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ZnO heterostructure with enhanced photocatalytic activity
Materials Letters 243 (2019) 183–186
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
A type-II MoS2/ZnO heterostructure with enhanced photocatalytic activity Rajkumar Selvaraj a,1, Kavi Rasu Kalimuthu b,1, Vijayarangamuthu Kalimuthu a,⇑ a b
Centre for Nanoscience and Technology, Pondicherry University, Puducherry, Puducherry – 605014, India Department of Physics, Faculty of Engineering, Karpagam Academy of Higher Education, Coimbatore, Tamilnadu – 641021, India
a r t i c l e
i n f o
Article history: Received 5 November 2018 Received in revised form 1 February 2019 Accepted 5 February 2019 Available online 13 February 2019 Keywords: ZnO MoS2 Charge separation Photocatalyst Nanoparticles Nanocomposites
a b s t r a c t ZnO and MoS2/ZnO composite were synthesized by co-precipitation method and their structural and optical properties were studied using XRD, Raman, and photoluminescence spectroscopy. The MoS2/ ZnO composite has shown 40% enhancement in photocatalytic degradation of an organic dye as compared to ZnO. The contribution of active species such as superoxide (O2 ), hydroxyl radical (OH), and hole (h+) in the photocatalytic process were tested and their role was ordered as follows O2 > OH > h+. In total, the enhancement in photocatalytic performance of composite was correlated to specific surface area, association of all active species, and the synergistic effect between MoS2 and ZnO. Ó 2019 Elsevier B.V. All rights reserved.
1. Introduction After the discovery of graphene, research on non-graphene 2D materials such as hexagonal boron nitride, phosphorene, and transition metal dichalcogenides (TMDCs), has attracted immense interest. Among TMDCs, molybdenum disulfide (MoS2) has been explored intensively due to its unique electrical, optical and chemical properties [1–3]. The indirect to direct band gap transition and large surface area with abundant edge active sites witnessed in few-layer MoS2 are strategic for photocatalytic, and photovoltaic applications nevertheless of its high electron-hole recombination rate [4,5]. On other-hand, oxide semiconductor like zinc oxide (ZnO) and titanium oxide (TiO2) were extensively investigated for photocatalytic application due to high stability, low cost and ecofriendly nature [6–10]. Among them, ZnO is more desirable for photocatalytic and photovoltaic applications owing to its direct band gap, high exciton binding energy, and high charge mobility. However, the poor charge transferability and high electron-hole recombination rate of ZnO limits its efficiency. The drawbacks resulted from the intrinsic properties of MoS2 and ZnO can be resolved by forming a heterostructure MoS2/ZnO with the appropriate interface and bandgap engineering. Thereon, the
⇑ Corresponding author. 1
E-mail address: [email protected] (V. Kalimuthu). Both authors contributed equally.
https://doi.org/10.1016/j.matlet.2019.02.022 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.
heterostructures of MoS2 with WS2, ZnO, TiO2, GaN, and graphene were studied intensively for various applications. [11–13]. Here, we report the synthesis of MoS2/ZnO composite (MZC) with a type-II heterostructure that features spatial separation of valence band (VB) and conduction band (CB). Such, spatial distribution of VB and CB offers more reactive sites for photocatalytic application, i.e., redox reactions can take place at both MoS2 and ZnO sites. The contribution of reactive species such as superoxide (O2 ), hydroxyl radical (OH), and hole (h+) involved in the photocatalytic performance was also explored. 2. Experimental section The synthesis of MoS2/ZnO composite (MZC) via hydrothermal method, laser irradiation have reported previous with an equimolar or large weight ratio of MoS2 in ZnO. Here, we report the synthesis of MZC via co-precipitation method with a very low weight ratio of MoS2, i.e., 1:1000 ratio of MoS2:Zn. Further, we eliminated the involvement of heavy organic solvents for exfoliation on MoS2. The zinc acetate and exfoliated MoS2 were used as precursors for the nanocomposite synthesis. Detailed methods are presented in Supplementary information. The photocatalytic performance of the samples was gauged by the degradation of aqueous methylene blue (MB) dye (10 mg/L) under UV light. The scavenger test was done by adding benzoquinone (BQ), isopropanol (IPA) and ethylenediaminetetraacetic acid (EDTA) as scavengers for O2 , OH, and h+ respectively.
