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HgS nanohybrid: An effective photocatalyst for degradation of methylene blue
Materials Letters 250 (2019) 5–8
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Facile synthesis of novel SWCNT/HgS nanohybrid: An effective photocatalyst for degradation of methylene blue Pramendra Kumar Saini a, Nitish Kumar a, Ramesh Chandra b, Mala Nath b,⇑, Ashwani Kumar Minocha a a b
Environmental Science and Technology Division, CSIR-Central Building Research Institute, Roorkee 247667, Uttarakhand, India Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247667, Uttarakhand, India
a r t i c l e
i n f o
Article history: Received 19 February 2019 Received in revised form 12 April 2019 Accepted 23 April 2019 Available online 25 April 2019 Keywords: Carbon nanotubes Nanocomposites HgS nanoparticles Hydrothermal synthesis Photocatalysis
a b s t r a c t Mercury sulphide (HgS) nanoparticles decorated single walled carbon nanotubes (SWCNT/HgS) were synthesized via a hydrothermal route. We investigated structural and photocatalytic properties of SWCNT/ HgS nanohybrid. The diameter of pure SWCNTs and SWCNT/HgS as determined by Raman spectroscopy was 1.607 nm and 1.646 nm, respectively. Hexagonal phase of a-HgS nanoparticles decorated on SWCNTs was evidenced in HRTEM, SAED, XPS and XRD spectra. An increase in photocatalytic efficiency of SWCNT/ HgS nanohybrid (99.08%) as compared to HgS NPs (73.0%) was observed for photodegradation of methylene blue (5 ppm) under sunlight. This study suggests a promising approach to rational design of advanced heterogeneous photocatalysts for environmental remediation. Ó 2019 Published by Elsevier B.V.
1. Introduction Globally, immense efforts have been devoted for the development of advanced oxidation processes (AOPs) involving semiconductor-based photocatalysts, particularly transition metal oxides/sulphides nanoparticles (TiO2, ZnO, Fe2O3, CuS, CdS and ZnS) for degradation of organic dyes and other pollutant molecules [1–5]. Further, the processes involving heterogeneous photocatalysis have some advantages, for example ambient operating temperature and pressure, complete mineralization and low operating cost. But the developed semiconductor-photocatalysts have low quantum yield with lower solar energy conversion efficiency, thus limiting their practical application [6]. Further, they are not very stable in aqueous medium under light and they may lead to corrosion, therefore, causing metal toxicity, and the typical examples are low band gap transition metal sulphides [7]. Furthermore, the agglomeration of semiconductor nanoparticles decreases their photocatalytic efficiency which can be partially improved by their immobilization via encapsulation into porous materials such as zeolites and metal organic frame works [7–11]. In addition, their photocatalytic activity may also be enhanced by the incorporation of carbon nanotubes (CNTs) [12]. Carbon nanotubes (CNTs) intrinsically have an extremely high surface area and exhibit electron acceptor properties [13]. ⇑ Corresponding author. E-mail address: [email protected] (M. Nath). https://doi.org/10.1016/j.matlet.2019.04.090 0167-577X/Ó 2019 Published by Elsevier B.V.
Single-walled carbon nanotubes (SWCNTs) are more conductive [14], automatically uniform and exhibit well defined fibrous structure as compared to multi-walled carbon nanotubes (MWCNTs). In addition, the predicting behaviour of MWCNTs is complex due to inner wall reaction, and SWCNTs have a fixed charge transport phenomenon across the length of tube [15]. Further, SWCNTs associated with chemical species via chemical reactions on their surface attain exclusive catalytic support. Furthermore, the adsorption capacity of functionalized or acid treated SWCNTs increases due to the generation of many functional groups on their sidewalls [16]. HgS NPs show an optical band gap between 1.9 and 2.6 eV, and a-HgS possesses a hexagonal structure whereas b-HgS shows cubic structure. The band gap of HgS NPs lies in UV–Visible region which can be tuned with their size [17], therefore, they are widely used in ultrasonic transducers, image sensor, electrostatic image materials and photo-electron conversion devices [18]. The photocatalytic activity of a semiconducting nanomaterial having small band gap is based on recombination rate of photo generated electrons and holes but there is a drawback of their fast recombination rate. The SWCNT/NPs nanohybrid creates a Schottky barrier junction, which is liable to increase the recombination time, as a result of charge separation between two materials through depletion layer [19]. Furthermore, electron comes from high fermi level to lower fermi level and therefore, the behaviour of SWCNTs and semiconducting nanoparticles has been found as p-type and n-type semiconductors, respectively [20] (Fig. 1).
