Magnetic BiFeO₃ grafted with MWCNT hybrids as advanced photocatalysts for removing organic contamination with a high concentration

Abstract

This work focuses on a novel visible light responsive photocatalysis system for removing organic contamination in a high concentration, employing MWCNTs as a scaffold and an efficient electron relay mediator and visible light and magnetically responsive BiFeO₃. The MWCNT grafted BiFeO₃ composites are fabricated by annealing an acid-functional MWCNT and Bi–Fe complex mixture at 800 °C, which induces the in situ formation of BiFeO₃ particles that are grafted with the MWCNT matrix. The optimal BiFeO₃/MWCNT photocatalyst exhibits enhanced photocatalytic activity under visible light, ascribed to efficient charge separation and transport which greatly extends the electron life time using MWCNTs as an electron acceptor. Simultaneously, the high surface area of the BiFeO₃/MWCNT photocatalyst can act as an adsorbent to reduce organic contamination of a high concentration under an external magnetic field. The two processes composed of physical adsorption and photocatalytic degradation result in the fast removal of organic contamination during a short reaction time compared to N-doped TiO₂. This work not only provides a new strategy for the fabrication of high performance multimetallic oxide grafted MWCNT photocatalysts, but also facilitates their practical application in environmental issues.

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1. Introduction

The increasing emissions of organic contamination from the papermaking and textile industries greatly threaten human health. A sustainable, green and recyclable system is required to purify and separate these organic contaminations. In recent years, semiconductor photocatalysis, as a “green” technology, has been investigated for treating polluted water via degradation or splitting processes.^1–12 In view of solar energy utilization, researchers have been focusing on the development of visible-light-responsive photocatalysts, because ultraviolet (UV) light only accounts for about 4% whereas visible light contributes to about 43%.^13–15 Stable multimetallic oxides have attracted much attention as novel visible light responsive semiconductors for the removal of organic contamination under visible light irradiation.^16–19 Electronic structures calculated using density functional theory (DFT) show that the d orbital of the transition metals contributes the bottom of the conduction band (CB), while the top of the valence band (VB) consists of an O 2p orbital.²⁰ In particular, a perovskite-type BiFeO₃ with 2.18 eV bandgap has received considerable attention in recent decades because of its good photochemical stability and low cost.^21–23 Moreover, BiFeO₃ is a magnetic semiconductor material, meaning that it could be magnetically separable in aqueous organic contamination.^16,24 However, due to its low surface area (nanoparticle: 8.3 m² g⁻¹, bulk: 1.2 m² g⁻¹) and high ratio of recombination of electron/hole pairs, it needs to be combined with foreign materials that could offer a high surface area and electron transport ability.^25–28 For instance, Li et al. synthesized a BiFeO₃/graphene nanohybrid via a hydrothermal reaction that presented a six times enhancement in photocatalytic Congo red degradation compared to pure BiFeO₃ particles.²⁵ Dai also combined BiFeO₃ with graphene which significantly enhanced the photocatalytic performance for methyl orange degradation.²⁶ However, compared to 2D graphene, growing BiFeO₃ on 1D MWCNTs is a challenging task because BiFeO₃ photocatalysts with a relatively larger size are difficult to coat or deposit on the MWCNT surface as well. Therefore, a tailor-made method is expected to control their size, structure, and morphology.^29–31

Herein, we report one-step calcination to directly fabricate magnetic MWCNT/BiFeO₃ composites using citric acid as a complexant in 2-methoxyethanol solution. The experimental results show that the as-prepared photocatalysts can serve triply as a physical and photochemical degrader for organic molecules of high concentration under visible light. Prior to irradiation, an adsorption experiment was carried out using mechanical separation using an external magnetic field. Followed by physical absorption, photocatalytic degradation was further carried out under visible light. In a short period of 2 h, RhB as a contamination model with a concentration of 10⁻³ M can be completely removed.

2. Experimental

2.1 Materials

Bismuth nitrate (Bi(NO₃)₃·5H₂O) and iron nitrate (Fe(NO₃)₃·9H₂O) were purchased from Sigma Aldrich. 2-Methoxyethanol (C₃H₈O₂), poly(ethylene glycol), ammonia solution (NH₃·H₂O, 3 mol L⁻¹) and citric acid were obtained from Shanghai Ansin Chemical Co. Ltd, Japan. Rhodamine B (RhB) was supplied by Shanghai Yiji Dye Chemicals.

