Materials Science & Engineering B 227 (2018) 116–121
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Synthesis and characterization of PbS nanoparticles in an ionic liquid using single and dual source precursors Zikhona Tshemese, Malik Dilshad Khan, Sixberth Mlowe, Neerish Revaprasadu
MARK
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Department of Chemistry, University of Zululand, Private Bag X1001, Kwa-Dlangezwa 3886, South Africa
A R T I C L E I N F O
A B S T R A C T
Keywords: Lead ethyl xanthogenate Ionic liquid 1-Ethyl-3-methylimidazolium methanesulfonate PbS Single source precursor
We report a green route for the synthesis of PbS nanoparticles in an imidazolium based ionic liquid (IL) (1-ethyl3-methylimidazolium methanesulfonate) using single source and dual source precursor methods. Lead ethyl xanthogenate complex was used as single source molecular precursor for the synthesis of PbS nanoparticles, whereas in dual source approach organic sulfur (1-dodecanethiol) and inorganic sulfur (sodium sulfide) sources were used to observe the suitability of different sulfur sources on formation of PbS nanoparticles. The results showed that the temperature has an effect on the as prepared nanoparticles for both routes. X-ray diffraction (XRD) studies confirmed formation of cubic phase of PbS from both routes. Electron microscopy techniques showed that the nanoparticle morphologies differed significantly depending on the synthetic factors such as temperature, nature of precursors and ranged from spherical to cubic shapes.
1. Introduction The use of ionic liquids in nanomaterial synthesis is in keeping with the principles of green chemistry [1]. These principles impart knowledge about the importance of designing chemical products and processes which are environmentally friendly and at the same time reducing negative impacts to human health [2]. Ionic liquids are associated with properties such as negligible vapour pressure, non-flammability, high ionic conductivity, good thermal and chemical stability, and tunable solubility for both organic and inorganic molecules [3]. These properties make them ideal substitutes for traditional organic solvents. The first use of ionic liquids as a solvent was reported by Dai and coworkers [4]. Green and co-workers reported the synthesis of CdSe nanocrystals passivated by an ionic liquid (trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentylphosphinate) [5]. Cubic PbS nanoparticles have been prepared in 1-butyl-3-methylimidazolium tetrafluoroborate using an ionic liquid as the reaction media [6]. Ionic liquids with BF6- as ions are known to decompose and produce toxic species such as hydrofluoric acid in the presence of water. Therefore the use of halogen free compounds such as alkyl sulfonate ions may be good alternatives [7]. Star-like PbS nanoclusters have been prepared in 1ethyl-3-methylimidazolium ethyl sulfate [8]. Similar ionic liquids such as 1-butyl-3-methylimidazolium bis(triflylmethyl-sulfonyl) imide have been used for the synthesis of iron oxide nanorods and nanocubes [9]. PbS is a IV–VI semiconductor that has a small direct band gap energy of 0.41 eV and large exciton Bohr radius of 18 nm [10–13]. Optical
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Corresponding author. E-mail address:
[email protected] (N. Revaprasadu).
http://dx.doi.org/10.1016/j.mseb.2017.10.018 Received 6 June 2017; Received in revised form 9 October 2017; Accepted 23 October 2017 0921-5107/ © 2017 Published by Elsevier B.V.