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dispersed ultrasonically in deionised water. To this was added 0.03 M mercury acetate and 0.06 M thiourea solution dropwise under constant stirring. The resulting reaction mixture was kept into 50 ml teflon lined autoclave in a controlled furnace at 160 °C for 5 h. After vacuum filtration, the product was washed thoroughly with deionised water, followed by ethanol, and finally dried at 60 °C for 12 h. The details of synthesis as well as other experimental methods for characterization are given in supporting file.
3. Results and discussion Fig. 1. A schematic representation of e /h+ transfer mechanism in photocatalytic degradation of MB by SWCNT/HgS nanohybrid in aqueous solution. The solid lines indicate the CB and VB of SWCNT/HgS hybrid whereas dashed lines indicate the generation of fermi levels. Here Eg represents the band gap energy.
The present study describes the facile synthesis of new SWCNT/ HgS nanohybrid via a hydrothermal route. SWCNT/HgS nanohybrid exhibits excellent photocatalytic activity for photodegradation of MB. To the best of our knowledge, no report has been found on the synthesis of SWCNT/HgS nanohybrid, and its application for photodegradation of MB. 2. Experimental SWCNT/HgS nanohybrid was synthesized by the decoration of HgS nanoparticles on SWCNTs. The functionalized SWCNTs were
The hydrothermal reaction of SWCNTs with HgS nanoparticles (formed in situ) in a 1:1 M ratio leads to the formation of SWCNT/HgS nanohybrid as shown in Scheme S1. XRD patterns of pristine SWCNTs and SWCNT/HgS nanohybrid are shown in Fig. 2 (i), while that of pure HgS nanoparticles is shown in Fig S1. The hexagonal phase of a-HgS (cinnabar) is clearly evident in XRD pattern of SWCNT/HgS. Further, the diffraction pattern illustrates the preferential alignment of hexagonal HgS NPs on the surface of SWCNTs which is also well supported by IR (Fig. 2(ii)), as new peaks are observed at 1109 and 1384 cm 1 corresponding to the sulphur bonding in nanohybrid [21]. The average size of HgS nanocrystallites (43.33 nm) has been calculated by using DebyeScherrer equation (Eq. S1) [21]. The back scattered electron (BSE) FESEM image (Fig. 2(iii)) clearly indicates encapsulation of HgS nanoparticles among the inter-tubular spacing of SWCNTs bundles which is confirmed by EDX spectrum (Fig. S2). TEM images (Fig. 2(iv–v)) also clearly
Fig. 2. (i) XRD patterns of (a) pristine SWCNT, (b) SWCNT/HgS hybrid. (ii) FTIR images of (a) pristine SWCNTs, (b) purified and functionalized SWCNTs, (c) SWCNTs/HgS hybrid. (iii) FESEM (BSE) image of SWCNT/HgS hybrid. TEM images of Pristine SWCNTs, Purified and functionalized SWCNTs, and SWCNT/HgS hybrid are shown as (iv), (v), and (vi), respectively, and (vii) Lattice fringe spacing in SWCNT/HgS hybrid.