2.2 Synthesis of MWCNT/BiFeO₃ photocatalysts

Acid-functional MWCNTs. The MWCNTs were first purified and functionalized using concentrated nitric acid. Typically, 500 mg of MWCNTs were dispersed in 150 mL of concentrated nitric acid and refluxed at 140 °C for 8 h. Then, the functional MWCNTs were filtrated and rinsed with water several times until pH = 7, and were finally dried for further use at 50 °C in an oven.

Synthesis of MWCNT/BiFeO₃ photocatalysts. 1 mM Bi(NO₃)₃·5H₂O and 1 mM Fe(NO₃)₃·9H₂O were firstly dissolved in 100 mL C₃H₈O₂ under vigorous stirring. The pH of the solution was controlled to be 4.5 by slowly adding nitric acid. 1 mM citric acid as a complexant and 0.005 g poly(ethylene glycol) as a dispersant were then added to the above solution. The mixture was further stirred for 2 h at 50 °C to obtain the sol, then, different amounts of MWCNTs were added into the above sol and kept at 80 °C for 4 days to form a xerogel. The final xerogel was ground into a fine powder and calcined at t = 800 °C for 2 h in N₂. N-Doped TiO₂ was synthesized by annealing anatase TiO₂ (∼40 nm) under NH₃ conditions for 2 h.

2.3 Photoelectrochemical measurements

Photocurrent responses were carried out using a three-electrode system with a potentiostat (CH Instruments, CHI 660), and Pt as a counter electrode, and Ag@AgCl as a reference electrode in 1 M Na₂SO₄ solution at 0.5 V potential bias under a solar simulator with a Xe lamp through a UV-cutoff filter (>420 nm). The electrodes were prepared by dropping 20 μL of aqueous slurry that consisted of 50 mg of prepared sample, 5 μL of acetylacetone, and 5 μL of Nafion in 200 μL of H₂O on a cleaned ITO glass substrate and annealing at 300 °C for 2 h in air.

2.4 Analysis instruments

Diffuse reflectance UV-visible absorption spectra were carried out using a spectrophotometer (Shimadzu UV-2401PC) with an integrating sphere attachment. BaSO₄ was used as the reference. XRD patterns were recorded with a diffractometer (Riguka, Japan, RINT 2500 V) using Cu Kα radiation. A N₂ adsorption isotherm was measured at 77 K using a BEL sorp Analyzer (BEL, Japan). Raman spectra were produced using a Horiba Jobin Yvon LabRAM using a 100× objective lens with a 532 nm laser excitation. XPS analysis was obtained using an ESCALAB-220I-XL (THERMO-ELECTRON, VG Company) device. Transmission electron microscopy (TEM) observations were performed on a JEOL, JEM-2100F (Japan) electron microscope with an accelerating voltage of 200 kV. The photoluminescence of powder samples was recorded at room temperature using a fluorescence spectrometer (Shimadzu, RF-5410PC).

2.5 Photocatalytic test

For the photodegradation of organic dyes, a decomposition reaction of a 50 mL RhB (1 mM) aqueous solution was carried out. A 30 mg powdered sample was dispersed in the RhB solution under ultrasonication for 1 min, and then was separated using an external magnetic field. The remaining RhB solution was added to 30 mg of refreshed sample under the irradiation of a 300 W Xe lamp with a 420 nm filter with a distance of 100 mm from the solution in a darkness box. The samples were then withdrawn regularly from the reactor and the dispersed powder was separated using a magnet. The clean transparent solution was analyzed using UV-vis spectroscopy (Shimadzu 2550). The concentration of RhB in the solution was determined as a function of irradiation time from the absorbance region at a functional wavelength.

3. Results and discussion

Fig. 1a presents the XRD patterns of pure BiFeO₃ and BiFeO₃/MWCNT photocatalysts. It reveals that the BiFeO₃ nanoparticles are highly crystallized and exhibit a single-phase perovskite structure in both BiFeO₃ and BiFeO₃/MWCNT photocatalysts. The crystal phase of BiFeO₃ is determined to be rhombohedral,³² which is obviously different from the tetragonal structure of BiFeO₃ films.³³

Fig. 1 (a) XRD patterns of BiFeO₃ and BiFeO₃/MWCNT, and (b) Raman shift of MWCNT and BiFeO₃/MWCNT.