absorption and emission in the near infrared region require semiconductors with small band gap energy, making PbS a potential candidate for electronic devices [2,14–18]. Synthetic routes to this material are of great importance especially in aiding desired properties which are influenced by the shape and size of the material. These synthetic routes differ by varying reaction parameters which include type of precursors, conditions of the reactions and techniques used. Many reports exist which make use of dual precursors in synthesizing PbS nanomaterials. Hardman et al. prepared PbS nanoparticles using PbO and bistrimethylsilyl sulfide as their starting materials [19]. Ultrasound transient absorption measurements were used to study multiple exciton generation in solutions of the synthesized nanoparticles in this study. PbBr2 has been used with sulfur as the precursor in the fabrication of PbS quantum dots [20]. Using the bromide precursor rendered slow reaction kinetics which led to smaller quantum dots compared to another halide precursor thus giving a possible way to control quantum dot growth. PbS nanowires with ellipse and parallelogram structures have been synthesized from Pb(NO3)2 and thiourea in ethylenediamine/H2O as a solvent [21]. Ji et al. used the hydrothermal route to control the morphology of PbS nanocrystals by varying the lead and sulfur sources, PbS particles in the form of star shapes and rods were obtained [14]. Using a hot injection technique enhances the quality of nanoparticles, this method has been used for the preparation of highly luminescent PbS nanocrystals [22]. Oleic acid capped PbS nanocrystals of 1–2 nm were prepared using colloidal techniques [23]. The use of single molecular precursors is a well reported route for
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thermometer and a rubber septum, 5.0 mL of an ionic liquid was heated to a desired temperature (150 °C and 200 °C). At the desired temperature, a suspension of the lead ethyl xanthogenate complex (0.130 g, 0.29 mmol) dispersed in 3.0 mL ionic liquid was injected into the hot liquid and the reaction was continued for 30 min, after which the contents were removed from heating, allowed to cool to room temperature and then washed several times with a mixture of acetone and ethanol. The product was then dried in a desiccator and used for further analysis. For method 2, two different sulfur sources were used and the details are explained as follows; in a three necked flask, lead acetate (0.098 g, 0.26 mmol) was added into 5.0 mL of the ionic liquid and the mixture was heated up to the desired temperature (200 °C and 250 °C) followed by an injection of 1.0 mL (4.2 mmol) dodecanethiol, then the reaction was allowed to continue for 1 h. The product was washed with a mixture of acetone and ethanol several times and water to ensure that the unreacted salts are removed. The product was then dried in a desiccator and dispersed in ethanol for further analysis. In another system, lead acetate (0.098 g, 026 mmol) and 5.0 mL 1-ethyl-3-methylimidazolium methanesulfonate were heated in a three necked flask. Sodium sulfide powder (0.020 g, 0.26 mmol) was dispersed in 3.0 mL of ionic liquid and injected into the hot solution of lead acetate at desired temperature (150 °C and 200 °C. After injection, the reaction was allowed to continue for 30 min, after which the reaction was stopped and allowed to cool naturally to room temperature. After cooling, the solution was washed with a mixture of acetone and ethanol and finally with water to ensure the complete removal of unreacted salts. The product was dried and dispersed in ethanol for further analysis.
synthesis of metal chalcogenide nanomaterials [24–26]. One of the advantages of the route is the elimination of defects as the metal is already coordinated to the chalcogenide source. Earlier research on the use of these precursors include a study by Fainer et al. where PbS thin films were grown by low pressure chemical vapour deposition using dithiocarbamates complexes [27]. Aerosol assisted chemical vapour deposition was used for PbS thin film synthesis using dithiocarbamate or xanthate precursors [28,29]. Trindade and co-workers reported nanodispersed PbS prepared from a thermolysis of lead dithiocarbamato complexes [30]. Different morphologies ranging from polyhedral to flower-like structures have been obtained from a solvothermal decomposition of lead diethyldithiocarbamate at 150–180 °C [31]. Apart from the extensively studied dithiocarbamate complexes as precursors, heterocyclic lead dithiocarbamates, [13] lead dialkyl dithiophos phates [32], lead hydroxy thiocyanate [33] and lead xanthates [34,35] have been investigated for the synthesis of lead sulfide nanomaterials. It is well stipulated that variation of growth temperature, reaction time or capping agent can result in different nanoparticle morphology as reported by Lee and others, where they demonstrated shape evolution of PbS nanocrystals from star-shaped structures to truncated octahedron and cubes [36]. The concentration of the precursor also has an effect on modifying the morphology of nanomaterials. Plante et al. found an increasing degree of branching with the increase in the precursor concentration for lead sulfide nanocrystals [37]. Herein we describe the synthesis of PbS nanoparticles in 1-ethyl-3methylimidazolium methanesulfonate, using single and dual source precursors. Furthermore, the effect of various parameters such as temperature, time and nature of precursors on as synthesized nanoparticles was also observed systematically. The use of this ionic liquid which is the green solvent for the preparation of PbS nanoparticles has been a success. To the best of our knowledge, this ionic liquid (1-ethyl3-methylimidazolium methanesulfonate) has never been reported for the synthesis of PbS nanoparticles. This approach provides clean and greener method for the synthesis of monodispersed, crystalline nanoparticles in an environmentally friendly matrix.