P.K. Saini et al. / Materials Letters 250 (2019) 5–8
7
Fig. 3. (i) Raman spectra of SWCNT/HgS hybrid: (a) shows RBM band and (b) shows D and G band (ii) Typical tauc plots of HgS, and SWCNT/HgS hybrid by direct method. (iii) Photo-degradation measurements of MB (5 ppm, pH 6.03) with respect to time in presence of HgS NPs, and SWCNT/HgS. (iv) Kinetics of photodegradation of cationic dye.
reveal the removal of amorphous carbon and metal impurities during the purification and functionalization of SWCNTs. Further, TEM micrograph (Fig. 2(vi)) of SWCNT/HgS depicted that a-HgS nanoparticles (5–10 nm) are decorated on the walls of SWCNTs. Additionally the inter-planar (‘d’) spacings of the microstructure measured through HRTEM (Fig. 2(vii)) correspond to (1 1 4) and (0 1 5) planes of a-HgS nanoparticles. The concentric rings and SAED pattern (Fig. S3) indicate the polycrystalline nature of nanohybrid [22]. XPS full scan spectrum of SWCNT/HgS nanohybrid (Fig. S4a) exhibits peaks of C 1s, O 1s, S 2p3/2, 2p1/2 and Hg 4f7/2, 4f5/2 corresponding to binding energy of 284.0 eV, 531.0 eV 161.20, 162.5 eV, and 101.15, 104.15 eV, respectively, with inset table showing their percentages. The details of XPS analysis with expanded orbital fitted spectra of each element are given in supporting file (Fig. S4). A band in Raman spectrum assigned to the radial breathing mode (RBM) is observed at 157.99 cm 1 (size 1.607 nm) in f-SWCNT which is red shifted to 154.30 cm 1 (size 1.646 nm) in SWCNT/HgS nanohybrid (Fig. 3(ia)). The diameter of SWCNT bundles of both bands is calculated by Eq. S2 [22]. D and G bands are shown in Fig. 3(ib), no appreciable shift of D band is observed which indicates that all SWCNT bundles (carbon species) remain in the similar order of lattice during the formation of nanohybrid. However, G band shows highly ordered graphite like sp2 hybridized species [22,23]. The ID/IG ratio of f-SWCNT and SWCNT/HgS nanohybrid is 0.17 and 0.054, respectively, and these values indicate that the crystallinity is further enhanced by encapsulation of nanoparticles in inter-tubular space [21,23]. The optical properties of SWCNT/HgS nanohybrid are investigated by UV-DRS. The band gap of HgS and SWCNT/HgS is 1.96 and 2.91 eV, respec-
tively, for direct transition (Fig. 3(ii)) (Eq. S3) and 1.67 and 2.11 eV, respectively, for indirect transition (Fig. S5) (Eq. S4), which can be attributed to the definite shift in microenvironment of HgS NPs decorated on the surface of SWCNTs. The photocatalytic performance of HgS and SWCNT/HgS nanohybrid has been investigated for photodegradation of methylene blue (MB) under sunlight (average intensity: 24675 ± 100 l) at 20 ± 5 °C (details of photodegradation are given in the supporting file). For the determination of an effective amount of photocatalysts, it is observed that HgS NPs (4 mg) photodegrade 73.0% of MB (5 ppm), whereas the same amount of SWCNT/HgS nanohybrid shows 99.08% photodegradation efficiency (Fig. 3(iii)) at pH 6.03 under similar conditions (Fig. S6, Table ST-1). Furthermore, it has also been experimentally verified that %photodegradation gradually decreases (99.08%–93.3%) with increasing the concentration of MB (from 5 to15 ppm) (Fig. S7 and Table ST-2). The reactive oxygen species (ROS) such as O2 , OH and O2 [11] are responsible for photodegradation of MB by SWCNT/HgS nanohybrid, which follows the first order kinetics (Eq. S7) as shown in Fig. 3(iv). The photodegraded products of MB were analysed and identified by GC– MS and its possible degradation pathway has been proposed (Scheme-S2). FTIR and XRD spectra of nanohybrid recorded after 6 months remain unchanged indicating its high stability (Figs. S9 and S10). 4. Conclusion The SWCNT/HgS nanohybrid has been prepared in one step by hydrothermal method and characterized as well. SWCNT/HgS
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P.K. Saini et al. / Materials Letters 250 (2019) 5–8
nanohybrid exhibits excellent photocatalytic activity and can be employed for the removal of MB dye. Due to lower band gap and chemical stability of SWCNT/HgS nanohybrid, it can also be used in broad areas as image sensors, biological display, photo detectors, and other optical devices. Conflict of interest None declared. Acknowledgements Dr. Pramendra Kumar Saini is thankful to DST-SERB, Government of India for financial support under Young Scientist scheme (SB/FT/CS-136/2014). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.matlet.2019.04.090. References [1] C. Yu, F. Wang, J. Chen, et al., J. Mol. Catal. A: Chem. 411 (2016) 1–8.