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3. Results and discussions Fig. 1a shows the XRD pattern of exfoliated MoS2, ZnO and MZC samples. All the diffraction patterns observed for bulk and exfoliated MoS2 samples were matched well with standard MaoS2 pattern (JCPDS: 77-1716). However, the XRD pattern of the exfoliated MoS2 sample has shown a strong orientation along (0 0 2) plane due to few-layer nature. The XRD patterns observed for ZnO and MZC samples were matched well with the standard pattern of wurtzite ZnO (JCPDS: 36-1451). Further, MZC didn’t show any characteristic patterns related to MoS2 due to the small weight fraction of MoS2 present in the composite. The average crystalline size was estimated to be 22 and 12 nm for ZnO and MZC, respectively. Fig. 1b shows the Raman spectra of exfoliated MoS2, ZnO and MZC samples. Exfoliated MoS2 sample exhibits characteristic MoS2 Raman modes at 380 and 405 cm 1 correspond to E12g and A1g modes, respectively with an interval of 23 cm 1 which indicate the few-layer nature [14]. The ZnO sample shows modes at 99 and 440 cm 1 corresponds to E2-low and E2-high modes of wurtzite ZnO [15,16]. The higher intensity of E2-Low and E2-High modes indicates the good crystallinity of the MZC without any posttreatment. The XRD and Raman results of exfoliated MoS2 sample confirm the few-layer nature. The co-existence of ZnO and MoS2 Raman modes in the MZC sample confirm the formation of the composite. Fig. 1c and d show the SEM images of ZnO and MZC samples with spherical-like and sheet-like morphological structures with ZnO grown over the MoS2 sheets, respectively. Fig. 2a shows the diffused reflectance spectra (DRS) of ZnO and MZC samples in UV-visible region. The ZnO sample shows strong absorption in the UV region and poor absorption in the visible
region. However, MZC shows strong absorption in the UV region along with small absorption in the visible region. Fig. 2b shows the estimation of bandgap via the Kubelka-Munk function (F(R)). Both ZnO and MZC show a direct bandgap around 3.3 eV due to ZnO but an additional bandgap around 2.2 eV was observed for MZC due to few-layer MoS2. The widening of the MoS2 band gap in MZC leads to formation of type-II (staggered) MoS2/ZnO heterostructure, i.e., the CB of exfoliated MoS2 was positioned above the CB of ZnO and the VB of ZnO was positioned below the VB of MoS2. The positioning of the bands was estimated based on bandgap calculated from UV visible and theoretical workfunctions [17]. Such a staggered arrangement is a promising way for efficient charge carrier separation due to the spatial distribution of bands. The widening of MoS2 bandgap also brings the redox potential in the bandgap region of few-layer MoS2 in the composite (Fig. 3e) which is a key strategy to achieve superior photocatalytic performance [17]. Fig. 2c shows the PL spectra of the exfoliated MoS2, ZnO and MZC samples. The ZnO and MZC samples show a broad PL emission with center around 390 nm due to near band edge (NBE) emission. Relatively a weak PL emission from MZC indicates a curtailed in the recombination of the electron-hole pair in MZC sample as compared to ZnO. Further, the decay lifetime of photo-generated charge carriers was estimated via PL decay spectra (Fig. 2d). The average lifetime was estimated to be 25 and 29 ns for ZnO and MZC samples, respectively and shows MZC sample exhibits longer lifetime compared to ZnO. The photocatalytic performance was assessed by photodegradation of MB under UV light. Fig. 3a and b show the UV-vis spectra of MB for various degradation time with ZnO and MZC as
Fig. 1. (a) XRD, (b) Raman spectra, (c and d) SEM images of MoS2, pure ZnO and MoS2/ZnO samples.