2.4. Characterization
Lead acetate trihydrate (≥99.9%, Sigma-Aldrich), 1-dodecanethiol (Sigma-Aldrich), 1-ethyl-3-methylimidazolium methanesulfonate (Sigma-Aldrich), sodium sulfide (60–62% Sigma Aldrich), potassium ethyl xanthogenate (96%, Sigma-Aldrich), acetone (Prestige laboratory) and ethanol (≥99.8%, Sigma-Aldrich), were used, as received, with no further purification.
Powder X-ray diffraction (p-XRD) patterns were recorded at room temperature in the high-angle 2θ range of 20–80° by using a Bruker AXS D8 Advance diffractometer equipped with nickel-filtered Cu-Kα radiation (λ = 1.5418 Å) at 40 kV and 40 mA. Transmission electron microscopy (TEM) was conducted with a JEOL 1400 instrument equipped with a Megaview III camera, at an accelerating voltage of 120 kV. Scanning electron microscopy measurements were carried out by a Zeiss Ultra Plus FEG equipped with an Oxford detector EDX at 20 kV. Thermogravimetric analysis (TGA) was carried out with a Perkin Elmer Pyris 6 Diamond TGA model at a heating rate of 10 °C min−1 under nitrogen atmosphere. Infrared radiation (IR) spectra were recorded with a Perkin–Elmer Spectrum Two FTIR spectrometer in the range 4000–450 cm−1. The elemental analysis of C, H, S and Pb was carried out on a thermos scientific flash 2000 organic elemental analyzer which had been calibrated with standard reference materials.
2.2. Synthesis of lead ethyl xanthogenate
3. Results and discussion
Lead ethyl xanthogenate was synthesized by the method of Iimura et al. [38] Yield: 74%. Elemental analysis: C6H10PbO2S4: Calculated (%): C, 16.01; H, 2.24; S, 28.16; Pb, 46.09. Found (%): C, 16.33; H, 2.44; S, 28.49; Pb, 46.18.
3.1. Synthesis of PbS using lead ethyl xanthogenate
2. Experimental 2.1. Materials
Lead ethyl xanthogenate complex has been used as a single source precursor and the ionic liquid, 1-ethyl-3-methylimidazolium methanesulfonate (ESI Fig. S1) was used as a reaction medium in this approach (method 1). The decomposition of lead ethyl xanthogenate complexes is known to follow Chugaev elimination mechanism [39]. A single step decomposition (ESI Fig. S2) was observed in the temperature range of 110 °C to 150 °C with major mass loss of 46%. The final residue was found to be 54%, which shows formation of PbS (ca. 53%). The decomposition of xanthogenate complexes usually undergo β-elimination mechanism, involving a six membered cyclic transition state. An olefin is formed as a by-product and sulfur of thiocarbonyl moiety is protonated. The transition state undergoes rearrangement with evolution of OCS molecule along with formation of metal–SH group. Similarly the second xanthogenate group undergoes decomposition in the same way and leads to the formation of Pb(SH)2, which transforms to PbS with
2.3. Synthesis of PbS nanoparticles Two approaches were used for the synthesis of PbS nanoparticles from of single source precursor and dual source precursors. Lead ethyl xanthogenate was used for the single source precursor route (method 1) while lead acetate and dodecanethiol/ sodium sulfide (method 2) were used as dual sources all in the presence of the ionic liquid (1-ethyl-3methylimidazolium methanesulfonate). In both methods reaction parameters (temperature, time) were varied to observe their effect on the formation of the nanoparticles. Details for method 1 are as follows: in a three necked flask which is equipped with a reflux condenser, 117
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H
H
S
O
S
Pb
O
G
Scheme 1. Proposed decomposition mechanism of lead ethyl xanthogenate complex.
SH
S S
Pb G
Pb
+O
S
ethene -COS
HS
G
-COS
PbS + H2S
Pb
SH
(511)
(422)
(331)
(400)
(222)
(420)
(311)
(200) (111)
Intensity (a.u.)