[2] A. Buthiyappan, A.R.A. Aziz, W.M.A.W. Daud, Rev. Chem. Eng. 32 (1) (2016) 1– 47. [3] (a) M.H. Alonso, F. Fresno, S. Suareza, J. Coronado, Energy Environ. Sci. 2 (2009) 1231–1257; (b) C. Lai, M. Zhang, B. Li, et al., Chem. Eng. J. 358 (2019) 891–902. [4] S. Dong, J. Feng, M. Fan, et al., RSC Adv. 5 (2015) 14610–14630. [5] D.A. Kean, K.G. McGuigan, P.F. Ibanez, et al., Catal. Sci. Tech. 4 (2014) 1211– 1226. [6] A. Ajmal, I. Majeed, R.N. Malik, et al., RSC Adv. 4 (2014) 37003–37026. [7] A. Mallik, M. Nath, S. Mohiyuddin, et al., ACS Omega 3 (2018) 8288–8308. [8] H. Faghihian, A. Bahranifard, Ira. J. Catal. 1 (2011) 45–50. [9] (a) S. Anandan, M. Yoon, J. Photochem. Photobiol. C: Photochem. Rev. 4 (2003) 5–18; (b) C.C. Wang, C.K. Lee, M.D. Lyu, et al., J. Dyes & Pigments xx (2007) 1–8. [10] R. Chandra, S. Mukhopadhyay, M. Nath, Mat. Lett. 164 (2016) 571–574. [11] (a) R. Chandra, M. Nath, Chem. Select. 2 (2017) 7711–7722; (b) B. Li, C. Lai, G. Zeng, et al., ACS Appl. Mater. Interfaces 10 (2018) 18824– 18836. [12] F. Taleshi, J. Appl. Spectrosc. 82 (2015) 303–306. [13] S. Iijima, T. Ichihashi, Nature 363 (1993) 603–605. [14] A. Kongkanand, R.M. Domínguez, P.V. Kamat, Nano Lett. 7 (2007) 676–680. [15] J.-P. Issi, J.-C. Charlier, Electrical transport properties in carbon nanotubes, in: K. Tanaka, T. Yamabe, K. Fukui (Eds.), The Science and Technology of Carbon Nanotubes, Elsevier Ltd., 1999, pp. 107–127. [16] S.K. Pillai, S.S. Ray, M. Moodley, J. Nanosci. Nanotechnol. 7 (2007) 3011–3047. [17] B.K. Patel, S. Rath, S.N. Sarangi, et al., Appl. Phys. A 86 (2007) 447–450. [18] S.A. Pande, B. Pandit, B.R. Shankpal, J. Colloid Interface Sci. (2017). [19] K. Woan, G. Pyrgiotakis, W. Sigmund, Adv. Mater. 21 (2009) 2233–2239. [20] A. Kongkanand, P.V. Kamat, ACS Nano 1 (2007) 13–21. [21] R. Paul, P. Kumbhakar, A.K. Mitra, J. Exp. Nanosci. 5 (2010) 363–373. [22] Y. Zhang, X. He, L. Wang, et al., Materials 9 (2016) 718–724. [23] M. Nath, P.V. Teredesai, D.V.S. Mathu, et al., Curr. Sci. 85 (2003) 956–960.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Facile synthesis of novel SWCNT/HgS nanohybrid: An effective photocatalyst for degradation of methylene blue Pramendra Kumar Saini a, Nitish Kumar a, Ramesh Chandra b, Mala Nath b,⇑, Ashwani Kumar Minocha a a b
Environmental Science and Technology Division, CSIR-Central Building Research Institute, Roorkee 247667, Uttarakhand, India Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247667, Uttarakhand, India
a r t i c l e
i n f o
Article history: Received 19 February 2019 Received in revised form 12 April 2019 Accepted 23 April 2019 Available online 25 April 2019 Keywords: Carbon nanotubes Nanocomposites HgS nanoparticles Hydrothermal synthesis Photocatalysis
a b s t r a c t Mercury sulphide (HgS) nanoparticles decorated single walled carbon nanotubes (SWCNT/HgS) were synthesized via a hydrothermal route. We investigated structural and photocatalytic properties of SWCNT/ HgS nanohybrid. The diameter of pure SWCNTs and SWCNT/HgS as determined by Raman spectroscopy was 1.607 nm and 1.646 nm, respectively. Hexagonal phase of a-HgS nanoparticles decorated on SWCNTs was evidenced in HRTEM, SAED, XPS and XRD spectra. An increase in photocatalytic efficiency of SWCNT/ HgS nanohybrid (99.08%) as compared to HgS NPs (73.0%) was observed for photodegradation of methylene blue (5 ppm) under sunlight. This study suggests a promising approach to rational design of advanced heterogeneous photocatalysts for environmental remediation. Ó 2019 Published by Elsevier B.V.
