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Experimental Evidence for the Carrier Transportation Enhanced Visible Light Driven Photocatalytic Process in Bismuth Ferrite (BiFeO) One-Dimensional Fiber Nanostructures 3

Sivakumar Bharathkumar, Mohan Sakar, and Subramanian Balakumar J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b04344 • Publication Date (Web): 29 Jul 2016 Downloaded from http://pubs.acs.org on July 30, 2016

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Experimental Evidence for the Carrier Transportation Enhanced Visible Light Driven Photocatalytic Process in Bismuth Ferrite (BiFeO3) OneDimensional Fiber Nanostructures S. Bharathkumar, M. Sakar and S. Balakumar* National Centre for Nanoscience and Nanotechnology, University of Madras, Guindy campus, Chennai 600025, India *E-mail: [email protected]; *Telephone number: +91-44-22202749 Abstract The effective carrier transportation schemes in one-dimensional (1D) nanofibers of bismuth ferrite (BFO) have been experimentally demonstrated in comparison with their 3D particulate nanostructures. The structural analysis using X-ray diffraction technique revealed the rhombohedral crystal structure with R3c space group of BiFeO3 particulate and fiber nanostructures. The influences of dimension on the optical properties are analyzed using UVvisible absorption/diffuse reflectance spectroscopy, where the band gap energy is found to be increased for fibers (~2.39 eV) as compared to the particulates (~2.32 eV). The photoluminescence (PL) spectroscopy analysis indicated a reduced radiative-emission in BFO fibers that could be attributed to the slower recombination of excited electron-hole pairs in fibers as compared to particulates, which is also experimentally confirmed by estimating their fluorescence lifetime measurements. The room temperature photo-current conductivity measurements showed an enhanced photocurrent for the fibers, which revealed that the transportation of charge carriers is improved in fibers due to the delocalization of electrons in its conduction band and subsequent delayed recombination owing to the 1D confinement. The

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photocatalytic efficiency on the degradation of organic dyes (methylene blue and rhodamine B) under the simulated solar light irradiation showed an enhanced degradation rate for BFO fibers as compared to particulates. This could be attributed to the observed modifications in band energy structures and enhanced photocurrent conductivity of the fibers. Further, the effective carrier transportation-induced photocatalytic reactions that resulted from the increased number of *OH radicals is also probed by PL spectroscopy using terephthalic acid as a probing molecule. 1. Introduction Materials that show multifunctional properties are being attractive for their dimension dependent properties.1,2 Dimensional effects could be exploited to enhance materials’ mechanical, electronic, magnetic, and other physical and chemical properties.3,4 Among such materials, multiferroics have gained significant attention due to their simultaneous existence of multiple ferroic orders such as magnetic, ferroelectric, ferroelastic, etc.5 Several materials that produced by combining the magnetic and ferroelectric phases have been reported to exhibit magnetoelectric coupling at room temperature.6 However, the availability of room temperature single phase multiferroics is very limited. Bismuth ferrite (BiFeO3/BFO) is one of the well-known multiferroic materials, which exhibits simultaneous ferroelectric and magnetic properties.7 BFO possesses rhombohedrally distorted perovskite crystal structure that belongs to R3c space group. BFO is typically an anti-ferromagnetic (AFM) and ferroelectric material with relatively very high Neel (TN ∼ 625 – 643 K) and Curie (TC ∼ 1083 – 1103 K) temperatures respectively.8 BFO can be used for numerous applications such as memory devices, sensors, spintronics, photovoltaic, photocatalytic water splitting, degradation of organic pollutants, etc.9-12 Among these applications, BFO is being a promising candidate for the photocatalytic applications that can be used to degrade the organic pollutants in water environments.13-15 Photocatalytic reactions 2 ACS Paragon Plus Environment

