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Electrospun BiOBr lamellae for efficient photocatalysis on ARS dye degradation
Author’s Accepted Manuscript Electrospun BiOBr lamellae for photocatalysis on ARS dye degradation
efficient
Veluru Jagadeesh Babu, Merum Sireesha, R S R Bhavatharini, Seeram Ramakrishna www.elsevier.com
PII: DOI: Reference:
S0167-577X(16)30075-1 http://dx.doi.org/10.1016/j.matlet.2016.01.074 MLBLUE20202
To appear in: Materials Letters Received date: 20 July 2015 Revised date: 6 January 2016 Accepted date: 16 January 2016 Cite this article as: Veluru Jagadeesh Babu, Merum Sireesha, R S R Bhavatharini and Seeram Ramakrishna, Electrospun BiOBr lamellae for efficient photocatalysis on ARS dye degradation, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2016.01.074 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 galley proof before it is published in its final citable 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.
Electrospun BiOBr lamellae for efficient photocatalysis on ARS dye degradation Veluru Jagadeesh Babu*, Merum Sireesha, R S R Bhavatharini, and Seeram Ramakrishna*, Department of Mechanical Engineering, Faculty of Engineering, National University of Singapore, Singapore-117576, Singapore. Center for Nanofibers and Nanotechnology, Nanoscience and Nanotechnology Initiative (NUSNNI), National University of Singapore, Singapore-117576, *
Corresponding Author: [email protected] (vjbabu); [email protected] (SRamakrishna);
ABSTRACT
Lamellar shaped BiOBr nanostructures were synthesized via electrospinning followed by calcination at 500 °C in air. These nano lamellae were comprised of miro/nano petals confirmed by electron microscopy. Lamellae crystal structures were identified as tetragonal by X-ray diffraction (XRD). X-ray photoelectron spectroscopy (XPS) was performed to identify the molecular composition, chemical interactions. These structures were utilized in photodegradation of Alizarin Red S (ARS) dye with respect to time. The half-life time of the dye was estimated about ~33.5 min under white light illumination. A plausible electron transfer mechanism was proposed with respect to standard hydrogen electrode (SHE) potential.
Key words: Electron microscopy, Porous materials, Nanocomposites, Semiconductors.
1. Introduction
Bismuth oxyhalides, BiOX (X = Cl, Br, I), have been received application potential for the efficient dye degradation [1, 2] due to their excellent photodecomposition ability against the removal of toxic organic pollutants [3, 4]. Especially, BiOBr has two discrete valence bands (VB) arising one from O2- and another from Br-, in addition, the band gap (indirect band gap) of BiOBr belong to visible region so it is able to effectively use sunlight to degrade pollutants. Shang et.al.,[5] synthesized highly anisotropic layered lamellar structures via hydrothermal route and reported higher degradation performance for BiOBr. In general, phtocatalytic activity (PCA) of a semiconductor is closely related to the positions of VB and conduction band (CB) and mobility of the photogenerated charge carriers of catalyst. In Bi based photocatalysts, the VB 1
contain O 2p and Bi 6s hybrid orbitals and the CB contains Bi 6p. Also the Bi 6s orbital is largely dispersed and is more useful to increase the mobility of photogenerated charge carriers. While, Electrospinning is adopted to produce nanofibers [6-8] due to their application potential in energy and environmental remediation related issues [9, 10]. In the present paper, the preparation of BiOBr (BiBr3 was a precursor material) nanostructures via electrospinning and post calcination at 500 °C in air. BiOBr catalysts were characterized by SEM, TEM, XRD and XPS techniques. The acquired BiOBr catalyst was used to perform photocatalytic degradation on Alizarin Red S (ARS) dye under white light (150W Xe lamp) illumination. The dye degradation was observed with respect to time. The better PCA was observed in presence of BiOBr catalyst due to the easy electron-hole separation and efficient charge transfer. We anticipate that these lamellae would be potential for environmental remediation application. 