The crystal structure of BiFeO₃ and the presence of MWCNTs are further investigated using Raman spectra, as shown in Fig. 1b. The broad peaks below 250 cm⁻¹ can be assigned to A1 (1TO), A1 (2TO), and A1 (3TO) modes for the rhombohedral BiFeO₃ system, which is consistent with what has been previously reported.³⁴ Moreover, a larger number of bands between 500 and 600 cm⁻¹ are found, which may reflect a space group with a lower symmetry.³⁵ Furthermore, two peaks of D and G corresponding to sp³ and sp² carbon are found in BiFeO₃/MWCNT, indicating the co-existence of BiFeO₃ and MWCNTs in the BiFeO₃/MWCNT photocatalysts.

The morphology of BiFeO₃/MWCNT is examined using transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM). As shown in Fig. 2a, the diameter of the MWCNTs is around 10–20 nm. The uniform growth of BiFeO₃ can be observed distinctly in Fig. 2b, where the BiFeO₃ grows homogeneously in the MWCNT matrix. Fig. 2c and d are typical TEM images that show the detailed structure of BiFeO₃/MWCNT. It can clearly be seen that BiFeO₃ particles with an average size of larger than 100 nm are distributed in the MWCNT matrix.

Fig. 2 (a) TEM image of MWCNT, (b) FE-SEM images of BiFeO₃/MWCNT, and (c) and (d) TEM images of BiFeO₃/MWCNT.

It was well-known that a strong interface could maximize charge transport and separation, while suppressing the recombination of the electron–hole pairs in hybrids. Hence, the chemical bonding between BiFeO₃ and MWCNTs could play an important role in promoting their photocatalytic activity. The chemical bonding between BiFeO₃ particles and MWCNTs is characterized using XPS analysis. The XPS signals from the high resolution C 1s and O 1s spectra are shown in Fig. 3a and b, respectively. Curve fittings have determined the exact peak location of C 1s in C–C, C–O, C=O and COOH bonds, which originate from the primary oxidation of MWCNTs. The XPS spectrum of O 1s in the BiFeO₃/MWCNT is then deconvoluted by four peaks of COOH, C=O, Bi–O, and Fe–O bonds, as shown in Fig. 3b. The two binding energies in BiFeO₃ usually exhibit 530.5 eV for Bi–O and 529.9 eV for Fe–O bonds.²⁸ Interestingly, the two binding energies obtained for the Bi–O position and Fe–O position are 531.7 eV and 530.1 eV, respectively, in the BiFeO₃/MWCNT, and the red-shift of several electron volts might imply the existence of C–O–Fe or C–O–Bi bonds. The Bi 4f and Fe 2p XPS spectra in Fig. 3c and d further indicate the formation of a BiFeO₃ crystal, where the peaks at 159.3 and 164.5 eV are respectively assigned to the Bi 4f_7/2 and Bi 4f_5/2 states for Bi³⁺, and both of the peaks at 711.1 and 724.4 eV correspond to Fe³⁺ in BiFeO₃/MWCNT.

Fig. 3 XPS spectra of BiFeO₃/MWCNT. (a) C 1s XPS spectrum, (b) O 1s XPS spectrum, (c) Bi 4f XPS spectrum, and (d) Fe 2p XPS spectrum.

Before the photocatalytic activity characterization, the optical absorption of the as-prepared BiFeO₃ particles was investigated which is relevant to their band gap. As shown in Fig. 4a, the absorption edge of the bare BiFeO₃ is about 600 nm, while that of 10 wt% BiFeO₃/MWCNT is red-shifted to longer than 700 nm. Fig. 4b shows the spectra transformation of the bare BiFeO₃ from the diffuse reflection spectra according to the Kubelka–Munk (K–M) theory.³⁶ The band gap for the bare BiFeO₃ is determined to be 2.15 eV, which is similar to a previous result.³⁷ A much smaller band gap of 1.7 eV is obtained for the 10 wt% BiFeO₃/MWCNT, which is agreement with a red shift in the absorption edge of BiFeO₃/MWCNT as compared to the bare BiFeO₃. Analogous band gap narrowing of BiFeO₃ is also found in the case of BiFeO₃/graphene^25,28,38 which could be attributed to the chemical bonding between BiFeO₃ and the specific sites of carbon.