R. Selvaraj et al. / Materials Letters 243 (2019) 183–186
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Fig. 2. (a) DRS spectra, and (b) bandgap estimation, (c) PL spectra, and (d) PL lifetime spectra of ZnO and MoS2/ZnO samples.
Fig. 3. (a), (b) UV-visible absorption spectra of MB with pure ZnO and MoS2/ZnO as a catalyst, (c) photocatalytic degradation cure, (d) scavenger test results, and (e) schematic representation of photocatalytic mechanism.
a catalyst, respectively. Fig. 3c shows the percentage of degradation for both samples along with commercial P25 TiO2. The ZnO and P25 TiO2 samples show photo-degradation of MB around 60% and 90% after 120 min respectively. In contrast, the MZC
sample shows 99% of degradation after 120 min, i.e., 40% enhancement in photocatalytic degradation as compared to ZnO. The enhancement in photocatalytic performance of MZC can be understood via (a) specific surface area, (b) active species involved,
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R. Selvaraj et al. / Materials Letters 243 (2019) 183–186
and (c) the charge separation/transfer. The Brunauer-EmmettTeller (BET) surface area of ZnO and MZC was estimated to be 10.1 m2/g and 34.5 m2/g, respectively. The smaller crystalline size along with the higher specific surface area of MZC provides more reaction sites for photocatalytic degradation as compared to ZnO. The active species such as O2 ,OH, and h+ that involved in the photocatalytic degradation were identified by adding a scavenger like BQ, IPA, and EDTA, respectively [18]. Fig. 3d shows the scavenger test results and the percentage of photo-degradation was estimated to be 30%, 45%, and 57% for BQ, IPA, and EDTA as scavengers, respectively. The scavenger results confirm that all the three species were involved in photocatalytic degradation and their contribution is ordered as follows O2 > OH > h+. The charge transfer in MZC can be predicted from the position of the CB minimum, VB maximum and the bandgap. In the staggered arrangement, during excitation, the electrons in the CB of MoS2 will jump to the CB of ZnO, while the holes created in VB of ZnO will jump up to VB of MoS2 (Fig. 3e) [17]. Further, the spatial distribution of charges in CB and VB bands at different energy levels will delay the recombination of the photo-generated electron-hole pair and increase the lifetime of excited electrons as supported by PL and lifetime study. Such an excited electron will create a large number of O2 radicals which facilitate the photodegradation. The formation of a large number of O2 radicals is supported by the scavenger test using BQ [18]. The BQ test confirms that O2 was the mainly involved in this photo-degradation, since only 30% of MB was degraded without O2 . Concurrently, the photogenerated holes at VB of MoS2 and ZnO sites form OH either by direct reaction with surface adsorbed OH or H2O [18]. The contribution of OH and h+ species in photo-degradation was also supported by the scavenger test. Further, MoS2 with edge active sites will act as hot spots and an increase in a number of reactive sites due to its CB position above the redox potential. 4. Conclusions The MoS2/ZnO composite synthesized by co-precipitation method shows 40% enhancement in photocatalytic as performance compared to ZnO. The role of various active species was discussed