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involve the absorption of photons to produce reactive electrons and holes to initiate the oxidation/reduction of the dye molecules that leads to their degradation.16 The narrow band gap (~2.3 eV) of BFO facilitates to be activated by visible light towards such photo-driven applications. To enhance the photocatalytic properties, BFO has been altered chemically through compositing, substitutions, etc., and physically alerted through changing its morphologies and sizes.17-20 The meticulous understanding of photo-catalytic mechanisms sheds lights on the design of nanomaterials towards enhancing their photocatalytic efficiencies. One of the very significant requirements for the enhanced photocatalytic reaction is the rapid separation of charge carriers and their slower recombination.21 To achieve this, the physical changes in nanomaterials offer a controlled way of electronic and optical transportations and thereby they become manipulative towards

the

required

functional

properties,

especially

photo-responsive

properties.22

Accordingly, in the process of altering the physical structures of BFO, one dimensional (1D) structures, particularly, fiber nanostructures have drawn significant attention due to their enhanced physical as well as chemical properties owing to their electronic confinements and subsequent changes in electronic properties of the materials. 1D nanostructures essentially reduces the recombination possibilities in the materials through the delocalization of electrons in their conduction bands.23-26 Accordingly, we herein report the fabrication of BFO 3D particulates and 1D fibrous nanostructures using sol-gel and electrospinning techniques respectively. The synthesized 3D and 1D BFO nanostructures were studied for their photocatalytic capabilities towards the degradation of organic dyes under simulated solar light irradiations and the photoresponsive properties of 1D BFO nanofibers have been experimentally demonstrated in comparison with 3D particulate BFO nanostructures. 3 ACS Paragon Plus Environment


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2. Experiment 2.1. Synthesis of BiFeO3 nanoparticulates BiFeO3 particulates were synthesized via sol–gel method. Bismuth nitrate (Bi(NO3)3.5H2O) and iron nitrate (Fe (NO3)3.9H2O) precursors in 1:1 ratio (3 mmol) were dissolved in 50 ml of double distilled (DD) water, containing 3 ml of ~65 – 70 % concentrated nitric acid (HNO3). To this, 3 mmol of tartaric acid solution was added for gelling purposes, and the solution was stirred well to obtain a homogeneous mixture. Then, the sol was heated to 80 °C to obtain a gel and dried to powder. Finally, the obtained as-prepared powder was annealed at 600 °C for 2 h to obtain the required BiFeO3 phase. 2. 2. Synthesis of BiFeO3 nanofibers Nanofibers of BiFeO3 were fabricated by electrospinning method. In the typical procedure, 0.8 M of bismuth nitrate (Bi(NO3)3.6H2O) and iron nitrate (Fe(NO3)3.9H2O) were dissolved in 5 ml of glacial acetic acid. To this, 1 g of polyvinyl pyrrolidone (PVP, M.W. 1 300 000) that dissolved in 5 ml of DD water was added. Thereafter, the solution was stirred homogenously for 6 h. This final solution was taken in a plastic syringe connected with a stainless needle and loaded into the electrospinning system. An optimized high electrical voltage of 15 kV was applied to the needle at a flow rate of 0.3 ml/h, and the fibers were collected in a plate collector. The as-spun BFO/PVP fibers were finally annealed at 600 °C for 2 h, to obtain the BiFeO3 phase. 2. 3. Photocatalytic Experiment The fabricated BiFeO3 particulate and fiber nanostructures are studied for their photocatalytic activity on the degradation of methylene blue (MB) and rhodamine B (RhB) under the simulated


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solar light irradiations. In the experiment, a stock of 10 ppm dye solution was prepared. From the stock, 100 ml of solution (1 ppm) was taken and the BFO photocatalyst was added such that the concentration of photocatalyst was 5 ppm in the 1 ppm of dye solution and the pH of the solution was adjusted to 1-2 by adding a small amount of HCl. This photocatalyst-dye mixture was further kept and stirred well under dark condition for an optimized time of 15 min to obtain the adsorption-desorption equilibrium. Finally, this mixture was exposed to the light source and the degradation of dye molecules was recorded in periodical intervals using UV-Visible absorption spectrometer. A commercialized photoreactor equipped with arc Xe lamp 500 W was used as the light source. The intensity of simulated solar-light is monitored using LT Lutron LX-10/A digital Lux meter. The average intensity over the whole duration of each experiment was calculated and found to be ~ 2 x 104 lux. 2. 4. Analysis of OH radical productions The photoluminescence spectroscopy technique with terephthalic acid (TA) as a probing molecule was employed to analyze the production of hydroxyl radicals by the synthesized BFO nanostructures. In this process, the conversion of TA into a fluorescent molecule known as 2hydroxyterephthalic acid (HTA) is taking place via the reaction of TA with the produced OH radicals. In the experiment, 100 ml of 0.025 mmol TA-NaOH solution was prepared by dissolving 0.1 mmol of TA in 100 ml of NaOH solution. To this, 0.25 g of BFO photocatalyst was added and sonicated under dark and the exposed under simulated solar light in the photoreactor. In periodical intervals, the light-irradiated solution was centrifuged to separate the catalyst from the solution mixture and the PL signals were recorded that excited at a wavelength of 315 nm.