2. Results and Discussion: The procedure for the electrospinning method and optimization of the fiber formation conditions were explained in our previous studies [8, 11, 12]. In brief, the complete experimental methods and characterization are given in supplementary information [SI†]. As shown in figure 1 (a1, b1, c1 and d1), the fibers are randomly oriented but the diameters are uniform throughout their lengths. It indicates that the viscoelastic force and electrostatic repulsions within precursor solution were successfully balanced by controlling the process parameters (humidity, flow rate, substrate rotation speed and high voltage) to suppress the influence of surface tension. While increasing concentration of the BiBr3 (as x=1 to 2, 3 and 4 %) in polymer (PAN), morphology does not change much. After the calcination the nanofibers were disintegrated to lamellar shapes with the thickness in range of 90 - 200 nm are presented in figure 1 (a2, b2, c2 and d2). Due to the decomposition of polymer, rough surfaced nano-petals were interconnected to form the fibrous morphology. From figure 1 (a2 and b2), the nano-petal morphology was not so prominent but lamellar got interwoven with each other resulting in pockets of many large microspores. These large microspores formed between stacked lamellae could be associated with increase in pore volume. The lamellar shapes starts to form from x = 1 to 3 %, at x=4%, the nanofibrous morphology completely lost and lamellar shapes are predominant and surface become rough. It is well-known that the self-aggregation effect of the Bi which turn into lamellar structure. Shang et.al [5] reported that the possible mechanism for lamellar shapes formation. In addition, the unit 2
cell of the BiOBr (tetragonal matlockite-type structure) composed of one ‘O’ atom and four ‘Br’ atoms coordinate with four ‘Bi’ atoms. One ‘Bi’ atom coordinates with four ‘O’ atoms and four ‘Br’ atoms in different bases. One [Bi2O2]2+ plates/petal is sandwiched between two Br slabs. Different petals are stacked together via weak van der Waals interactions [13]. Thus the lamellar shaped nanostructure formed with Bi-O weak interactions. The TEM image (SI-figure 1a†) shows the lamellar shapes clearly. From SI-figure 1b†, all the XRD peaks are perfectly identified as tetragonal structure of BiOBr and there are no peaks related to other impurities. Just for comparison, a thin film and the fiber sample (x=4%) is used in XRD. The lattice parameters are obtained as a = 11.42 Å and c = 8.0723Å. The calculated crystallite size is 3.017 Å. The calculated surface area (Sa) of the lamellar structures is about 77.22 m2/g (calculated from the eqn. given in Ref.[14]) is in close agreement with literature reported [15]. The composition of the product was determined by X-ray energy-dispersive spectroscopy (EDS), as shown in SI-Figure 2†, which unambiguously demonstrates the existence of Bi, O and Br elements in the product, along with C element attributing to the conductive carbon tape substrate used for the SEM observations. The EDS elemental mapping (SI-Figure 2†) further confirms that these elements were uniformly distributed in the BiOBr lamellar shaped catalysts. The XPS survey spectrum (SI-figure 3a†) is composed of elements Bi, O, Br and C. The peak at the binding energy about 284.95 eV is attributed to C 1s. In the high resolution spectrum (SIfigure 3b†) of Bi 4f revealed that the strong peaks of 4f7/2 at 159.2 eV and 4f5/2 at 164.4 eV doublet (Δso = 5.2 eV) is due to spin–orbit coupling. The band shape is in agreement with the presence of Bi3+ species in a BiOBr network according to the previous reports [16]. The peak at 530.5 eV in O 1s, assigns O-2 in BiOBr (SI-figure 3c†). From SI-figure 3d†, a strong peak at 68.1 eV (Br 3d5/2) and a broad peak 69.0 eV (Br 3d3/2) identified as Br-1 anion. Then PCA experiments were carried out and presented in SI-figure 4†. The photodecomposition of the sample at x=3% is faster within 80 min, but the best PCA was noticed at x=2% where mere complete degradation occurred. This might be due to pocket like microspores having high surface area which can facilitate more available active sites for the photocatalysis reaction. ARS dye degradation with respect to time at different concentrations of the BiOBr is presented in SIfigure 5†. From figure 2, the black dye (without catalyst), there is no indication of degradation. After adding the BiOBr catalyst, the PCA performance is enhanced approximately 83.89% (x= 2%) within 100 min. Therefore, the enhanced PCA was because of the lamellar shaped BiOBr 3