using scavenger test results. Finally, the significant improvement in photocatalytic performance of MoS2/ZnO was attributed to the factors, like (a) increase in the specific surface area along with a number of edge reactive sites, (b) involvement of all three active species, and (c) the charge separation/transfer as a result of type -II heterojunction. Acknowledgement Vijayarangamuthu acknowledges Department of Science and Technology (DST), India for the DST-Inspire Faculty award and grant (Grant No: DST/INSPIRE/04/2015/002060). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.matlet.2019.02.022. References [1] Q.H. Wang, K. Kalantar-Zadeh, et al., Nat. Nanotechnol. 7 (2012) 699–712. [2] Z. Wang, B. Mi, Environ. Sci. Technol. 51 (2017) 8229–8244. [3] F. Haque, T. Daeneke, K. Kalantar-zadeh, J.Z. Ou, Nano-Micro Lett. 10 (2018) 1– 27. [4] K.F. Mak, C. Lee, J. Hone, J. Shan, T.F. Heinz, Phys. Rev. Lett. 105 (2010) 2–5. [5] Y. Liu, S. Xie, H. Li, X. Wang, ChemCatChem. 6 (2014) 2522–2526. [6] K. Vijayarangamuthu, J.-S. Youn, C.-M. Park, K.-J. Jeon, Catal. Today. (2018). [7] A. Das, P. Malakar, R.G. Nair, Mater. Lett. 219 (2018) 76–80. [8] E. Han, K. Vijayarangamuthu, J. sang Youn, et al., Catal. Today. 303 (2018) 305– 312. [9] C.B. Ong, L.Y. Ng, A.W. Mohammad, Renew. Sustain. Energy Rev. 81 (2018) 536–551. [10] V. Kalimthu, E. Han, K.-J. Jeon, J. Nanosci. Nanotechnol. 16 (2016) 4399–4404. [11] Z. Zhang, Q. Qian, B. Li, K.J. Chen, A.C.S. Appl, Mater. Interfaces. 10 (2018) 17419–17420. [12] G.P. Awasthi, S.P. Adhikari, S. Ko, et al., J. Alloys Compd. 682 (2016) 208–215. [13] N. Shen, G. Tao, Adv. Mater. Interfaces. 4 (2017) 1601083. [14] C. Lee, H. Yan, L.E. Brus, et al., ACS Nano. 4 (2010) 2695–2700. [15] T.C. Damen, S.P.S. Porto, B. Tell, Phys. Rev. 142 (1966) 570–574. [16] S. Ahn, V. Kalimuthu, K.-J. Jeon, J. Nanosci. Nanotechnol. 16 (2016) 4417–4421. [17] S.S. Lo, T. Mirkovic, C.H. Chuang, et al., Adv. Mater. 23 (2011) 180–197. [18] J. Schneider, M. Matsuoka, M. Takeuchi, et al., Chem. Rev. 114 (2014) 9919– 9986.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
A type-II MoS2/ZnO heterostructure with enhanced photocatalytic activity Rajkumar Selvaraj a,1, Kavi Rasu Kalimuthu b,1, Vijayarangamuthu Kalimuthu a,⇑ a b
Centre for Nanoscience and Technology, Pondicherry University, Puducherry, Puducherry – 605014, India Department of Physics, Faculty of Engineering, Karpagam Academy of Higher Education, Coimbatore, Tamilnadu – 641021, India
a r t i c l e
i n f o
Article history: Received 5 November 2018 Received in revised form 1 February 2019 Accepted 5 February 2019 Available online 13 February 2019 Keywords: ZnO MoS2 Charge separation Photocatalyst Nanoparticles Nanocomposites
a b s t r a c t ZnO and MoS2/ZnO composite were synthesized by co-precipitation method and their structural and optical properties were studied using XRD, Raman, and photoluminescence spectroscopy. The MoS2/ ZnO composite has shown 40% enhancement in photocatalytic degradation of an organic dye as compared to ZnO. The contribution of active species such as superoxide (O2 ), hydroxyl radical (OH), and hole (h+) in the photocatalytic process were tested and their role was ordered as follows O2 > OH > h+. In total, the enhancement in photocatalytic performance of composite was correlated to specific surface area, association of all active species, and the synergistic effect between MoS2 and ZnO. Ó 2019 Elsevier B.V. All rights reserved.