(b)
(a) 20
30
40
50 60 2q (deg)
70
80
Pb G
study the effect of temperature on the properties of the synthesized PbS nanoparticles. The synthetic protocol of the hot injection route to nanomaterials was carried out, whereby an ionic liquid (1-ethyl-3-methylimidazolium methanesulfonate) pre-heated at desired temperatures is used as a thermolysis solvent, followed by the injection of the complex (dispersed in the same solvent). Ionic liquids are known to mechanistically stabilize nanoparticles in three ways namely electrostatic, steric and a combination of both steric and electrostatic [40]. P-XRD for the ionic liquid capped PbS nanoparticles synthesized from lead ethyl xanthogenate complex is shown in Fig. 1. From both temperatures, a pure product of PbS nanoparticles was obtained and the peaks confirm formation of the expected cubic rock salt phase of PbS. O’Brien and co-workers studied the effect of temperature on the crystalline nature of the synthesized PbS using lead(II) n-butylxanthogenate, (Pb(S2COBu)2) complex from ambient temperature to 275 °C, they found that low temperatures resulted in partial decomposition of the precursors [41]. This phase has characteristic planes of (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (3 3 1), (4 2 0), (4 2 2) and (5 1 1) (ICDD Card #: 00-005-0592). The crystal growth preference is towards the (2 0 0) plane for the nanoparticles prepared at 150 °C, whereas an increase in temperature to 200 °C, (2 2 0) growth orientation was preferred. Furthermore, the increase in temperature (200 °C) resulted in the formation of highly crystalline nanoparticles. TEM images of PbS nanoparticles synthesized at 150 °C and 200 °C using lead ethyl xanthogenate as a single source precursor, in imidazolium based ionic liquid are presented in Fig. 2. Well-defined cubic shaped particles with average particle sizes of 102 ± 4.2 nm were observed for the 150 °C reaction temperature (Fig. 2a). At an elevated temperature of 200 °C, the PbS nanoparticles appear agglomerated
(220)
HS
90
Fig. 1. Powder XRD patterns of ionic liquid capped PbS nanoparticles synthesized at temperatures of (a) 150 °C and (b) 200 °C from lead ethyl xanthogenate complex.
release of hydrogen sulfide gas. The whole mechanism is shown in Scheme 1. Previous studies on the lead ethyl xanthogenate complexes also showed a one-step decomposition with a rapid weight loss between 129 °C and 156 °C [34,35]. From the basis of the complex’s decomposition temperature, thermolysis reactions were carried out at temperatures 150 °C and 200 °C to
Fig. 2. TEM (a and b) and their corresponding SEM (c and d) images of ionic liquid capped PbS nanoparticles synthesized from lead ethyl xanthogenate complex at 150 °C and 200 °C respectively.
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of smaller particles, and thereby both the average size and the size distribution of the particles are increased. SEM images confirm the formation of cubic shaped nanoparticles for PbS obtained at 150 °C (Fig. 2c) while those obtained at 200 °C showed mixed cubic to spherical shaped morphology (Fig. 2d). The size of these nanoparticles was estimated to be 64 ± 18 nm and 55 ± 29 nm for 150 °C and 200 °C reaction temperatures respectively. EDX composition showed a 1:1 ratio of Pb: S (Table 1).