nanocatalysts. The adsorption capacity of these structures exhibit nearly 20 % is stronger than that of 2D nanoparticles [17]. In general, the PCA performance of the semiconductors is closely related to its characteristics of crystallinity, electronic structure (VB and CB), mobility of the carriers and band gap energy among other. In case of Bi based catalysts, the VB consists of hybridized orbitals of 2p of oxygen and 6s of Bi, and the CB is composed of the 6p orbital of Bi which possesses the highly reductive capability. VB and CB could determine the reductive and oxidative ability of the catalyst, respectively, while the mobility of the charge carriers could determine the photocatalytic efficiency [18]. Thus, once the photon (h) of energy illuminated on BiOBr catalyst, the effective separation of photogenerated charge carriers are favourable for the PCA. During the photocatalysis, Bi-O square anti-prisms could generate many oxygen defects because of the unstable bonding between Bi and O. The BiOBr exhibited (at x=3% of BiBr3 in PAN) the effective photon utilization and allowed the photoabsorption to increase more (e-) and hole (h+) pairs, to promote the PCA. The experimentally observed parameters are listed in Table 1. From table 1, x=3% of BiBr3 doping gives the fast reaction. The PCA performance was observed in presence of BiOBr lamellae shaped catalyst. Table 1: parameters obtained from the photocatalysis experiments. Concen(x%)
Rate of activity
Reaction efficiency (%)
Rate constant
-1
(min )
Half life time (t1/2) (min)
1
0.197
80.35
0.0191
36.6
2
0.161
83.89
0.0190
37.1
3
0.321
82.27
0.0210
33.49
4
0.177
67.94
0.0101
70.5
The possible band alignment and charge carrier mechanism is proposed in figure 3. Once the photon of light illuminate on the BiOBr catalyst, the free electron will be (energy E > Eg = 2.88 eV) excited. The excited charge carriers (e-) move to the CB. The electrons accumulated on the CB of BiOBr react with O2 adsorbed on the surface of nanocatalyst to generate oxygen radical (∙O2). This reactive radical, eventually leads to the degradation of ARS. From the figure 3, the CB and VB edges estimated by using electronegativity theory as proposed by the several groups [19, 20] and modified by the Liang Kong et.al [21]. According to the electronegativity theory photogenerated holes have a strong oxidation potentials and serve as best active sites for photodegradation. The better PCA was observed for the 3% BiBr3 doping is assumed as critical 4
concentration. Beyond this critical concentration (x=4%) excessive of BiBr3 can object the active sites of BiOBr and hinders light penetration, that is why PCA is poor at x=4%. 3. Conclusions: BiOBr lamellae shaped nano structures are successfully synthesized via electrospinning and post calcination at 500 C in air. These lamellae confirmed with SEM and TEM and other series of characterization techniques. From the XRD, tetragonal crystal structure of BiOBr is confirmed and the surface area is calculated and compared with literature reported value. The concentration level of BiBr3 in PAN at about x=3 %, BiOBr exhibited the better PCA performance in order to degrade the ARS dye. The best catalytic activity within lower reaction times has been estimated and its half life time is about ~ 33.49 min calculated. A plausible VB and CB band alignment structure is proposed and discussed with respect to SHE standard potential.