1. Introduction After the discovery of graphene, research on non-graphene 2D materials such as hexagonal boron nitride, phosphorene, and transition metal dichalcogenides (TMDCs), has attracted immense interest. Among TMDCs, molybdenum disulfide (MoS2) has been explored intensively due to its unique electrical, optical and chemical properties [1–3]. The indirect to direct band gap transition and large surface area with abundant edge active sites witnessed in few-layer MoS2 are strategic for photocatalytic, and photovoltaic applications nevertheless of its high electron-hole recombination rate [4,5]. On other-hand, oxide semiconductor like zinc oxide (ZnO) and titanium oxide (TiO2) were extensively investigated for photocatalytic application due to high stability, low cost and ecofriendly nature [6–10]. Among them, ZnO is more desirable for photocatalytic and photovoltaic applications owing to its direct band gap, high exciton binding energy, and high charge mobility. However, the poor charge transferability and high electron-hole recombination rate of ZnO limits its efficiency. The drawbacks resulted from the intrinsic properties of MoS2 and ZnO can be resolved by forming a heterostructure MoS2/ZnO with the appropriate interface and bandgap engineering. Thereon, the
⇑ Corresponding author. 1
E-mail address: [email protected] (V. Kalimuthu). Both authors contributed equally.
https://doi.org/10.1016/j.matlet.2019.02.022 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.
heterostructures of MoS2 with WS2, ZnO, TiO2, GaN, and graphene were studied intensively for various applications. [11–13]. Here, we report the synthesis of MoS2/ZnO composite (MZC) with a type-II heterostructure that features spatial separation of valence band (VB) and conduction band (CB). Such, spatial distribution of VB and CB offers more reactive sites for photocatalytic application, i.e., redox reactions can take place at both MoS2 and ZnO sites. The contribution of reactive species such as superoxide (O2 ), hydroxyl radical (OH), and hole (h+) involved in the photocatalytic performance was also explored. 2. Experimental section The synthesis of MoS2/ZnO composite (MZC) via hydrothermal method, laser irradiation have reported previous with an equimolar or large weight ratio of MoS2 in ZnO. Here, we report the synthesis of MZC via co-precipitation method with a very low weight ratio of MoS2, i.e., 1:1000 ratio of MoS2:Zn. Further, we eliminated the involvement of heavy organic solvents for exfoliation on MoS2. The zinc acetate and exfoliated MoS2 were used as precursors for the nanocomposite synthesis. Detailed methods are presented in Supplementary information. The photocatalytic performance of the samples was gauged by the degradation of aqueous methylene blue (MB) dye (10 mg/L) under UV light. The scavenger test was done by adding benzoquinone (BQ), isopropanol (IPA) and ethylenediaminetetraacetic acid (EDTA) as scavengers for O2 , OH, and h+ respectively.
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R. Selvaraj et al. / Materials Letters 243 (2019) 183–186
3. Results and discussions Fig. 1a shows the XRD pattern of exfoliated MoS2, ZnO and MZC samples. All the diffraction patterns observed for bulk and exfoliated MoS2 samples were matched well with standard MaoS2 pattern (JCPDS: 77-1716). However, the XRD pattern of the exfoliated MoS2 sample has shown a strong orientation along (0 0 2) plane due to few-layer nature. The XRD patterns observed for ZnO and MZC samples were matched well with the standard pattern of wurtzite ZnO (JCPDS: 36-1451). Further, MZC didn’t show any characteristic patterns related to MoS2 due to the small weight fraction of MoS2 present in the composite. The average crystalline size was estimated to be 22 and 12 nm for ZnO and MZC, respectively. Fig. 1b shows the Raman spectra of exfoliated MoS2, ZnO and MZC samples. Exfoliated MoS2 sample exhibits characteristic MoS2 Raman modes at 380 and 405 cm 1 correspond to E12g and A1g modes, respectively with an