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2. 5. Materials Characterization The fabricated materials were characterized for their thermal properties using thermo-gravimetric analyzer (TGA-NETZSCH STA449 F1 Jupiter), structural analysis using X-ray diffraction technique (XRD-PANalytical Instruments), chemical state analysis using X-ray photoelectron spectroscopy (XPS-Scienta Omicron Nanotechnologies), morphological analysis using field emission scanning electron microscope (FESEM-Hitachi High Tech SU6600), high resolution transmission electron microscope (HRTEM-Techni G2 S TWIN,FEI), magnetic characteristics using vibrating sample magnetometer (VSM) and optical characteristics using UV-Visible absorption/diffuse

reflectance

spectroscopy

(UV-Vis

Abs/DRS-Perkin

Elmer)

and

photoluminescence spectroscopy (PL-HORIBA Jobin Yvons LabRAM systems) techniques. Fluorescence decay measurements were carried out using an IBH time correlated single photon counting spectrometer with micro channel plate photomultiplier tube as detector and a femtosecond laser with second and third harmonic laser output at 375 nm and at 430 nm was used as the excitation source. The photo-catalytic studies were carried out using a commercial photo-reactor (HEBER Photo-reactor, India). Photocurrent conductivity studies were carried out using Kiethely 485 pico-ammeter coupled with a DC power supply unit. 3. Result and Discussion 3. 1. Structure and phase analysis The annealing temperature towards the required phase formation of the materials was estimated from their corresponding thermo-gravimetric weight loss curves as given in the supporting information (Fig. S1). The X-ray diffraction patterns of the obtained BFO particulate and fiber nanostructures are shown in Fig. 1. 6 ACS Paragon Plus Environment

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Fig. 1 XRD pattern of BiFeO3 particulate and fiber nanostructures The XRD patterns are found to be well agreed with JCPDS data (Card #71-2494) and it revealed the rhombohedral crystal structure with R3c space group of BiFeO3 phase. There are no peaks corresponding to any secondary or impurity phases appeared in the XRD patterns of the synthesized BFO nanostructures. The average crystallite size of BFO nanostructures corresponding to (104) and (110) planes is calculated using Scherrer’s formula d = kλ/βCosθ, where d-crystallite size, k-0.9 (shape factor), λ-wavelength of Cu kα (1.5406 Å), β-full with half maximum (FWHM) and θ-Bragg’s angle.27 The average crystallite size of particulates and fibers is found to be 60 nm and 24 nm respectively. The clear and distinct XRD peaks of BFO particulates indicated their enhanced crystalline nature, while the broaden XRD peaks of fibers represented reduced crystallite size28 of the fibers as compared to the particulates. This can also be corroborated with the calculated crystallite sizes from Scherer’s formula.28



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3.2 Morphology analysis The morphology of BFO particulates and fibers are analyzed using FESEM and HRTEM techniques and the obtained images are shown in Fig. 2(a)-(d) and 3(a)-(d) respectively.

Fig. 2 FESEM images of BiFeO3 (a)-(b) particulates and (c)-(d) fibers The obtained electron microscopic images of the BFO particulates revealed that the particles are aggregated in nature and possessed random/irregular shapes. The average size of the particles is found to be 128 nm (Fig. 3(e)). The high surface energy of the particles led to the random shapes and aggregations of the particles towards attaining a thermodynamically stable structure.