4. References: [1] Babu VJ, Bhavatharini RSR, Ramakrishna S. Electrospun BiOI nano/microtectonic plate-like structure synthesis and UV-light assisted photodegradation of ARS dye. RSC Adv. 2014;4:19251-6. [2] An H, Du Y, Wang T, Wang C, Hao W, Zhang J. Photocatalytic properties of BiOX (X = Cl, Br, and I). Rare Metals. 2008;27:243-50. [3] Wang W, Huang F, Lin X, Yang J. Visible-light-responsive photocatalysts xBiOBr–(1−x)BiOI. CatalCommun. 2008;9:8-12. [4] Shenawi-Khalil S, Uvarov V, Fronton S, Popov I, Sasson Y. A novel class of heterojunction photocatalysts with highly enhanced visible light photocatalytic performances: yBiO(ClxBr1−x)–(1 − y) bismuth oxide hydrate ApplCatalB. 2012;117-118:148-55. [5] Shang M, Wang W, Zhang L. Preparation of BiOBr lamellar structure with high photocatalytic activity by CTAB as Br source and template. JHazardMater. 2009;167:803–9. [6] Babu VJ, Satheesh KK, Trivedi DC, Murthy VRK, Natarajan TS. Electrical Properties of Electrospun Fibers of PANI-PMMA Composites. JEngdFiberFabrics. 2007;2:25-31. [7] Babu VJ, Kumar VSP, Subha GJ, Kumari RV, Natarajan TS, Nair AS, et al. AC Conductivity Studies on PMMA-PANI (HCl) Nanocomposite Fibers Produced by Electrospinning. JEngdFiberFabrics. 2011;6:54-9. [8] Babu VJ, Rao RP, Nair AS, Ramakrishna S. Nitrogen-doped rice grain-shaped titanium dioxide nanostructures by electrospinning: frequency and temperature dependent conductivity. JApplPhys. 2011;110:064327. [9] Wang C, Shao C, Liu Y, Zhang L. Photocatalytic properties BiOCl and Bi2O3 nanofibers prepared by electrospinning. Scripta Materiallia. 2008;59:332-5. [10] Liu Z, Sun DD, Guo P, Leckie JO. An Efficient Bicomponent TiO2/SnO2 Nanofiber Photocatalyst Fabricated by Electrospinning with a Side-by-Side Dual Spinneret Method. Nano Lett. 2007;7:1081-5. [11] Babu VJ, Nair AS, Peining Z, Ramakrishna S. Synthesis and characterization of rice grains like Nitrogen-doped TiO 2 nanostructures by electrospinning–photocatalysis. MaterLett. 2011;65:3064-8. [12] Babu VJ, Kumar MK, Murugan R, Khin MM, Rao RP, Nair AS, et al. Photocatalytic hydrogen generation by splitting of water from electrospun hybrid nanostructures. IntJHydrogen Energy. 2013;38:4324–33. 5
[13] Zhang D, Li J, Wang Q, Wu Q. High {001} facets dominated BiOBr lamellas: facile hydrolysis preparation and selective visible-light photocatalytic activity. J MaterChem A. 2013;1:8622-9. [14] babu VJ, Vempati S, Ertas Y, Uyar T. Excitation dependent recombination studies on SnO2/TiO2 electrospun nanofibers. RSC Adv. 2015;5:66367-75. [15] Wei X-X, Cui H, Guo S, Zhao L, Li W. Hybrid BiOBr–TiO2 nanocomposites with high visible light photocatalytic activity for water treatment. JHazardMater. 2013;263:650-8. [16] Shi X, Chen X, Chen X, Zhou S, Lou S, Wang Y, et al. PVP assisted hydrothermal synthesis of BiOBr hierarchical nanostructures and high photocatalytic capacity. ChemEngJ. 2013;222:120–7. [17] Dai K, Peng T, Ke D, Wei B. Photocatalytic hydrogen generation using a nanocomposite of multiwalled carbon nanotubes and TiO2 nanoparticles under visible light irradiation. Nanotechnology. 2009;20:125603. [18] Li H, Liu J, Liang X, Hou W, Tao X. Enhanced visible light photocatalytic activity of bismuth oxybromide lamellas with decreasing lamella thicknesses. J MaterChem A. 2014;2:8926-32. [19] Mulliken RS. Electronic Structures of Molecules XI. Electroaffinity, Molecular Orbitals and Dipole Moments. JChemPhys. 1935;3:573. [20] Nethercot AH. Prediction of Fermi Energies and Photoelectric Thresholds Based on Electronegativity Concepts. Phys Rev Lett. 1974;33:1088–91. [21] Kong L, Jiang Z, Xiao T, Lu L, Jones MO, Edwards PP. Exceptional visible-light-driven photocatalytic activity over BiOBr–ZnFe2O4 heterojunctions. Chem Commun. 2011;47:5512–4.
Figure Captions Figure 1: SEM images a1, b1, c1 and d1 shows as-spun nanofibers and a2, b2, c2, and d2 shows after sintering lamellar structures of BiOBr concentrations varied from 1, 2, 3, and 4% (w/v) respectively. Figure 2: Photocatalytic reactivity of BiOBr at (a) x=1%, (b) x=2%, (c) x=3% and (d) x=4% (w/v) under white light irradiation. Figure 3: Schematic representation for energy-band diagram of BiOBr and ARS dye degradation over BiOBr.
6
Figure:1
Figure:2
7
Figure 3:
8
Graphical Abstract
Highlights BiOBr lamellae nanostructures are synthesized via electrospinning and post calcination. The time dependent photocatalytic activity under white light illumination found. The time dependent photocatalytic activity is shown first order pseudo rate kinetics. The possible band structure alignment is proposed with respect to SHE.