Table 1 Elemental composition of as synthesized PbS nanoparticles, obtained by EDX analysis at different conditions. S. No
Method
Reagents
Temp. °C
Pb
S
Pb:S
1
Single source route
Lead ethyl xanthate
150
52.77
47.23
1:0.9
2
Dual source route
DT Na2S
200 150
67.43 57.17
32.57 42.83
1:0.5 1:0.8
(200)
Attempts were also carried, using dual precursor sources instead of single source precursors. Two different sulfur sources (organic and inorganic) were used for the PbS preparation to observe the interaction and its effect on the morphological properties of the nanoparticles (method 2). 1-dodecanethiol was used as the organic sulfur source while sodium sulfide was used as the inorganic source. Each of these two sulfur sources was used with lead acetate and 1-ethyl-3-methylimidazolium methanesulfonate. Lead acetate and the ionic liquid were preheated to a stable desired temperature followed by an injection of 1dodecanethiol or sodium sulfide (dispersed in IL). It is observed that the reaction between lead acetate and dodecanethiol was not thermodynamically favored at temperatures below 200 °C. The addition of the thiol may lead to the formation of a lead thiolate complex because of the strong coordination between the mercapto group and lead ions [42]. This thiolate complex might not decompose easily to PbS, requiring elevated reaction temperatures. Thus decomposition of Pb-thiolate at elevated temperatures lead to the formation of PbS and R-S-R [43]. The possible mechanism of PbS formation are presented in Eqs. (1) and (2). As shown in Fig. 3a, the presence of extra peaks along with those of pure PbS indicate the partial decomposition of the lead thiolate complex at 200 °C. Furthermore, the peaks were of low intensity which indicates an amorphous product at 200 °C. When the reaction was performed at an elevated temperature of 250 °C (Fig.3b), complete decomposition of the lead thiolate complex was observed and pure crystalline PbS was obtained. The XRD pattern shows prominent peaks that can be indexed to exclusively cubic PbS without the presence of any other impurity for the reaction carried at 250 °C.
(511)
(422)
(331) (420)
(400)
(222)
(311)
Intensity (a.u.)
(220)
(111)
3.2. Synthesis of PbS nanoparticles using dual source precursors
(b)
(a) 20
30
40
50 60 2q (deg)
70
80
90
Fig. 3. Powder XRD pattern of ionic liquid capped PbS nanoparticles synthesized at temperature of (a) 200 °C and (b) 250 °C from an organic sulfur source (1-dodecanethiol).
forming large irregular shaped particles with an average diameter of 160 ± 7.2 nm (Fig. 2b). This is in agreement with the XRD results where the peaks are relatively narrow, suggesting larger sizes. Due to faster depletion of the precursor, elevated temperatures promote the Ostwald ripening process, in which larger particles grow at the expense
PBb(CH3 COO)2 + 2R−SH → Pb(S−R)2 + 2Ac−H
(1)
Fig. 4. (a) and (c) TEM at 200 °C and 250 °C and their corresponding (b) SAED and (d) SEM images of ionic liquid capped PbS nanoparticles synthesized from the organic sulfur source respectively.
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ionic nature and high reactivity. The reaction was performed at comparatively lower temperatures of 150 and 200 °C. The powder XRD patterns indicating the formation of a well-defined PbS fcc structure shown in Fig. 5. The high intensity of the (2 0 0) peak shows that the nanoparticles have a preferred orientation towards the (2 0 0) plane which makes it the dominant reflection in the diffraction pattern at both temperatures and differs from diffraction pattern of PbS synthesized by using the xanthogenate complex which shows a different preferred orientation i.e. (2 2 0) at higher temperature. Pure PbS nanoparticles were obtained at both temperatures and the XRD pattern did not show any trace of impurities. This suggests that use of sodium sulfide is more favorable and efficient as compared to the thiol, probably due to its high reactivity. The change in morphology was also confirmed by the TEM images. At low temperature, rectangular particles were observed; whereas at higher temperature comparatively smaller sized, forming branched/nanodendrites-like clusters were observed (Fig. 6). The morphology of the PbS particles obtained at both temperatures had a broad size distribution i.e. 38 ± 11.3 nm (150 °C) to 37 ± 6.8 nm (200 °C). SEM images also show that the as prepared PbS nanoparticles were small in size with almost spherical morphology (Fig. 6). On increasing the reaction temperature to 200 °C, the formation of even smaller nanoparticles was observed. The EDX analysis showed that the sample consisted of only lead and sulfur in the ratio 1:0.8 (Pb:S) as shown on Table 1. FTIR spectra of PbS nanoparticles in an ionic liquid and pure ionic liquid are shown in Fig. 7. The FTIR analysis was done for the material prepared from dual sources (lead acetate and sodium sulfide). The band observed at 3095 cm−1 can be attributed to CeH stretching of the imidazolium ring [45]. The vibrational band around 1574 cm−1 is ascribed to C ] N stretching and its shift (1244 cm−1) on the PbS spectrum represents a change in bonds which are due to new bond formation between the ionic liquid and the PbS nanoparticles. A symmetric eSO3 stretching vibration has been observed on both spectra at 1086–1177 cm−1 [46]. Again the adsorption bands found at 1460 cm−1 (IL) and its shift to 1422 cm−1 (IL capped PbS) are attributed to the skeleton stretching vibrations of the imidazole ring [47]. Qi et al. reported synthesis of star shaped PbS, and explained the strong interaction of the ionic binding heads to stabilize the high energy and charged (1 1 1) faces than uncharged (1 0 0) faces [48]. The mesylate ion probably behaves in a similar manner.