Fig. 4 (a) UV-vis DRS spectra of BiFeO₃ and BiFeO₃/MWCNT. (b) Plot of the transformed Kubelka–Munk function versus the energy of light.

As mentioned above, the magnetic properties of BiFeO₃ make it a good absorbent for organic contamination removal using magnetic separation techniques. As shown in Fig. 5, the characteristic UV-vis absorption peak of rhodamine B is located at 553 nm, and when the amount of 10 wt% BiFeO₃/MWCNT is established to be 1 g L⁻¹, over 41% of rhodamine B can be removed within 10 min at room temperature. Adsorption efficiencies express the relationship between the MWCNT content in BiFeO₃/MWCNT and the concentration changes of adsorbate in the bulk solution at a given temperature under equilibrium conditions, as shown in Fig. 5b. The BiFeO₃/MWCNT with 30% BiFeO₃/MWCNT content possesses the maximum adsorption efficiency for RhB. The increased adsorption efficiency with increasing MWCNT content up to 30 wt% could be ascribed to their rising BET surface area (Table 1), which is beneficial for RhB uptake, as is well-known. The decreased adsorption efficiency of the BiFeO₃/MWCNT photocatalysts with 50 wt% content can be ascribed to excess MWCNTs that lead to difficult magnetic separation due to weakly magnetic BiFeO₃/MWCNT.

Fig. 5 (a) UV-Vis spectra of RhB solution before and after adsorption by 10 wt% BiFeO₃/MWCNT. The inset shows the adsorption process of BiFeO₃/MWCNT for RhB. (b) Adsorption efficiencies of RhB over BiFeO₃/MWCNT with different MWCNT content.

Table 1 BET surface area and degradation ratio of various samples

Samples	Surface area (m^2 g^−1)	Degradation ratio (min^−1)
MWCNT	212
BiFeO3	8.9	0.00466
5 wt% BiFeO3/MWCNT	25.6	0.0096
10 wt% BiFeO3/MWCNT	47.8	0.02441
20 wt% BiFeO3/MWCNT	65.4	0.00738
30 wt% BiFeO3/MWCNT	84.6	0.00536
50 wt% BiFeO3/MWCNT	102.9	0.00208

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The photocatalytic activities of the BiFeO₃/MWCNT photocatalysts with different MWCNT content for the degradation of RhB were performed under visible-light irradiation. Fig. 6a shows the RhB concentration decrease against irradiation time over BiFeO₃ and various BiFeO₃/MWCNT composites. The initial RhB concentration is based on each adsorption efficiency. Under irradiation for 120 min, the RhB solution is completely bleached over 10% BiFeO₃/MWCNT. Fig. 6b further shows that the photodegradation rates of RhB decreased in the following order: 10 wt% BiFeO₃/MWCNT > 5 wt% BiFeO₃/MWCNT > 20 wt% BiFeO₃/MWCNT > BiFeO₃ > 30 wt% BiFeO₃/MWCNT > 50 wt% BiFeO₃/MWCNT. Clearly, when the weight addition ratio of MWCNTs is increased to 20%, the activity is lower than the 10% BiFeO₃/MWCNT, although it is higher than BiFeO₃. A further increase of the weight addition ratio of MWCNTs will lead to a significant decrease of photocatalytic activity. The results demonstrated that 10 wt% BiFeO₃/MWCNT could be the optimal condition for organic contamination removal, considering its sustainability. The pseudo-first-order kinetics of photocatalytic degradation ratio for these BiFeO₃/MWCNT composites are shown in Fig. 6c, and the equation could be expressed as follows:

−ln C/C₀ = kT

where C and C₀ are the concentration and initial concentration of RhB, respectively, T is the irradiation time and k is the degradation rate constant. By plotting ln(C/C₀) as a function of time, the k (min⁻¹) values are obtained from the slopes of the lines of best fit and listed in Table 1. The RhB removal stability of the optimized 10 wt% BiFeO₃/MWCNT composite is evaluated by repeating the performance four times, as shown in Fig. 6d. It can be clearly seen that the removal efficiency only reveals a slight fading after four continuous times, indicating a high practicability for organic contamination removal.