9
efficient
Veluru Jagadeesh Babu, Merum Sireesha, R S R Bhavatharini, Seeram Ramakrishna www.elsevier.com
PII: DOI: Reference:
S0167-577X(16)30075-1 http://dx.doi.org/10.1016/j.matlet.2016.01.074 MLBLUE20202
To appear in: Materials Letters Received date: 20 July 2015 Revised date: 6 January 2016 Accepted date: 16 January 2016 Cite this article as: Veluru Jagadeesh Babu, Merum Sireesha, R S R Bhavatharini and Seeram Ramakrishna, Electrospun BiOBr lamellae for efficient photocatalysis on ARS dye degradation, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2016.01.074 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 galley proof before it is published in its final citable 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.
Electrospun BiOBr lamellae for efficient photocatalysis on ARS dye degradation Veluru Jagadeesh Babu*, Merum Sireesha, R S R Bhavatharini, and Seeram Ramakrishna*, Department of Mechanical Engineering, Faculty of Engineering, National University of Singapore, Singapore-117576, Singapore. Center for Nanofibers and Nanotechnology, Nanoscience and Nanotechnology Initiative (NUSNNI), National University of Singapore, Singapore-117576, *
Corresponding Author: [email protected] (vjbabu); [email protected] (SRamakrishna);
ABSTRACT
Lamellar shaped BiOBr nanostructures were synthesized via electrospinning followed by calcination at 500 °C in air. These nano lamellae were comprised of miro/nano petals confirmed by electron microscopy. Lamellae crystal structures were identified as tetragonal by X-ray diffraction (XRD). X-ray photoelectron spectroscopy (XPS) was performed to identify the molecular composition, chemical interactions. These structures were utilized in photodegradation of Alizarin Red S (ARS) dye with respect to time. The half-life time of the dye was estimated about ~33.5 min under white light illumination. A plausible electron transfer mechanism was proposed with respect to standard hydrogen electrode (SHE) potential.
Key words: Electron microscopy, Porous materials, Nanocomposites, Semiconductors.
1. Introduction
Bismuth oxyhalides, BiOX (X = Cl, Br, I), have been received application potential for the efficient dye degradation [1, 2] due to their excellent photodecomposition ability against the removal of toxic organic pollutants [3, 4]. Especially, BiOBr has two discrete valence bands (VB) arising one from O2- and another from Br-, in addition, the band gap (indirect band gap) of BiOBr belong to visible region so it is able to effectively use sunlight to degrade pollutants. Shang et.al.,[5] synthesized highly anisotropic layered lamellar structures via hydrothermal route and reported higher degradation performance for BiOBr. In general, phtocatalytic activity (PCA) of a semiconductor is closely related to the positions of VB and conduction band (CB) and mobility of the photogenerated charge carriers of catalyst. In Bi based photocatalysts, the VB 1
contain O 2p and Bi 6s hybrid orbitals and the CB contains Bi 6p. Also the Bi 6s orbital is largely dispersed and is more useful to increase the mobility of photogenerated charge carriers. While, Electrospinning is adopted to produce nanofibers [6-8] due to their application potential in energy and environmental remediation related issues [9, 10]. In the present paper, the preparation of BiOBr (BiBr3 was a precursor material) nanostructures via electrospinning and post calcination at 500 °C in air. BiOBr catalysts were characterized by SEM, TEM, XRD and XPS techniques. The acquired BiOBr catalyst was used to perform photocatalytic degradation on Alizarin Red S (ARS) dye under white light (150W Xe lamp) illumination. The dye degradation was observed with respect to time. The better PCA was observed in presence of BiOBr catalyst due to the easy electron-hole separation and efficient charge transfer. We anticipate that these lamellae would be potential for environmental remediation application. 