could be attributed to the inescapable loss of catalyst during the recycling process. The result indicated that photocatalysts based on Bi₂₅FeO₄₀/rGO nanocomposites could be reused completely for wastewater treatment.

Fig. 8 Recyclability of the Bi₂₅FeO₄₀/rGO photocatalyst in four successive experiments for the photocatalytic degradation of MO under visible light irradiation.

Fig. 9 displays the photoluminescence (PL) spectra of the as-prepared BiFeO₃ and the Bi₂₅FeO₄₀/rGO nanocomposites excited by 568 nm. The main emission peak is centered at about 440 nm for the pure BiFeO₃ sample. Compared with BiFeO₃, the PL spectra intensity of the Bi₂₅FeO₄₀/rGO nanocomposites is significantly decreased in the same position, which indicated that the photogenerated charges recombination rate in Bi₂₅FeO₄₀/rGO composites was much lower than that in BiFeO₃ samples. That is, the photogenerated electron–hole pairs can efficiently transfer at the interface of Bi₂₅FeO₄₀/rGO composites, resulting in the highest photocatalytic activity under visible light irradiation.

Fig. 9 PL spectra of the as-prepared BiFeO₃ and the Bi₂₅FeO₄₀/rGO nanocomposites.

The enhancement of the photocatalytic activity should be mainly attributed to the increase of the light absorption and the synergistic effect in the Bi₂₅FeO₄₀/rGO nanocomposites, a proposed mechanism is discussed as shown in Fig. 10. Under visible light irradiation, Bi₂₅FeO₄₀ and rGO can be excited to create free electrons and holes. The photogenerated electrons in Bi₂₅FeO₄₀/rGO can easily move towards the surface of rGO sheets. The conjugated sp²-hybridized structure of rGO composite materials provides abundance of delocalized electrons to enhance the transport of photogenerated electrons.³⁸ Accordingly, the superoxide anion radicals are formed in rGO sheets by a reaction between photoinduced electrons and adsorbed oxygen, while the hydroxyl radicals are formed in the Bi₂₅FeO₄₀ surface by a reaction between photoinduced holes and water.³⁹ Hence, MO molecules were degraded by serials reactions with holes, superoxide anion radical and hydroxyl radical.³¹ One other important role in enhancing the photocatalytic efficiency is the surface area of catalyst. The specific surface area for BG3 was determined by BET (Brunauer–Emmett–Teller) measurements and the result showed that the specific surface area is 7.945 m² g⁻¹. The effective surface area which organic dyes are in direct contact with catalyst could increase the adsorption of holes. Bi₂₅FeO₄₀ nanoparticles are well spread over the rGO sheets with very minor agglomeration, as shown in Fig. 4. As a result, the photogenerated electrons and holes are efficiently separated between Bi₂₅FeO₄₀ and rGO, which reduces the electron–hole recombination in the hybrid composite photocatalysts.⁴⁰ Meanwhile, O₂ absorbed on the surface of rGO could capture e⁻ and form ˙O₂⁻ which then oxidized MO directly on the surface. Both of these two predominance lead to an increase in the photo-conversion efficiency of the hybridized photocatalyst.

Fig. 10 The proposed photocatalytic mechanism for MO degradation over the Bi₂₅FeO₄₀/rGO nanocomposites.

4. Conclusions

In conclusion, Bi₂₅FeO₄₀/rGO photocatalysts with varied Bi₂₅FeO₄₀ contents were synthesized by a liquid phase approach. At room temperature, a phase transition from rhombohedral BiFeO₃ to sillenite Bi₂₅FeO₄₀ took place with adding rGO in the solution, while the spindle-like structures were maintained after the reaction. The improved photo-degradation activity toward methyl orange can be mainly ascribed to the structural characteristics of sillenite type Bi₂₅FeO₄₀ and the great charge mobility of rGO, which retard the recombination of photoexcited pairs. Therefore, the Bi₂₅FeO₄₀/rGO nanocomposite is a promising photocatalytic material for environmental applications as well as water splitting.

Acknowledgements

We acknowledge the financial support from the Ministry of Education of China (No. IRT1148), Jiangsu Synergistic Innovation Center for Advanced Materials (SICAM), the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD, YX03001), the National Natural Science Foundation of China (51372119, 61377019, 61136003, 51173081, 51602161), College Postgraduate Research and Innovation Project of Jiangsu Province (KYLX_0794, KYLX15_0848), the Natural Science Foundation of Jiangsu Province (KZ0070715050), the Seed Project Funded by Introducing Talent of NJUPT (XK0070915022) and the Natural Science Foundation of NJUPT (NY214129, NY214130, NY214181, NY215176, NY215077).

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