interval of 23 cm 1 which indicate the few-layer nature [14]. The ZnO sample shows modes at 99 and 440 cm 1 corresponds to E2-low and E2-high modes of wurtzite ZnO [15,16]. The higher intensity of E2-Low and E2-High modes indicates the good crystallinity of the MZC without any posttreatment. The XRD and Raman results of exfoliated MoS2 sample confirm the few-layer nature. The co-existence of ZnO and MoS2 Raman modes in the MZC sample confirm the formation of the composite. Fig. 1c and d show the SEM images of ZnO and MZC samples with spherical-like and sheet-like morphological structures with ZnO grown over the MoS2 sheets, respectively. Fig. 2a shows the diffused reflectance spectra (DRS) of ZnO and MZC samples in UV-visible region. The ZnO sample shows strong absorption in the UV region and poor absorption in the visible
region. However, MZC shows strong absorption in the UV region along with small absorption in the visible region. Fig. 2b shows the estimation of bandgap via the Kubelka-Munk function (F(R)). Both ZnO and MZC show a direct bandgap around 3.3 eV due to ZnO but an additional bandgap around 2.2 eV was observed for MZC due to few-layer MoS2. The widening of the MoS2 band gap in MZC leads to formation of type-II (staggered) MoS2/ZnO heterostructure, i.e., the CB of exfoliated MoS2 was positioned above the CB of ZnO and the VB of ZnO was positioned below the VB of MoS2. The positioning of the bands was estimated based on bandgap calculated from UV visible and theoretical workfunctions [17]. Such a staggered arrangement is a promising way for efficient charge carrier separation due to the spatial distribution of bands. The widening of MoS2 bandgap also brings the redox potential in the bandgap region of few-layer MoS2 in the composite (Fig. 3e) which is a key strategy to achieve superior photocatalytic performance [17]. Fig. 2c shows the PL spectra of the exfoliated MoS2, ZnO and MZC samples. The ZnO and MZC samples show a broad PL emission with center around 390 nm due to near band edge (NBE) emission. Relatively a weak PL emission from MZC indicates a curtailed in the recombination of the electron-hole pair in MZC sample as compared to ZnO. Further, the decay lifetime of photo-generated charge carriers was estimated via PL decay spectra (Fig. 2d). The average lifetime was estimated to be 25 and 29 ns for ZnO and MZC samples, respectively and shows MZC sample exhibits longer lifetime compared to ZnO. The photocatalytic performance was assessed by photodegradation of MB under UV light. Fig. 3a and b show the UV-vis spectra of MB for various degradation time with ZnO and MZC as
Fig. 1. (a) XRD, (b) Raman spectra, (c and d) SEM images of MoS2, pure ZnO and MoS2/ZnO samples.
R. Selvaraj et al. / Materials Letters 243 (2019) 183–186
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Fig. 2. (a) DRS spectra, and (b) bandgap estimation, (c) PL spectra, and (d) PL lifetime spectra of ZnO and MoS2/ZnO samples.
Fig. 3. (a), (b) UV-visible absorption spectra of MB with pure ZnO and MoS2/ZnO as a catalyst, (c) photocatalytic degradation cure, (d) scavenger test results, and (e) schematic representation of photocatalytic mechanism.
a catalyst, respectively. Fig. 3c shows the percentage of degradation for both samples along with commercial P25 TiO2. The ZnO and P25 TiO2 samples show photo-degradation of MB around 60% and 90% after 120 min respectively. In contrast, the MZC
sample shows 99% of degradation after 120 min, i.e., 40% enhancement in photocatalytic degradation as compared to ZnO. The enhancement in photocatalytic performance of MZC can be understood via (a) specific surface area, (b) active species involved,