1. Introduction Globally, immense efforts have been devoted for the development of advanced oxidation processes (AOPs) involving semiconductor-based photocatalysts, particularly transition metal oxides/sulphides nanoparticles (TiO2, ZnO, Fe2O3, CuS, CdS and ZnS) for degradation of organic dyes and other pollutant molecules [1–5]. Further, the processes involving heterogeneous photocatalysis have some advantages, for example ambient operating temperature and pressure, complete mineralization and low operating cost. But the developed semiconductor-photocatalysts have low quantum yield with lower solar energy conversion efficiency, thus limiting their practical application [6]. Further, they are not very stable in aqueous medium under light and they may lead to corrosion, therefore, causing metal toxicity, and the typical examples are low band gap transition metal sulphides [7]. Furthermore, the agglomeration of semiconductor nanoparticles decreases their photocatalytic efficiency which can be partially improved by their immobilization via encapsulation into porous materials such as zeolites and metal organic frame works [7–11]. In addition, their photocatalytic activity may also be enhanced by the incorporation of carbon nanotubes (CNTs) [12]. Carbon nanotubes (CNTs) intrinsically have an extremely high surface area and exhibit electron acceptor properties [13]. ⇑ Corresponding author. E-mail address: [email protected] (M. Nath). https://doi.org/10.1016/j.matlet.2019.04.090 0167-577X/Ó 2019 Published by Elsevier B.V.
Single-walled carbon nanotubes (SWCNTs) are more conductive [14], automatically uniform and exhibit well defined fibrous structure as compared to multi-walled carbon nanotubes (MWCNTs). In addition, the predicting behaviour of MWCNTs is complex due to inner wall reaction, and SWCNTs have a fixed charge transport phenomenon across the length of tube [15]. Further, SWCNTs associated with chemical species via chemical reactions on their surface attain exclusive catalytic support. Furthermore, the adsorption capacity of functionalized or acid treated SWCNTs increases due to the generation of many functional groups on their sidewalls [16]. HgS NPs show an optical band gap between 1.9 and 2.6 eV, and a-HgS possesses a hexagonal structure whereas b-HgS shows cubic structure. The band gap of HgS NPs lies in UV–Visible region which can be tuned with their size [17], therefore, they are widely used in ultrasonic transducers, image sensor, electrostatic image materials and photo-electron conversion devices [18]. The photocatalytic activity of a semiconducting nanomaterial having small band gap is based on recombination rate of photo generated electrons and holes but there is a drawback of their fast recombination rate. The SWCNT/NPs nanohybrid creates a Schottky barrier junction, which is liable to increase the recombination time, as a result of charge separation between two materials through depletion layer [19]. Furthermore, electron comes from high fermi level to lower fermi level and therefore, the behaviour of SWCNTs and semiconducting nanoparticles has been found as p-type and n-type semiconductors, respectively [20] (Fig. 1).