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Fig. 3 HRTEM images of BiFeO3 (a)-(b) particulates and (c)-(d) fibers; histograms show the size distribution of (e) particulates and (f) fibers In a typical sol-gel process, formation kinetics of morphology and size of nanoparticles often depend upon gelling agent and annealing processes. Gellants act as a matrix for the ion species and control the growth of particles by means of their binding habits on the growing host particles.29 Though the gelling agents control the particles growth, they will be decomposed during annealing process and therefore the resultant particle size and morphology would be influenced by the annealing parameters such as annealing-temperature, -time and -rate.30 The size parameters of BFO nanofibers such as length and average width of the fibers are found to be few micrometers and 285 nm respectively (Fig. 3(f)). The surface texture of BFO fibers clearly reveals that they are consisting of smaller particles that stacked into one dimensional direction.



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3. 3. X-ray photoelectron spectroscopic analysis 3.3.1. Chemical state analysis: The oxidation state of the elements presence in the synthesized BFO particulates and fibres was analyzed using XPS technique. The XPS full survey spectrum of particulate and fibers are shown in Fig. 4(a). The narrow scan spectra of Bi4f, Fe2p and O1s peaks are de-convoluted using software (CasaXPS) that supplied with the XPS instrument. The obtained binding energies are normalized using C1s peak at 284.6 eV. The bismuth as found in the narrow scan spectrum given in Fig. 4(b) shows the doublet peak at binding energy (BE) of 158.3 eV, 164.0 eV for particulates and 158.6 eV, 163.9 eV for fibers corresponding to Bi 4f7/2 and Bi 4f5/2. This confirmed that the Bi ions possess the intrinsic/native oxidation state of +3. In addition, the calculated spin-orbit splitting energy (∆E) of these doublet Bi 4f lines is around 5.3 eV, which is also consistent with the reported theoretical value of 5.4 eV.31 The oxidation state of Fe ions are volatile related to other elements in BFO and it often possesses multiple oxidation states that primarily indicates the existence of defects such as oxygen vacancies in BFO.32 In a stoichiometric BiFeO3 phase, the stable oxidation state of Fe is +3, which is also confirmed from their Fe spectra of BFO nanostructures as shown in Fig. 4(c). The core peaks corresponding to Fe 2p3/2 and Fe 2p1/2 lines are appeared at 710.73 eV, 724.33 eV for particulates and at 711.2 eV, and 724.8 eV for fibers. This indicates the stable +3 oxidation state of Fe ions in BFO. Further, the spin-orbit splitting energy of Fe 2p3/2 and Fe 2p1/2 is deduced to be 13.6 eV, which is also further confirmed the +3 oxidation state of Fe ions in the synthesized BFO nanostructures.33


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Fig. 4 (a) Survey spectrum, (b) Bi 4f, (c) Fe 2p, and (d) O 1s XPS spectrum of BiFeO3 particulate and fiber nanostructures As aforementioned, the existence of Fe ions with multiple oxidation (Fe2+/Fe3+) states often leads to the oxygen vacancies in BFO34, which could be validated from its XPS spectrum. Accordingly, the oxygen core spectra, as shown in Fig. 4(d), show the peaks corresponding to lattice oxygen (OL) and chemisorbed oxygen/other species (Oads) that adsorbed on the surface of the BFO nanostructures. The peaks appeared at 530.1 eV, 531.9 eV for particulates and 530.4 eV, 532.3 eV for fibers indicate the lattice (OL) and chemisorbed oxygen species (Oads) respectively. These results signify that the fabricated particulate and fiber nanostructures are stoichiometric BiFeO3 phase, possibly without any anionic/cationic defects in the material or they may exist well below the threshold level of detectable traces.32,35 11 ACS Paragon Plus Environment


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3.3.2. Valence band structure analysis: Positioning of energy band edges in materials is the most versatile modification as far as the optical and photocatalytic process is concerned. This can be greatly accomplished by modifying the dimension of materials at nanoscale; especially, structuring the materials into nanoscale fibers offers effective ways to shift the position of band edges in a material. Accordingly, the band edge energy of valence band (VB) is effectively determined using XPS technique and the obtained spectra of BFO particulate and fiber nanostructures are shown in Fig. 5.

