Facile synthesis of BiFeO3 nanoparticles by modified microwave-assisted hydrothermal method as visible light driven photocatalysts

Accepted Manuscript Facile synthesis of BiFeO3 nanoparticles by modified microwave-assisted hydrothermal method as visible light driven photocatalysts Xiaoyan Sun, Zhongwu Liu, Hongya Yu, Zhigang Zheng, Dechang Zeng PII: DOI: Reference:

S0167-577X(18)30258-1 https://doi.org/10.1016/j.matlet.2018.02.052 MLBLUE 23878

To appear in:

Materials Letters

Received Date: Revised Date: Accepted Date:

11 November 2017 27 January 2018 12 February 2018

Please cite this article as: X. Sun, Z. Liu, H. Yu, Z. Zheng, D. Zeng, Facile synthesis of BiFeO3 nanoparticles by modified microwave-assisted hydrothermal method as visible light driven photocatalysts, Materials Letters (2018), doi: https://doi.org/10.1016/j.matlet.2018.02.052

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Facile synthesis of BiFeO3 nanoparticles by modified microwave-assisted hydrothermal method as visible light driven photocatalysts Xiaoyan Sun (X.Y. Sun), Zhongwu Liu (Z.W. Liu), Hongya Yu (H.Y. Yu), Zhigang Zheng (Z.G. Zheng) *, and Dechang Zeng (D.C. Zeng)* School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China *

Corresponding authors, E-mail: [email protected] (D.C. Zeng); [email protected] (Z.G. Zheng)

Abstract: BiFeO3 nanoparticles (BFO-NP) have been successfully synthesized through modified microwave-assisted hydrothermal method. The obtained BFO-NP was hexagonal phase with R3c structure and covered with much surface hydroxyl groups. Compared with the bulk BiFeO3 microsphere, BFO-NP has much higher purity and similar band gap energy, about 2.10 eV. The photocatalytic performance of BFO-NP was evaluated by Rhodamine B (RhB) removal without any sacrificial agent under visible-light irradiation. A fivefold increase in the rate of RhB degradation and mineralization was obtained, which should

result

from

effective

separation

of

the

electron-hole

pairs.

Furthermore,

the

scavengers-experiments reveal that photo-generated holes and hydroxyl radicals were the primary reactive species. This study opens a new pathway to synthesis BiFeO3 nanoparticle by hydrothermal method and provides a referential route for other perovskite photocatalysts. Keywords: semiconductors; bismuth ferrite; nanoparticle; microwave-assisted hydrothermal process; visible light catalytic; optical materials and properties

1.

Introduction

Bismuth ferrite (BiFeO3) is an intriguing material, not only for its multiferroic property [1] but also 1/ 8

promising optical performance. With a narrow and regulable band gap (Eg) of ~2.2 eV and good chemical stability, BiFeO3 [2] can be used as an effective photocatalyst under visible light compared with TiO2-based and ZnO-based photocatalysts. Despite of high efficient solar optical absorption, BiFeO3 is not immune to poor carrier mobility and rapid electron-hole recombination, which limit its widely application. In order to overcome this issue, many methods were proposed to improve the performance on the nanoscale, such as microstructural adjustments (morphology and grain sizes), heterojunction construction [3] or even introduction of suitable defects [4]. Nanoscale BiFeO3 powders are often prepared by solid-state reaction, sol-gel method [5,6], electrospinning method [7] or traditional hydrothermal method [8]. However, these processes need high-temperature calcination or long-time consumption. In fact, the microwave-assisted hydrothermal method [9] can speed up the synthesis process to get the crystalline phase in short time, saving energy and homogenizing products. However, in the microwave-assisted synthesis of BiFeO3, the single perovskite phase can only be formed under supercritical conditions, where the chemical potential and ionic motion in the precursor solution are needed to modulate accurately by adding alkaline mineralizer. A strong alkaline environment is beneficial for good crystallization but also leads BFO nanoparticles to reunite into microspheres or microcubes [10-12]. This uncontrollable large size limits further regulation and application in photocatalytic process. Herein, we successfully synthesized BiFeO3 microsphere with loosened shell structure in presence of CTAB by hydrothermal method. Therefore, it was easier to disintegrate the loose shell to release inner nanoparticles. The photocatalytic activity was investigated with the degradation rhodamine B (RhB) without any sacrificial agent under the visible-light irradiation. The photocatalytic mechanism is also investigated. 2. Experiment 2/ 8