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dispersed ultrasonically in deionised water. To this was added 0.03 M mercury acetate and 0.06 M thiourea solution dropwise under constant stirring. The resulting reaction mixture was kept into 50 ml teflon lined autoclave in a controlled furnace at 160 °C for 5 h. After vacuum filtration, the product was washed thoroughly with deionised water, followed by ethanol, and finally dried at 60 °C for 12 h. The details of synthesis as well as other experimental methods for characterization are given in supporting file.
3. Results and discussion Fig. 1. A schematic representation of e /h+ transfer mechanism in photocatalytic degradation of MB by SWCNT/HgS nanohybrid in aqueous solution. The solid lines indicate the CB and VB of SWCNT/HgS hybrid whereas dashed lines indicate the generation of fermi levels. Here Eg represents the band gap energy.
The present study describes the facile synthesis of new SWCNT/ HgS nanohybrid via a hydrothermal route. SWCNT/HgS nanohybrid exhibits excellent photocatalytic activity for photodegradation of MB. To the best of our knowledge, no report has been found on the synthesis of SWCNT/HgS nanohybrid, and its application for photodegradation of MB. 2. Experimental SWCNT/HgS nanohybrid was synthesized by the decoration of HgS nanoparticles on SWCNTs. The functionalized SWCNTs were
The hydrothermal reaction of SWCNTs with HgS nanoparticles (formed in situ) in a 1:1 M ratio leads to the formation of SWCNT/HgS nanohybrid as shown in Scheme S1. XRD patterns of pristine SWCNTs and SWCNT/HgS nanohybrid are shown in Fig. 2 (i), while that of pure HgS nanoparticles is shown in Fig S1. The hexagonal phase of a-HgS (cinnabar) is clearly evident in XRD pattern of SWCNT/HgS. Further, the diffraction pattern illustrates the preferential alignment of hexagonal HgS NPs on the surface of SWCNTs which is also well supported by IR (Fig. 2(ii)), as new peaks are observed at 1109 and 1384 cm 1 corresponding to the sulphur bonding in nanohybrid [21]. The average size of HgS nanocrystallites (43.33 nm) has been calculated by using DebyeScherrer equation (Eq. S1) [21]. The back scattered electron (BSE) FESEM image (Fig. 2(iii)) clearly indicates encapsulation of HgS nanoparticles among the inter-tubular spacing of SWCNTs bundles which is confirmed by EDX spectrum (Fig. S2). TEM images (Fig. 2(iv–v)) also clearly
Fig. 2. (i) XRD patterns of (a) pristine SWCNT, (b) SWCNT/HgS hybrid. (ii) FTIR images of (a) pristine SWCNTs, (b) purified and functionalized SWCNTs, (c) SWCNTs/HgS hybrid. (iii) FESEM (BSE) image of SWCNT/HgS hybrid. TEM images of Pristine SWCNTs, Purified and functionalized SWCNTs, and SWCNT/HgS hybrid are shown as (iv), (v), and (vi), respectively, and (vii) Lattice fringe spacing in SWCNT/HgS hybrid.