Fig. 5 Valence band spectra indicate the shift observed in BiFeO3 fiber as compared to the particulate nanostructures The essential need for positioning the band edges is to control the separation of charge carriers as to achieve the required redox potential in a photocatalyst.36 From the obtained data, the observed relative increment in the VB energy of BFO fibers (1.53 ± 0.022 eV) as compared


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to particulates (1.13 ± 0.041 eV) signifies the shift of VB edge position towards higher energy, which could be attributed to their one-dimensional confinements in BFO. The shift of VB towards higher potentials enhances the charge separation energy and it augments the kinetic energy of the separated carriers thereby it leads to the effective and vibrant productions of redox species during the photocatalytic process. 3. 4. Optical properties 3.4.1 UV-Visible absorption properties: Figure 6(a) shows the UV-visible absorption spectrum of the fabricated BFO particulates and fibers. The absorption profile reveals a significant change in the absorption characteristics of the fibers as compared to particulates. The absorption profile of BFO articulates shows a distinct absorption in the visible region, while it is slightly shifted to UV region in the case of BFO fibers, which could be attributed to their reduced crystallite sizes.37 It should be noted that the 1D systems are anisotropic structures and therefore, the electronic confinement would vary across their length and diameter of the fibers. For example, the electronic transition across the diameter of fibers behaves like discrete transition as occurs in typical nanomaterials, while it completely behaves like a continuous-band like transition along the length of the fibers. Thereby, these anisotropic nanostructures collectively exhibit both nanoand bulk-effects in the materials. Therefore, the observed optical properties could be attributed to the synergistic effect due to the fettered and unfettered dimensions of 1D BFO.



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Fig. 6 UV-visible (a) absorption and (b) diffuse reflectance spectrum of BiFeO3 particulate and fiber nanostructures. (Insert: Kubelka–Munk function to deduce band gap energy of nanostructures) The band gap energy of the BFO particulate and fiber is obtained from DRS spectra by applying the Kubelka–Munk function38, 39 as shown in Fig. 6(b) and its insert image respectively. The band gap energy of BFO particulate and fibers is deduced to be 2.324 ± 0.013 eV and 2.391 ± 0.021 eV, respectively. It is realized that the modification of materials through doping or controlling their sizes at nanoscale would help tuning the band energy of the materials. On the other hand, controlling the morphology of materials would also considerably influence their band gap energy as it leads to the confinements in materials.40 These confinements primarily cause the change in electronic transportations through the shifting of band edge positions and thereby it leads to the changes in their band gap energies.41 Therefore, in this present case, the observed changes in the band gap energy of BFO fibers could be attributed to the 1D confinements. This can also be corroborated with the results obtained from the valance band analysis using XPS technique, where the valance band energy of fibers is slightly shifted towards higher binding


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energy. These results indicate that the 1D nanostructuring considerably modifies the band edge position in BFO. 3.4.2 Photoluminescence properties: Engineering the band structure in a photocatalyst is essentially to delay or reduce the recombination possibilities during the photocatalytic process. As to study this, the photoluminescence (PL) spectroscopy can be effectively used as a probe because of the fact that the origin of luminescence property largely associated with the recombination processes in semiconductor materials.42 Accordingly, PL spectrum of BFO particulates and fibers are acquired at room temperature at the excitation wavelength at 430 nm and the obtained spectra are showed in Fig. 7. From Fig. 7, in the case of particulates, the characteristic PL signal at 494 nm is attributed to the radiative-emission of BFO during the recombination of electron-hole pairs, while the other two signals at 556 nm and 588 nm are due to the emission as a result of the recombination due to defects and donor-acceptor interactions/secondary/impurity components in BFO respectively.36,43 These characteristic signals are blue-shifted in the case of BFO fibers that could be due to their 1D confinement effects. It should be noted that the intensity of the major signal at 492 nm is greatly reduced and the profile is also slightly modified for BFO fibers as compared to that of BFO particulates. This signifies that the radiative emission in BFO fibers is greatly minimized due to the improved recombination resistance as a result of the delocalization of excited electrons in the conduction band BFO.



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Fig. 7 Photoluminescence spectra of BiFeO3 particulate and fiber nanostructures 3.4.3 Fluorescence life time measurements: In addition, the ascribed enhanced recombination resistance that delayed the recombination possibilities in BFO nanostructures is further experimentally analyzed through the fluorescence lifetime decay measurements.44 The obtained fluorescence decay spectrum of BFO particulate and fiber nanostructures that excited at 430 nm is shown in Fig. 8. The life time data of the nanostructures were very well fitted to a two exponential function as given in eqn (1) and the average lifetime was calculated to be 0.812 ± 0.027 ns and 0.986 ± 0.016 ns respectively with 95% confidence limits.45 The observed lifetimes with standard deviation, relative amplitude values and the obtained reduced chi square values, which indicate the Goodness of Fit, are given in Table 1.