Briefly, 20 mL aqueous solution of 0.259 g Bi(NO3)3·5H2O, 0.205 g Fe(NO3)3·9H2O, 0.0025 g hexadecyl trimethyl ammonium bromide (CTAB) and 9 M KOH were mixed by magnetic force stirring for 30min then transferred to a 60 mL Teflon vessel. The vessel was microwave heated at 180°C for 90 min to obtain BiFeO3 phase. A part of samples was washed directly with diluted HNO3 and distilled water subsequently dried at 80°C. The atropurpureus powder was obtained and labeled BFO-MC. The other part was wet-ground in an agate mortar and collected the suspension after 1 minute. The suspension was centrifugally separated and washed to obtain rufous powder, which is labeled as BFO-NP. The photocatalytic reaction of BFO-NP and BFO-MC were investigated through the degradation of Rhodamin B (RhB) solution under visible-light irradiation at 25 °C. Photocatalysts were added into RhB (30 mg/L) solution with the concentration of 0.3 g/L. Before light irradiation, the suspension was stirred for 30 minutes in the dark to make sure the adsorption-desorption equilibrium. A 300 W xenon lamp was used as the visible light source. The intensity of visible light used was fixed at 25 mW/cm2. At given time intervals, 2 mL aliquots were filtered through a nylon syringe filter to remove the catalyst particulates. The concentration of Rh B was determined by UV-visible spectrophotometer with λ=554nm. The rate constant (k) was calculated following pseudo-first-order equation (min-1), as the research from Ma et al.[13]. In scavenger experiments, the corresponding active species scavengers, benzoquinone (BQ) (0.05 mM), methanol (MeOH) (0.5 mM), isopropanol (IPA) (0.5 mM) or N2 airflow, were added before light irradiation. The morphologies of samples were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) using NOVA NANOSEM 430 and JEOL 2100F instruments, respectively. The powder X-ray diffraction (XRD) measurements were performed with a Rigaku Ultima IV diffractometer using Cu-Kα radiation. Fourier transform infrared (FT-IR) spectra of samples were recorded 3/ 8

on a VECTOR33 spectrometer. UV-vis absorbance spectra (UV-vis) were measured by a Perkin-Elmer Lambda 900 UV/vis/NIR spectrometer. 3. Results and discussion The SEM image of BFO-MC exhibits microsphere about 10 µm in diameter, as shown in Fig.1a, which consisted of faceted subunits. The section images (Fig. 1b and c) present that the inner of a microsphere is the aggregation of irregular nanoparticles ranged from 50 nm to 500 nm. The further away from the center, the bigger the nanoparticle size is. The faceted subunits are smashed after ground (Fig.1d). The crystallinity of BFO-NP is further confirmed by the HR-TEM images where the interplanar spacing is 0.394 and 0.276 nm, corresponding to the (012) and (110) planes for the hexagonal structure (JCPDS 71-2494), respectively. In addition, the experimental data (not shown here) revealed that CTAB, a typical cationic surfactant, can refine grained crystalloid but too much amount will lead to the formation of Bi2Fe4O 9 parasitic phase. Fig.2a shows the XRD patterns of as-prepared BFO-MC and BFO-NP. The diffraction peaks can be indexed to BiFeO3 (JCPDS 71-2494) with a hexagonal R3c structure. FT-IR was used to characterize the local structure and constituent of samples (Fig.2b). For both samples, the absorption bands at 444 and 550 cm-1 characterize the asymmetrical stretching of Fe-O-Fe bonds and Bi-O bonding for the perovskite-type BiFeO3[8]. The broad bands at 3440 and 1638 cm-1 can be ascribed to ν(O-H) stretching and δ(O-H) bending vibration respectively due to surface-adsorbed hydroxyl groups. Surface-adsorbed hydroxyl coordinated to surface cation was commonly observed in nanostructured metal oxide materials and played an important role in photocatalysis, which may have high activity and can be trapped by holes to produce ·OH for oxidizing organics [14]. For BFO-NP, the bending vibrations at 1384 cm-1 of nitrate attained from precursors is completely removed [15]. As presented in XRD, it is an effective way to leach out the minor 4/ 8

impurity using grinding in diluted nitric acid [16]. For the O 1s core level spectra, three fitting peaks can be clearly identified (Fig.2c). In detail, the peak at 529.79 eV is due to oxygen ions in the crystal lattice (O2-), the peak at 532.27 eV is associated with hydroxy species of surface-adsorbed hydroxyl group (-OH), and the peak at 530.77 eV is attributed to defect sites with low oxygen coordination (VO). The results indicate that the grinded BFO-NP possesses more oxygen vacancies and surface-adsorbed hydroxyl group [6]. UV-Vis diffuse reflectance spectra of BFO-MC and BFO-NP are demonstrated in Fig.2d. Both samples own a distinct absorption in the visible region. The corresponding band gap energy of BFO-MC and BFO-NP deduced by using Kubelka-Munk function as shown in the inset of Fig.2d are found to be 2.02 and 2.10 eV, respectively. Fig. 3a displays the photodegradation efficiencies of RhB (pH 6.0) as a function of illumination time with different photocatalysts. Blank test (RhB without any catalyst) shows that the self-degradation of RhB is extremely slow and only 9% of RhB is photolyzed after 4h illumination. The degradation of RhB with the BFO-NP is about 70% after 4 h illumination while that is 20% conversion was observed in presence of BFO-MC. The decolorization rate for blank, BFO-MC and BFO-NP treatment is 0.00049, 0.00093 and 0.0047 min-1, respectively. Although there may be photolysis and adsorption of RhB, the above comparative results signify that the enhanced RhB degradation with BFO-NP is indeed originated from a photocatalytic process or photocatalysis together with sensitization. To better understand the mechanism of photocatalysis, a series of scavengers is employed during the photodegradation processes. MeOH, BQ, N2 and IPA are used as scavengers for h+, superoxide radicals (·O2-), O2 and ·OH, respectively. The depressed degradation rate of RhB with MeOH supports high electron-hole separation efficiency. The degradation rate has no decrease with BQ and N2 while photocatalytic activity is remarkably suppressed by the addition of IPA and even lower than that adding MeOH, suggesting that ·OH are the primary and immediate oxidant. 5/ 8