P.K. Saini et al. / Materials Letters 250 (2019) 5–8
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Fig. 3. (i) Raman spectra of SWCNT/HgS hybrid: (a) shows RBM band and (b) shows D and G band (ii) Typical tauc plots of HgS, and SWCNT/HgS hybrid by direct method. (iii) Photo-degradation measurements of MB (5 ppm, pH 6.03) with respect to time in presence of HgS NPs, and SWCNT/HgS. (iv) Kinetics of photodegradation of cationic dye.
reveal the removal of amorphous carbon and metal impurities during the purification and functionalization of SWCNTs. Further, TEM micrograph (Fig. 2(vi)) of SWCNT/HgS depicted that a-HgS nanoparticles (5–10 nm) are decorated on the walls of SWCNTs. Additionally the inter-planar (‘d’) spacings of the microstructure measured through HRTEM (Fig. 2(vii)) correspond to (1 1 4) and (0 1 5) planes of a-HgS nanoparticles. The concentric rings and SAED pattern (Fig. S3) indicate the polycrystalline nature of nanohybrid [22]. XPS full scan spectrum of SWCNT/HgS nanohybrid (Fig. S4a) exhibits peaks of C 1s, O 1s, S 2p3/2, 2p1/2 and Hg 4f7/2, 4f5/2 corresponding to binding energy of 284.0 eV, 531.0 eV 161.20, 162.5 eV, and 101.15, 104.15 eV, respectively, with inset table showing their percentages. The details of XPS analysis with expanded orbital fitted spectra of each element are given in supporting file (Fig. S4). A band in Raman spectrum assigned to the radial breathing mode (RBM) is observed at 157.99 cm 1 (size 1.607 nm) in f-SWCNT which is red shifted to 154.30 cm 1 (size 1.646 nm) in SWCNT/HgS nanohybrid (Fig. 3(ia)). The diameter of SWCNT bundles of both bands is calculated by Eq. S2 [22]. D and G bands are shown in Fig. 3(ib), no appreciable shift of D band is observed which indicates that all SWCNT bundles (carbon species) remain in the similar order of lattice during the formation of nanohybrid. However, G band shows highly ordered graphite like sp2 hybridized species [22,23]. The ID/IG ratio of f-SWCNT and SWCNT/HgS nanohybrid is 0.17 and 0.054, respectively, and these values indicate that the crystallinity is further enhanced by encapsulation of nanoparticles in inter-tubular space [21,23]. The optical properties of SWCNT/HgS nanohybrid are investigated by UV-DRS. The band gap of HgS and SWCNT/HgS is 1.96 and 2.91 eV, respec-
tively, for direct transition (Fig. 3(ii)) (Eq. S3) and 1.67 and 2.11 eV, respectively, for indirect transition (Fig. S5) (Eq. S4), which can be attributed to the definite shift in microenvironment of HgS NPs decorated on the surface of SWCNTs. The photocatalytic performance of HgS and SWCNT/HgS nanohybrid has been investigated for photodegradation of methylene blue (MB) under sunlight (average intensity: 24675 ± 100 l) at 20 ± 5 °C (details of photodegradation are given in the supporting file). For the determination of an effective amount of photocatalysts, it is observed that HgS NPs (4 mg) photodegrade 73.0% of MB (5 ppm), whereas the same amount of SWCNT/HgS nanohybrid shows 99.08% photodegradation efficiency (Fig. 3(iii)) at pH 6.03 under similar conditions (Fig. S6, Table ST-1). Furthermore, it has also been experimentally verified that %photodegradation gradually decreases (99.08%–93.3%) with increasing the concentration of MB (from 5 to15 ppm) (Fig. S7 and Table ST-2). The reactive oxygen species (ROS) such as O2 , OH and O2 [11] are responsible for photodegradation of MB by SWCNT/HgS nanohybrid, which follows the first order kinetics (Eq. S7) as shown in Fig. 3(iv). The photodegraded products of MB were analysed and identified by GC– MS and its possible degradation pathway has been proposed (Scheme-S2). FTIR and XRD spectra of nanohybrid recorded after 6 months remain unchanged indicating its high stability (Figs. S9 and S10). 4. Conclusion The SWCNT/HgS nanohybrid has been prepared in one step by hydrothermal method and characterized as well. SWCNT/HgS
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nanohybrid exhibits excellent photocatalytic activity and can be employed for the removal of MB dye. Due to lower band gap and chemical stability of SWCNT/HgS nanohybrid, it can also be used in broad areas as image sensors, biological display, photo detectors, and other optical devices. Conflict of interest None declared. Acknowledgements Dr. Pramendra Kumar Saini is thankful to DST-SERB, Government of India for financial support under Young Scientist scheme (SB/FT/CS-136/2014). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.matlet.2019.04.090. References [1] C. Yu, F. Wang, J. Chen, et al., J. Mol. Catal. A: Chem. 411 (2016) 1–8.