τ =

α 1τ 12 + α 2τ 22 = f1τ 1 + f 2τ 2 α1τ 1 + α 2τ 2


(1)

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Fig. 8 Fluorescence life time decay in BiFeO3 particulate and fiber nanostructures that excited at 430 nm (IRF – Instrument response function)

Material

T1 (ns)

Standard Relative deviation amplitude (ns) (%)

BFO 0.621 particulates

0.024

70.25

BFO fibers 0.698

0.016

66.73

Standard Relative Average deviation amplitude lifetime (ns) (%) (ns) 0.812 ± 6.245 0.041 29.75 0.027 0.986 ± 6.711 0.024 33.27 0.016 T2 (ns)

Res. χ2 1.16 1.03

Table 1 Obtained lifetime parameters of BiFeO3 nanostructures It is evident that, in contrast to the BFO particulates, the fibers exhibit slower decay kinetics. The fitted decay spectrum reveals two radiative lifetimes with different percentages. Accordingly, the shorter lifetime of 0.621 ns in particulates is increased up to 0.698 ns in fibers, where the corresponding percentages are found to be 70.25% and 66.73% respectively. It should be noted that, in fibers, the percentage of the charge carriers with shorter life time (0.698 ns) is



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slightly low (66.73%) as compared to the charger carriers with shorter life time (0.621 ns – 70.25%) in particulates. However, the time difference is only 0.077 ns. Similarly, the longer life time of 6.245 ns of particulates is also increased to 6.711 ns in fibers and the respective percentages are found to be increased from 29.75% to 32.27%. These observed results signify that the lifetime of the charge carriers in BFO system can be increased by 28% merely by changing its dimensions from 3D particulates to 1D fibers. The 1D confinement induced enhanced carrier lifetime in BFO fibers could be attributed to the improved delocalization of electrons in the conduction band of BFO as depicted in Fig. 9 (a)-(b).

Fig. 9 Dimension induced charge carrier transportation schemes in BiFeO3 (a) particulate and (b) fiber nanostructures 3. 5. Photocatalytic studies The synthesized BFO particulates and fibers were investigated for their photocatalytic ability to degrade the organic dyes methylene blue (MB) and rhodamine B (RhB) under the simulated solar light irradiations. The degradation curves and the deduced C/C0 ratio graphs are shown in Fig. 10(a)-(f) and the observed degradation percentages of the dyes are depicted in Fig. 11. It 18 ACS Paragon Plus Environment

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could be observed that the fibers of BFO are found to degrade the dyes effectively as compared to the particulates. The observed enhanced photocatalytic efficiency of fibers could be attributed to its 1D nanostructure that facilitated the following characteristics in BFO, (i) increased recombination resistance, (ii) enhanced charge separation, (iii) extended lifetime of charge carriers, (iv) improved light absorption, and (v) increased surface area and interfacial interactions.47

Fig. 10 (a)-(d) Photocatalytic degradation of methylene blue and rhodamine B dyes and (e)-(f) degradation efficiency (C/Co ratio) graphs of by BiFeO3 particulate and fiber nanostructures. 19 ACS Paragon Plus Environment


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Fig. 11 Photocatalytic degradation percentages of methylene blue and rhodamine B dyes at the end of 50 min by BiFeO3 particulate and fiber nanostructures In addition, the ascribed enhanced carrier transportation and redox reactions of BFO fibers were further validated through analyzing their photocurrent properties and their ability to produce OH radicals during the photocatalytic process. 3.5.1 Photo-conductivity measurements: The photo-current conductivity of the prepared samples was studied under dark and light exposed conditions. Figure 12(a)-(b) shows the response of BFO particulate and fiber nanostructures under dark-current (Id) and photo-current (Ip), where both Id and Ip of the samples show a linear increment with increasing applied field. From the graphs, the photo-current of the samples is found to be higher than that of the dark current, which signifies the photo-mediated enhanced conductivity of synthesized BFO nanostructures under the exposure of light. In general, the photoconductivity in a solid is due to the increase of charge carriers or their lifetime in the presence of a radiation. The process of