The above trapping experiments preliminarily reveal that the predominant reactive species for RhB photodegradation by the BFO-NP catalyst were photogenerated holes and hydroxyl radicals rather than superoxide radicals (as shown in fig.3c). The verification of h+ proves the effective charge separations. As a p-type semiconductor, BFO is a holes-mediated photocatalyst rather than electron-mediated. The slow recombination should be attributed to delocalization of excited electrons in the conduction band [17]. Based on above results and photocatalytic mechanism, it demonstrates this is an effective method to prepare BFO nanoparticles with improved solar catalytic performance. 4. Conclusion In conclusion, BFO nanoparticles are rapidly and simply synthesized by the microwave assisted hydrothermal method using CTAB as surfactant combined further grinding. The BFO-NP owns wide visible light response and effective separation of photo-excited electron-hole pairs. The synergistic effect of small size and surface hydroxyl groups was conducive to improve the carrier transfer and electron-hole separation, which result in the enhancement of •OH yield and incident photocatalytic efficiency. Acknowledge This work was supported by Guangdong Provincial Science and Technology Program (Grant No. 2015A050502015), the Guangzhou Municipal Science and Technology Program (No. 2016201604030070, 201707010056, 201604016103) and Natural Science Foundation of Guangdong Province (No. 2016A030313494). References [1] D.Y. Chen, C.T. Nelson, Y. GaO, S.L. Hsu, D.G. Schlom, R. Ramesh, et al. Nano Lett. 17(2017) 5823-5829 [2] F. Gao, X.Y. Chen, K.B. Yin, S. Dong, Z.F. Ren, F. Yuan, et al. Adv. Mater. 19 (2007) 2889-2892 6/ 8

[3] W.Q. Fan, X.Q. Yu, H.C. Lu, H.Y. Bai, C. Zhang, W.D. Shi, Appl. Catal. B 181(2016) 7-15 [4] S. Wang, D. Chen, F. Niu, N. Zhang, L.S. Qin, Y.X. Huang, J. Alloys Compd. 688(2016) 399-406 [5] T. Soltani, B.K. Lee, Chem. Eng. J. 313(2017) 1258-1268 [6] M. Sakar, S. Balakumar, P. Saravanan, S. Bharathkumar, Nanoscale. 8(2016) 1147-1160 [7] M. Sakar, S. Balakumar, Nano-Structures & Nano-Objects 12(2017) 188-193 [8] R. Dhanalakshmi, M. Muneeswaran, K. Shalini, N.V. Giridharan, Mater. Lett. 165(2016) 205-209 [9] Y. Meng, D. Chen, Y. Sun, D. Jiao, D. Zeng, Z. Liu, Appl. Surf. Sci. 324(2015) 745-750 [10] L.F. Fei, J.K. Yuan, Y.M. Hu, C.Z. Wu, J.L. Wang, Y. Wang, Cryst. Growth Des. 11(2011) 1049-1053 [11] J. Prado-Gonjal, D. Ávilaa, M.E. Villafuerte-Castrejónb, F. González-Garcíac, L. Fuentes , R.W. Gómeze, et al. Solid State Sci.13(2011) 2030-2036 [12] W.Q. Cao, Z. Chen, T. Gao, D.T. Zhou, X.N. Leng, F. Niu, et al. Mater. Chem.Phys. 175(2016) 1-5 [13] X.Y. Ma, Y.Q. Cheng, Y.J. Ge, H.D. Wu, Q.S. Li, N.Y. Gao, J. Denga, Ultrasonics-Sonochemistry 40 (2018) 763-772 [14] A. Maldotti, A. Molinari, and R. Amadelli, Chem. Rev. 102 (10) (2002) 3811-3836 [15] W.D. Ji, M.M. Li, G.K. Zhang and P. Wang, Dalton Trans.46 (2017) 10586-10593 [16] J.Y. Wang, Y.W. Wei, J.J. Zhang, L.D. Ji, Y.X. Huang, Z. Chen, Mater. Lett. 124 (2014) 242-244 [17] S. Bharathkumar, M. Sakar, and S. Balakumar, J. Phys. Chem. C 120 (2016) 18811-18821

Figure captions

7/ 8

Fig.1 The SEM and HR-TEM images of the BFO-MC (a-c) and BFO-NP (d-f)

Fig.2 (a)XRD patterns, (b)FTIR spectra, (c) O 1s XPS core-level spectra and (d) UV-vis specra of BFO-MC and BFO-NP

Fig.3 (a) The photocatalytic activity of BFO-MC and BFO-NP; (b) Scavenging experiment of BFO-NP; (c) Schematic illustration of photocatalytic mechanism. 8/ 8