2. Results and Discussion: The procedure for the electrospinning method and optimization of the fiber formation conditions were explained in our previous studies [8, 11, 12]. In brief, the complete experimental methods and characterization are given in supplementary information [SI†]. As shown in figure 1 (a1, b1, c1 and d1), the fibers are randomly oriented but the diameters are uniform throughout their lengths. It indicates that the viscoelastic force and electrostatic repulsions within precursor solution were successfully balanced by controlling the process parameters (humidity, flow rate, substrate rotation speed and high voltage) to suppress the influence of surface tension. While increasing concentration of the BiBr3 (as x=1 to 2, 3 and 4 %) in polymer (PAN), morphology does not change much. After the calcination the nanofibers were disintegrated to lamellar shapes with the thickness in range of 90 - 200 nm are presented in figure 1 (a2, b2, c2 and d2). Due to the decomposition of polymer, rough surfaced nano-petals were interconnected to form the fibrous morphology. From figure 1 (a2 and b2), the nano-petal morphology was not so prominent but lamellar got interwoven with each other resulting in pockets of many large microspores. These large microspores formed between stacked lamellae could be associated with increase in pore volume. The lamellar shapes starts to form from x = 1 to 3 %, at x=4%, the nanofibrous morphology completely lost and lamellar shapes are predominant and surface become rough. It is well-known that the self-aggregation effect of the Bi which turn into lamellar structure. Shang et.al [5] reported that the possible mechanism for lamellar shapes formation. In addition, the unit 2
cell of the BiOBr (tetragonal matlockite-type structure) composed of one ‘O’ atom and four ‘Br’ atoms coordinate with four ‘Bi’ atoms. One ‘Bi’ atom coordinates with four ‘O’ atoms and four ‘Br’ atoms in different bases. One [Bi2O2]2+ plates/petal is sandwiched between two Br slabs. Different petals are stacked together via weak van der Waals interactions [13]. Thus the lamellar shaped nanostructure formed with Bi-O weak interactions. The TEM image (SI-figure 1a†) shows the lamellar shapes clearly. From SI-figure 1b†, all the XRD peaks are perfectly identified as tetragonal structure of BiOBr and there are no peaks related to other impurities. Just for comparison, a thin film and the fiber sample (x=4%) is used in XRD. The lattice parameters are obtained as a = 11.42 Å and c = 8.0723Å. The calculated crystallite size is 3.017 Å. The calculated surface area (Sa) of the lamellar structures is about 77.22 m2/g (calculated from the eqn. given in Ref.[14]) is in close agreement with literature reported [15]. The composition of the product was determined by X-ray energy-dispersive spectroscopy (EDS), as shown in SI-Figure 2†, which unambiguously demonstrates the existence of Bi, O and Br elements in the product, along with C element attributing to the conductive carbon tape substrate used for the SEM observations. The EDS elemental mapping (SI-Figure 2†) further confirms that these elements were uniformly distributed in the BiOBr lamellar shaped catalysts. The XPS survey spectrum (SI-figure 3a†) is composed of elements Bi, O, Br and C. The peak at the binding energy about 284.95 eV is attributed to C 1s. In the high resolution spectrum (SIfigure 3b†) of Bi 4f revealed that the strong peaks of 4f7/2 at 159.2 eV and 4f5/2 at 164.4 eV doublet (Δso = 5.2 eV) is due to spin–orbit coupling. The band shape is in agreement with the presence of Bi3+ species in a BiOBr network according to the previous reports [16]. The peak at 530.5 eV in O 1s, assigns O-2 in BiOBr (SI-figure 3c†). From SI-figure 3d†, a strong peak at 68.1 eV (Br 3d5/2) and a broad peak 69.0 eV (Br 3d3/2) identified as Br-1 anion. Then PCA experiments were carried out and presented in SI-figure 4†. The photodecomposition of the sample at x=3% is faster within 80 min, but the best PCA was noticed at x=2% where mere complete degradation occurred. This might be due to pocket like microspores having high surface area which can facilitate more available active sites for the photocatalysis reaction. ARS dye degradation with respect to time at different concentrations of the BiOBr is presented in SIfigure 5†. From figure 2, the black dye (without catalyst), there is no indication of degradation. After adding the BiOBr catalyst, the PCA performance is enhanced approximately 83.89% (x= 2%) within 100 min. Therefore, the enhanced PCA was because of the lamellar shaped BiOBr 3