(511)
(422)
(331) (420)
(400)
Intensity (a.u.)
(222)
(311)
(220)
(200)
(111)
Z. Tshemese et al.
(b)
(a) 20
30
40
50 60 2q (deg)
70
80
90
Fig. 5. XRD pattern of ionic liquid capped PbS nanoparticles synthesized at temperature of (a) 150 °C and (b) 200 °C from an inorganic sulfur source (sodium sulfide).
Pb(S−R)2 → PbS + R-S-R
(2)
TEM image of PbS nanoparticles synthesized at lower temperature (200 °C) (Fig. 4a) using dodecanethiol show a mixture of cubic and hexagonal shapes (most with sharp corners of hexagons) with an average size of 86 ± 27.2 nm. They are almost uniform in shape/size unlike those obtained at a higher temperature (250 °C), where particles appear agglomerated (Fig. 4c). It is clear that aggregation and particle size increases with an increase in temperature. Zhao et al. [44] reported the use of 1-n-butyl-3-methylimidazolium hexafluorophosphate ionic liquid as a coordinating solvent for the synthesis of crystalline, cubic shaped PbS nanoparticles in the range of 100 nm. The selective area electron diffraction (SAED) pattern (Fig. 4b) showed formation of highly crystalline PbS nanoparticles. SEM further showed formation of irregular porous-cubic shaped PbS particles with an average particle size of 45 ± 10 nm (Fig. 4d). The EDX results showed formation of close to 1:1 (Pb:S) (Table 1). We further explored the use of an inorganic sulfur source, sodium sulfide which has good solubility in the ionic liquid, and because of its
Fig. 6. TEM (a and b) and their corresponding SEM (c and d) images of ionic liquid capped PbS nanoparticles synthesized from the inorganic sulfur source at 150 °C and 200 °C respectively.
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% Transimittance
(1244) (1133)
(b)
(a)
9) 8) 17 3 (1 (10
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Wavenumber (cm ) Fig. 7. IR spectra of (a) pure ionic liquid and (b) ionic liquid capped PbS nanoparticles synthesized from the inorganic sulfur source.
4. Conclusions PbS nanoparticles capped with ionic liquid (1-ethyl-3-methylimidazolium methanesulfonate) were successfully prepared using lead ethyl xanthogenate complex as a single source precursor and an organic (1-dodecanethiol) and inorganic (sodium sulfide) sulfur sources and lead acetate as dual source precursors. Different reaction parameters were established and used for this study to investigate their influence on the morphology and structural properties of the as-synthesized PbS nanoparticles. An increase in the reaction temperature resulted in the formation of larger particle sizes. The use of single source precursor produced well defined shaped PbS nanoparticles than dual sources route. For dual source precursors, thiol requires relatively high temperatures for a pure product to be obtained whereas sodium sulfide reacts readily even at low temperatures. The present study further confirmed the effectiveness of an ionic liquid (1-ethyl-3-methylimidazolium methanesulfonate) as a green coordinating solvent for high quality nanoparticles synthesis. We intend to further investigate the surface chemistry of the imidazolium based capped PbS nanoparticles. Acknowledgments The authors are grateful to the Department of Science and Technology (DST), the National Research Foundation (NRF) (NRF Grant number 64820), South Africa for financial support. The authors also thank the Microscopy and Microanalysis Unit (MMU) of the University of KwaZulu-Natal South Africa for transmission electron microscopy imaging. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.mseb.2017.10.018. References [1] G. Nagaraju, T. Ravishankar, K. Manjunatha, S. Sarkar, H. Nagabhushana,
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