Fig. 6 (a) Photocatalytic removal of RhB over BiFeO₃/MWCNT with different MWCNT content under visible light irradiation. (b) Photocatalytic degradation rates of BiFeO₃/MWCNT with different MWCNT content under visible light irradiation. (c) The kinetics of photocatalytic degradation of RhB over BiFeO₃/MWCNT with different MWCNT content under visible light irradiation. (d) Durability study of 10 wt% BiFeO₃/MWCNT used as both photocatalyst and absorbent.

To demonstrate the efficient removal of high concentrations of RhB from solution, the photocatalytic performance of the optimized 10 wt% BiFeO₃/MWCNT composite is compared with N doped TiO₂. As shown in Fig. 7a, the overall removal of 1 mM RhB over 10% BiFeO₃/MWCNT could be carried out within 130 min, which includes 10 of min physical adsorption and 120 of min photocatalytic degradation. However, the overall removal of 1 mM RhB over N doped TiO₂ needs to take more than 310 min under the same conditions as the process for the 10% BiFeO₃/MWCNT. The results indicate that BiFeO₃/MWCNT as an advanced photocatalyst could efficiently remove highly concentrated RhB through the means of physical adsorption and photocatalytic degradation.

Fig. 7 (a) Comparison of removal of RhB through both physical adsorption and photocatalytic degradation between 10 wt% BiFeO₃/MWCNT and N-doped TiO₂. (b) Transient photocurrent responses of the 10 wt% BiFeO₃/MWCNT and N-doped TiO₂ in 1 M Na₂SO₄ aqueous solution under visible-light irradiation at 0.5 V vs. Ag/AgCl.

To give further evidence to support the above comparison, the transient photocurrent responses were recorded for several on–off cycles of irradiation. Fig. 7b shows photocurrent curves for the aforementioned two samples. It is clear that the sensitive photocurrent response against light turns on/off actions which indicate that most of the photogenerated electrons are transported to the FTO and generate photocurrent under visible-light irradiation. Notably, the 10% BiFeO₃/MWCNT electrode exhibits a higher photocurrent density than N-doped TiO₂, suggesting a higher charge carrier generation and transport under visible light, corresponding to high photocatalytic activity.

Except for the limited effects on adsorption and light harvesting capacities, the dominant contribution of MWCNTs in BiFeO₃/MWCNT composites to photoactivity is determined to be charge separation. A schematic illustration for the mechanism of enhanced photoactivity of the BiFeO₃/MWCNT nanocomposites is shown in Fig. 8. Considering the electron affinity of BiFeO₃ is ∼3.3 eV,³⁹ and the bandgap of BiFeO₃ is 2.15 eV, the CB and VB edges of BiFeO₃ could be established at −0.9 and 1.25 eV, respectively. Generally, the work function of CNT is located at −0.30 eV vs. NHE.⁴⁰ Therefore, under light illumination, electrons (e⁻) are excited from the VB to the CB, leaving holes (h⁺) in the VB. Normally, these e⁻/h⁺ pairs tend to recombine rapidly along with only a fraction taking part in the photocatalytic reaction, resulting in a low reactivity. However, when BiFeO₃ particles are chemically bonded to the surface of MWCNTs, these photo-generated electrons could be easily transferred to MWCNTs, resulting in efficient hole–electron separation. These separated electrons and holes are directly trapped by some surface adsorbates or generate highly reactive radical species.

Fig. 8 Proposed mechanism for the enhanced electron transfer in the BiFeO₃/MWCNT composites.

4. Conclusion

In this study, we successfully grafted magnetically responsive BiFeO₃ particle into multiwalled carbon nanotube matrix (BiFeO₃/MWCNT) composites with different loadings (5–50 wt%) of MWCNTs by annealing a MWCNT and Bi–Fe complex mixture at 800 °C. Moreover, chemical bonds are formed at the interface between MWCNTs and BiFeO₃. It is shown that a 10 wt% MWCNT loading significantly influences the textural properties including physical adsorption and photodegradation of highly concentrated RhB (1 mM). The dominant factor in enhancing the photocatalytic performance of the BiFeO₃/MWCNT composite is identified as the interfacial electron transfer and separation. It clearly indicates that only these BiFeO₃/MWCNT composites with appropriate CNT loadings (10 wt%) have a much higher efficiency for RhB removal as compared with N-doped TiO₂.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (NSFC) (Grants no. 21271010) and the Shanghai Municipal Education Commission (No. 15ZZ088 and No. 14JC1402500) and the Science and Technology Commission of Shanghai Municipality (No. 14DZ2261000).

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