nanocatalysts. The adsorption capacity of these structures exhibit nearly 20 % is stronger than that of 2D nanoparticles [17]. In general, the PCA performance of the semiconductors is closely related to its characteristics of crystallinity, electronic structure (VB and CB), mobility of the carriers and band gap energy among other. In case of Bi based catalysts, the VB consists of hybridized orbitals of 2p of oxygen and 6s of Bi, and the CB is composed of the 6p orbital of Bi which possesses the highly reductive capability. VB and CB could determine the reductive and oxidative ability of the catalyst, respectively, while the mobility of the charge carriers could determine the photocatalytic efficiency [18]. Thus, once the photon (h) of energy illuminated on BiOBr catalyst, the effective separation of photogenerated charge carriers are favourable for the PCA. During the photocatalysis, Bi-O square anti-prisms could generate many oxygen defects because of the unstable bonding between Bi and O. The BiOBr exhibited (at x=3% of BiBr3 in PAN) the effective photon utilization and allowed the photoabsorption to increase more (e-) and hole (h+) pairs, to promote the PCA. The experimentally observed parameters are listed in Table 1. From table 1, x=3% of BiBr3 doping gives the fast reaction. The PCA performance was observed in presence of BiOBr lamellae shaped catalyst. Table 1: parameters obtained from the photocatalysis experiments. Concen(x%)
Rate of activity
Reaction efficiency (%)
Rate constant
-1
(min )
Half life time (t1/2) (min)
1
0.197
80.35
0.0191
36.6
2
0.161
83.89
0.0190
37.1
3
0.321
82.27
0.0210
33.49
4
0.177
67.94
0.0101
70.5
The possible band alignment and charge carrier mechanism is proposed in figure 3. Once the photon of light illuminate on the BiOBr catalyst, the free electron will be (energy E > Eg = 2.88 eV) excited. The excited charge carriers (e-) move to the CB. The electrons accumulated on the CB of BiOBr react with O2 adsorbed on the surface of nanocatalyst to generate oxygen radical (∙O2). This reactive radical, eventually leads to the degradation of ARS. From the figure 3, the CB and VB edges estimated by using electronegativity theory as proposed by the several groups [19, 20] and modified by the Liang Kong et.al [21]. According to the electronegativity theory photogenerated holes have a strong oxidation potentials and serve as best active sites for photodegradation. The better PCA was observed for the 3% BiBr3 doping is assumed as critical 4
concentration. Beyond this critical concentration (x=4%) excessive of BiBr3 can object the active sites of BiOBr and hinders light penetration, that is why PCA is poor at x=4%. 3. Conclusions: BiOBr lamellae shaped nano structures are successfully synthesized via electrospinning and post calcination at 500 C in air. These lamellae confirmed with SEM and TEM and other series of characterization techniques. From the XRD, tetragonal crystal structure of BiOBr is confirmed and the surface area is calculated and compared with literature reported value. The concentration level of BiBr3 in PAN at about x=3 %, BiOBr exhibited the better PCA performance in order to degrade the ARS dye. The best catalytic activity within lower reaction times has been estimated and its half life time is about ~ 33.49 min calculated. A plausible VB and CB band alignment structure is proposed and discussed with respect to SHE standard potential.