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and (c) the charge separation/transfer. The Brunauer-EmmettTeller (BET) surface area of ZnO and MZC was estimated to be 10.1 m2/g and 34.5 m2/g, respectively. The smaller crystalline size along with the higher specific surface area of MZC provides more reaction sites for photocatalytic degradation as compared to ZnO. The active species such as O2 ,OH, and h+ that involved in the photocatalytic degradation were identified by adding a scavenger like BQ, IPA, and EDTA, respectively [18]. Fig. 3d shows the scavenger test results and the percentage of photo-degradation was estimated to be 30%, 45%, and 57% for BQ, IPA, and EDTA as scavengers, respectively. The scavenger results confirm that all the three species were involved in photocatalytic degradation and their contribution is ordered as follows O2 > OH > h+. The charge transfer in MZC can be predicted from the position of the CB minimum, VB maximum and the bandgap. In the staggered arrangement, during excitation, the electrons in the CB of MoS2 will jump to the CB of ZnO, while the holes created in VB of ZnO will jump up to VB of MoS2 (Fig. 3e) [17]. Further, the spatial distribution of charges in CB and VB bands at different energy levels will delay the recombination of the photo-generated electron-hole pair and increase the lifetime of excited electrons as supported by PL and lifetime study. Such an excited electron will create a large number of O2 radicals which facilitate the photodegradation. The formation of a large number of O2 radicals is supported by the scavenger test using BQ [18]. The BQ test confirms that O2 was the mainly involved in this photo-degradation, since only 30% of MB was degraded without O2 . Concurrently, the photogenerated holes at VB of MoS2 and ZnO sites form OH either by direct reaction with surface adsorbed OH or H2O [18]. The contribution of OH and h+ species in photo-degradation was also supported by the scavenger test. Further, MoS2 with edge active sites will act as hot spots and an increase in a number of reactive sites due to its CB position above the redox potential. 4. Conclusions The MoS2/ZnO composite synthesized by co-precipitation method shows 40% enhancement in photocatalytic as performance compared to ZnO. The role of various active species was discussed
using scavenger test results. Finally, the significant improvement in photocatalytic performance of MoS2/ZnO was attributed to the factors, like (a) increase in the specific surface area along with a number of edge reactive sites, (b) involvement of all three active species, and (c) the charge separation/transfer as a result of type -II heterojunction. Acknowledgement Vijayarangamuthu acknowledges Department of Science and Technology (DST), India for the DST-Inspire Faculty award and grant (Grant No: DST/INSPIRE/04/2015/002060). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.matlet.2019.02.022. References [1] Q.H. Wang, K. Kalantar-Zadeh, et al., Nat. Nanotechnol. 7 (2012) 699–712. [2] Z. Wang, B. Mi, Environ. Sci. Technol. 51 (2017) 8229–8244. [3] F. Haque, T. Daeneke, K. Kalantar-zadeh, J.Z. Ou, Nano-Micro Lett. 10 (2018) 1– 27. [4] K.F. Mak, C. Lee, J. Hone, J. Shan, T.F. Heinz, Phys. Rev. Lett. 105 (2010) 2–5. [5] Y. Liu, S. Xie, H. Li, X. Wang, ChemCatChem. 6 (2014) 2522–2526. [6] K. Vijayarangamuthu, J.-S. Youn, C.-M. Park, K.-J. Jeon, Catal. Today. (2018). [7] A. Das, P. Malakar, R.G. Nair, Mater. Lett. 219 (2018) 76–80. [8] E. Han, K. Vijayarangamuthu, J. sang Youn, et al., Catal. Today. 303 (2018) 305– 312. [9] C.B. Ong, L.Y. Ng, A.W. Mohammad, Renew. Sustain. Energy Rev. 81 (2018) 536–551. [10] V. Kalimthu, E. Han, K.-J. Jeon, J. Nanosci. Nanotechnol. 16 (2016) 4399–4404. [11] Z. Zhang, Q. Qian, B. Li, K.J. Chen, A.C.S. Appl, Mater. Interfaces. 10 (2018) 17419–17420. [12] G.P. Awasthi, S.P. Adhikari, S. Ko, et al., J. Alloys Compd. 682 (2016) 208–215. [13] N. Shen, G. Tao, Adv. Mater. Interfaces. 4 (2017) 1601083. [14] C. Lee, H. Yan, L.E. Brus, et al., ACS Nano. 4 (2010) 2695–2700. [15] T.C. Damen, S.P.S. Porto, B. Tell, Phys. Rev. 142 (1966) 570–574. [16] S. Ahn, V. Kalimuthu, K.-J. Jeon, J. Nanosci. Nanotechnol. 16 (2016) 4417–4421. [17] S.S. Lo, T. Mirkovic, C.H. Chuang, et al., Adv. Mater. 23 (2011) 180–197. [18] J. Schneider, M. Matsuoka, M. Takeuchi, et al., Chem. Rev. 114 (2014) 9919– 9986.