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photo-current in the materials generally involves the injection of electrons into the conduction band of the semiconductor and leads to the enhancement in the current under the irradiation.48

Fig. 12 Photo-current and dark-current conductivity of BiFeO3 (a) particulate and (b) fiber nanostructures It could be observed that both Id and Ip of fiber nanostructures is found to be enhanced as compared to that of the particulate nanostructures. This could be due to the improved delocalization of excited carriers in the conduction band of BFO nanofibers owing to their 1D confinement. Such delocalization of carriers offers (i) prolongation of lifetime of excited carriers, (ii) enhanced recombination resistance and (iii) effective transfer of these carriers to the surroundings. This could be explained as follows. The anisotropic confinements in nanostructures give rise to the electronic properties of materials and it essentially redesigns the energy modules in the systems.49 For instance, in an anisotropic nanostructure such as 1D nanofibers, the energy equation along ‘length’ of the fibers follows the bulk phenomenon while ‘diameter’ and ‘thickness’ follows nano-phenomenon. These so called bulk and nanophenomenon is originated due to unfettered and fettered dimensions respectively. Therefore, the



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energy of the system as a whole would be complex and it requires extensive energy operations to determine the total energy of the systems. It should be noted that the confinement effects principally reflect in the optical characteristics of the materials apart from their electronic properties and therefore the fibers tend to show an enhanced photo-current conductivity upon the irradiation of light. Eventually, these synergistic operations of confinements and optical characteristics of BFO fibers resulted to an enhanced photocatalytic activity towards the degradation of dyes. 3.5.2. Evaluation of *OH radical productions: As described in the experimental section, the production of OH radicals by the samples under visible light was analyzed using terephthalic acid (TA) as a probing molecule. From the obtained graphs as shown in Fig. 13(a)-(b), the plateau line that represents ‘blank’ signifies no emission from the bare TA solution. The succeeding curves with a broad peak at 425 nm indicate the formation of 2-hydroxyterephthalic acid (HTA) molecules upon the reaction between TA molecules and •OH radicals that produced due to the photo-generated charge carriers in the solution.50 The increasing intensity of the peaks with irradiation time indicates the increasing production rate of •OH radicals in the solution. Further, it could be seen that the intensity of HTA-BFO fiber mixture (Fig. 13(b)) is found to be enhanced as compared to the intensity of HTA-BFO particulates mixture (Fig. 13(a)). This clearly shows that the BFO fibers produce more •OH radicals than that of particulates. The general pathway of photo-catalytic reaction can be explained as follows. Upon the irradiation of photocatalyst by photons with suitable wavelength excites the electrons to the conduction band (CB) and leaving the corresponding holes in valence band (VB). When this charge separation is sustained in a photocatalyst, the dye molecules either reduced or oxidized through donation or acceptance of electrons.51 However, the key species of photocatalytic 22 ACS Paragon Plus Environment

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process, i.e. the redox reaction would principally determined by the band edge position of a photocatalyst.52 Accordingly, bismuth ferrite as being a p-type semiconductor,16, 53 it is reported to be a holes-mediated photocatalyst rather than electron-mediated. Accordingly, the degradation mechanism involves the generation of photo-induced electron-hole pairs and the subsequent reaction of holes with OH‾ ions leads to the conversion into ●OH radicals. Further, these ●OH radicals reacted with dye molecules and leads to the degradation of dyes.54 In addition to this, the holes could directly react and degrade the dye molecules effectively.55 Therefore, the observed over-all photocatalytic enhancement in BFO fiber nanostructures could be attributed to the effective charge separations, in which the delocalization process of electrons in CB limited their recombination with holes. Thereby, these separated holes effectively interacted with the surrounding medium and led to the effective of degradation dye molecules.

Fig. 13 Photoluminescence spectrum of 2-hydroxyterephthalic acid that formed upon the reaction between the OH radicals produced by BiFeO3 (a) particulates and (b) fibers and terapthalic acid



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However, as compared to the literature,56-59 the observed photocatalytic efficiency of the fibers could be attributed to several parameters such as the fiber length and diameter, homogeneity of the fibers, concentration of dye and photocatalyst, wavelength of visible light sources, pH of the dye solutions, etc.