4. References: [1] Babu VJ, Bhavatharini RSR, Ramakrishna S. Electrospun BiOI nano/microtectonic plate-like structure synthesis and UV-light assisted photodegradation of ARS dye. RSC Adv. 2014;4:19251-6. [2] An H, Du Y, Wang T, Wang C, Hao W, Zhang J. Photocatalytic properties of BiOX (X = Cl, Br, and I). Rare Metals. 2008;27:243-50. [3] Wang W, Huang F, Lin X, Yang J. Visible-light-responsive photocatalysts xBiOBr–(1−x)BiOI. CatalCommun. 2008;9:8-12. [4] Shenawi-Khalil S, Uvarov V, Fronton S, Popov I, Sasson Y. A novel class of heterojunction photocatalysts with highly enhanced visible light photocatalytic performances: yBiO(ClxBr1−x)–(1 − y) bismuth oxide hydrate ApplCatalB. 2012;117-118:148-55. [5] Shang M, Wang W, Zhang L. Preparation of BiOBr lamellar structure with high photocatalytic activity by CTAB as Br source and template. JHazardMater. 2009;167:803–9. [6] Babu VJ, Satheesh KK, Trivedi DC, Murthy VRK, Natarajan TS. Electrical Properties of Electrospun Fibers of PANI-PMMA Composites. JEngdFiberFabrics. 2007;2:25-31. [7] Babu VJ, Kumar VSP, Subha GJ, Kumari RV, Natarajan TS, Nair AS, et al. AC Conductivity Studies on PMMA-PANI (HCl) Nanocomposite Fibers Produced by Electrospinning. JEngdFiberFabrics. 2011;6:54-9. [8] Babu VJ, Rao RP, Nair AS, Ramakrishna S. Nitrogen-doped rice grain-shaped titanium dioxide nanostructures by electrospinning: frequency and temperature dependent conductivity. JApplPhys. 2011;110:064327. [9] Wang C, Shao C, Liu Y, Zhang L. Photocatalytic properties BiOCl and Bi2O3 nanofibers prepared by electrospinning. Scripta Materiallia. 2008;59:332-5. [10] Liu Z, Sun DD, Guo P, Leckie JO. An Efficient Bicomponent TiO2/SnO2 Nanofiber Photocatalyst Fabricated by Electrospinning with a Side-by-Side Dual Spinneret Method. Nano Lett. 2007;7:1081-5. [11] Babu VJ, Nair AS, Peining Z, Ramakrishna S. Synthesis and characterization of rice grains like Nitrogen-doped TiO 2 nanostructures by electrospinning–photocatalysis. MaterLett. 2011;65:3064-8. [12] Babu VJ, Kumar MK, Murugan R, Khin MM, Rao RP, Nair AS, et al. Photocatalytic hydrogen generation by splitting of water from electrospun hybrid nanostructures. IntJHydrogen Energy. 2013;38:4324–33. 5
[13] Zhang D, Li J, Wang Q, Wu Q. High {001} facets dominated BiOBr lamellas: facile hydrolysis preparation and selective visible-light photocatalytic activity. J MaterChem A. 2013;1:8622-9. [14] babu VJ, Vempati S, Ertas Y, Uyar T. Excitation dependent recombination studies on SnO2/TiO2 electrospun nanofibers. RSC Adv. 2015;5:66367-75. [15] Wei X-X, Cui H, Guo S, Zhao L, Li W. Hybrid BiOBr–TiO2 nanocomposites with high visible light photocatalytic activity for water treatment. JHazardMater. 2013;263:650-8. [16] Shi X, Chen X, Chen X, Zhou S, Lou S, Wang Y, et al. PVP assisted hydrothermal synthesis of BiOBr hierarchical nanostructures and high photocatalytic capacity. ChemEngJ. 2013;222:120–7. [17] Dai K, Peng T, Ke D, Wei B. Photocatalytic hydrogen generation using a nanocomposite of multiwalled carbon nanotubes and TiO2 nanoparticles under visible light irradiation. Nanotechnology. 2009;20:125603. [18] Li H, Liu J, Liang X, Hou W, Tao X. Enhanced visible light photocatalytic activity of bismuth oxybromide lamellas with decreasing lamella thicknesses. J MaterChem A. 2014;2:8926-32. [19] Mulliken RS. Electronic Structures of Molecules XI. Electroaffinity, Molecular Orbitals and Dipole Moments. JChemPhys. 1935;3:573. [20] Nethercot AH. Prediction of Fermi Energies and Photoelectric Thresholds Based on Electronegativity Concepts. Phys Rev Lett. 1974;33:1088–91. [21] Kong L, Jiang Z, Xiao T, Lu L, Jones MO, Edwards PP. Exceptional visible-light-driven photocatalytic activity over BiOBr–ZnFe2O4 heterojunctions. Chem Commun. 2011;47:5512–4.
Figure Captions Figure 1: SEM images a1, b1, c1 and d1 shows as-spun nanofibers and a2, b2, c2, and d2 shows after sintering lamellar structures of BiOBr concentrations varied from 1, 2, 3, and 4% (w/v) respectively. Figure 2: Photocatalytic reactivity of BiOBr at (a) x=1%, (b) x=2%, (c) x=3% and (d) x=4% (w/v) under white light irradiation. Figure 3: Schematic representation for energy-band diagram of BiOBr and ARS dye degradation over BiOBr.
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Figure 3:
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Graphical Abstract
Highlights BiOBr lamellae nanostructures are synthesized via electrospinning and post calcination. The time dependent photocatalytic activity under white light illumination found. The time dependent photocatalytic activity is shown first order pseudo rate kinetics. The possible band structure alignment is proposed with respect to SHE.
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