[2] A. Buthiyappan, A.R.A. Aziz, W.M.A.W. Daud, Rev. Chem. Eng. 32 (1) (2016) 1– 47. [3] (a) M.H. Alonso, F. Fresno, S. Suareza, J. Coronado, Energy Environ. Sci. 2 (2009) 1231–1257; (b) C. Lai, M. Zhang, B. Li, et al., Chem. Eng. J. 358 (2019) 891–902. [4] S. Dong, J. Feng, M. Fan, et al., RSC Adv. 5 (2015) 14610–14630. [5] D.A. Kean, K.G. McGuigan, P.F. Ibanez, et al., Catal. Sci. Tech. 4 (2014) 1211– 1226. [6] A. Ajmal, I. Majeed, R.N. Malik, et al., RSC Adv. 4 (2014) 37003–37026. [7] A. Mallik, M. Nath, S. Mohiyuddin, et al., ACS Omega 3 (2018) 8288–8308. [8] H. Faghihian, A. Bahranifard, Ira. J. Catal. 1 (2011) 45–50. [9] (a) S. Anandan, M. Yoon, J. Photochem. Photobiol. C: Photochem. Rev. 4 (2003) 5–18; (b) C.C. Wang, C.K. Lee, M.D. Lyu, et al., J. Dyes & Pigments xx (2007) 1–8. [10] R. Chandra, S. Mukhopadhyay, M. Nath, Mat. Lett. 164 (2016) 571–574. [11] (a) R. Chandra, M. Nath, Chem. Select. 2 (2017) 7711–7722; (b) B. Li, C. Lai, G. Zeng, et al., ACS Appl. Mater. Interfaces 10 (2018) 18824– 18836. [12] F. Taleshi, J. Appl. Spectrosc. 82 (2015) 303–306. [13] S. Iijima, T. Ichihashi, Nature 363 (1993) 603–605. [14] A. Kongkanand, R.M. Domínguez, P.V. Kamat, Nano Lett. 7 (2007) 676–680. [15] J.-P. Issi, J.-C. Charlier, Electrical transport properties in carbon nanotubes, in: K. Tanaka, T. Yamabe, K. Fukui (Eds.), The Science and Technology of Carbon Nanotubes, Elsevier Ltd., 1999, pp. 107–127. [16] S.K. Pillai, S.S. Ray, M. Moodley, J. Nanosci. Nanotechnol. 7 (2007) 3011–3047. [17] B.K. Patel, S. Rath, S.N. Sarangi, et al., Appl. Phys. A 86 (2007) 447–450. [18] S.A. Pande, B. Pandit, B.R. Shankpal, J. Colloid Interface Sci. (2017). [19] K. Woan, G. Pyrgiotakis, W. Sigmund, Adv. Mater. 21 (2009) 2233–2239. [20] A. Kongkanand, P.V. Kamat, ACS Nano 1 (2007) 13–21. [21] R. Paul, P. Kumbhakar, A.K. Mitra, J. Exp. Nanosci. 5 (2010) 363–373. [22] Y. Zhang, X. He, L. Wang, et al., Materials 9 (2016) 718–724. [23] M. Nath, P.V. Teredesai, D.V.S. Mathu, et al., Curr. Sci. 85 (2003) 956–960.