Fig. 14 Photocatalytic recyclability efficiency of BiFeO3 particulate and fiber nanostructures on the degradation of (a) methylene blue and (b) rhodamine B dye molecules 3.5.3. Photocatalytic recyclability studies: The synthesized BiFeO3 nanostructures were also studied for their photocatalytic recyclable efficiencies. Figure 14(a)-(b) shows 5 cycles of photocatalytic degradation of dyes by BFO particulate and fiber nanostructures respectively. From the obtained graphs, degradation efficiency of BFO nanostructures is found to be consistent in all the 5 cycles. The separation of fibers from the dye solution, apart from magnetic separation (the obtained room temperature magnetic hysteresis (M-H) curves of the BFO particulates and fibers are given in supporting information in Fig. S2), is found to be relatively easier due to its stripes like nature, while it is harder for particulates due to their powder nature.18 Further, the XRD pattern and electron microscopy images (FESEM and HRTEM) of the photocatalytically recycled BFO particulate and fiber nanostructures are shown in Fig. S3 and 24 ACS Paragon Plus Environment

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Fig. S4(a)-(d) respectively. It is evident from the obtained XRD patterns and electron microscopy images that the crystal phase and morphology of the BFO photocatayst were not affected even after 5 cycles of photocatalytic degradation process. 4. Conclusion Bismuth ferrite particulates and fibers nanostructures were fabricated through the sol-gel and electrospinning technique respectively. Thermo-gravimetric weight loss curves indicated the characteristic decomposition of the as-prepared samples, through which the appropriate annealing temperature was estimated for the required phase formation of the materials. The phase and crystal structure of the fabricated particulates and fiber were identified to be BiFeO3 with rhombohedral crystal structure with R3c space group. XPS studies revealed the characteristic binding energy of the respective elements that confirmed the existence of elements with native oxidation states as in a stoichiometric BiFeO3 phase. The XPS-valence band spectrum of BFO fibers showed that the VB energy is increased due to the 1D structuring of BFO that supports the observed increased band gap energy. The photoluminescence spectrum revealed the reduced radiative-emission of BFO fibers, which indicated the slower recombination of the excited electron-hole pairs in the system. Further, the fluorescence decay measurements revealed an increased lifetime of the excited charge carriers in fibers that attributed to the 1D nanostructure-induced delocalization of electrons in the conduction band of BFO fibers. The magnetic property analysis by VSM suggested that the origin of the magnetic properties in 1D BFO could be due to the emergence of canted spins that are manifested by the suppression of cycloidal spin structures in the system. The photocatalytic degradation of fiber was found to be greater as compared to the particulates. Accordingly, the improved photo-current conduction mediated charge carrier transportations and thereby the enhanced production of •OH radicals as 25 ACS Paragon Plus Environment


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well as the enhanced interfacial interactions between BFO fibers and dye molecules favored the improved photocatalytic degradations of dyes under visible light irradiations. The reusability studies further confirmed the consistent photocatalytic recyclable ability and better stability of the particulate and fiber nanostructures of BiFeO3. Supporting Information The supporting information contains (i) the thermogravimetric analysis (TGA) results of the asprepared BiFeO3 particulates and fiber nanostructure (Fig. S1), (ii) room temperature magnetic hysteresis (M-H) curves of phase BiFeO3 particulates and fibers, obtained using Vibrating Sample Magnetometer (VSM) technique (Fig. S2), and (iii) XRD patterns (Fig. S3) and electron microscopy (FESEM and HRTEM) images of the 5th time photo-catalytically recycled BiFeO3 particulate and fiber nanostructures (Fig. S4(a)-(d)). This material is available free of charge via the Internet at http://pubs.acs.org/. Acknowledgements Authors gratefully acknowledge the Council of Scientific and Industrial Research (CSIR) for funding support (80(0074)/10/EMR-II, dt. 30-12-2010), and one of the authors, M. Sakar gratefully acknowledges the MHRD-NCNSNT for the postdoctoral research fellowship (C2/NSNT/pdf/2014/044, dt. 24-01-2014), to carry out this research work. Authors also thankfully acknowledge Prof. P. Ramamurthy, National Centre for Ultrafast Processes, University of Madras for extending the PL and fluorescence lift time measurement facilities.


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