Journal of Colloid and Interface Science 451 (2015) 216–220
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Short Communication
High density decoration of noble metal nanoparticles on polydopamine-functionalized molybdenum disulphide Muhammad Asif Hussain a,b,1, MinHo Yang c,1, Tae Jae Lee d, Jung Won Kim a,⇑, Bong Gill Choi a,⇑ a
Department of Chemical Engineering, Kangwon National University, Samcheok 245-711, Republic of Korea Department of Advanced Materials Engineering, Kangwon National University, Samcheok 245-711, Republic of Korea c Graduate School of Nanoscience and Technology, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea d Department of Nano Bio Research, National NanoFab Center (NNFC), Daejeon 305-701, Republic of Korea b
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 28 November 2014 Accepted 31 March 2015 Available online 8 April 2015 Keywords: Composite materials Nanostructures Polymers Chemical synthesis Ultrasonic techniques
a b s t r a c t Here, we report a highly stable colloidal suspension of nanoparticles (i.e., Pt and Au)-deposited MoS2 sheets, in which polydopamine (PD) serves as surface functional groups. The adoption of polydopamine coating onto the MoS2 surface enables homogeneous deposition of nanoparticles in an aqueous solution. As-synthesized nanohybrids are thoroughly characterized by transmission electron microscopy (TEM), Raman spectroscopy, and X-ray diffraction (XRD) measurement. These intensive investigations reveal that noble metal nanocrystals are uniformly distributed on the surface of ultrathin MoS2 sheets (4 layers). Moreover, as-prepared Au/PD/MoS2 nanohybrids can be applied as a heterogeneous catalyst for reduction of 4-nitrophenol to 4-aminophenol, and they exhibit an excellent catalytic activity. Ó 2015 Elsevier Inc. All rights reserved.
1. Introduction Recent developments in nanostructured metal chalcogenides (e.g., MoS2, WS2, VS2, and TiS2) have shown promising electronic, ⇑ Corresponding authors. Fax: +82 33 570 6535. E-mail addresses: [email protected] (J.W. Kim), [email protected] (B.G. Choi). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.jcis.2015.03.062 0021-9797/Ó 2015 Elsevier Inc. All rights reserved.
optical, catalytic, and mechanical properties [1,2]. These investigations have triggered wide interest owing to the potential for their use in many applications, including electronics, light-emitting diodes, sensors, and energy conversion and storage devices [3–5]. Several methods have been developed to prepare exfoliated metal disulfide sheets (MDS), including mechanical exfoliation, chemical vapor deposition, electrochemical exfoliation, and liquid exfoliation methods [4]. Utilization of strong reducing agents such as n-butyl lithium enables a lithium ion-intercalation and exfoliation
M.A. Hussain et al. / Journal of Colloid and Interface Science 451 (2015) 216–220
process in two-dimensional (2D) nanosheets, leading to a simple, reliable, and scalable fabrication of single layer nanosheets [6]. Although single components of exfoliated MDS have shown interesting results in many applications, the intrinsic properties of MDS may not be limited to meet the strict requirements for further applications. One of the most promising strategies is integration of nanoparticles (NPs) and MDS into a nanohyrbid structure, which is a benchmarking process for graphene/NP nanohybrids [7–10]. The resulting nanohybrids can provide multiple functionalities and even novel properties, leading to enhanced device performance. Several approaches have been employed to fabricate MDS-based nanohybrids including a chemical reducing method, direct deposition, physical vapor deposition, and microwave treatment [7–10]. However, in most of these methods, the 2D nanosheets tend to agglomerate due to their van der Waals interactions, which are responsible for significant reduction of the surface area. Hence, it is still necessary to advance synthetic method for the production of stable colloidal MDS-based nanohybrids in an aqueous solution. Here, we report a simple, but versatile strategy for preparation of a stable dispersion of NP-deposited MDS nanohybrids in an aqueous solution. Polydopamine (PD), inspired by an adhesive protein in mussels [11], was adopted as a coating material to introduce functional groups onto the MDS surface. PD shells formed on individual MDSs effectively prevented re-stacking of MDSs while providing direct growth of NPs on 2D sheets under ultrasonic irradiation.
2. Experimental Prior to preparation of NP (i.e., Pt and Au)/PD/MoS2 nanohybrids, ultrathin MoS2 samples were synthesized by lithium intercalation and exfoliation method [6]. In a typical experiment, MoS2 flakes (0.1 g, Sigma–Aldrich) were purged by Ar gas for 4 h, after which n-butyl lithium (1.6 M, 1 mL, Sigma–Aldrich) was added into the sealed tube and the mixture was stirred at room temperature (RT) for 48 h under Ar. The resultant LixMoS2 samples were then washed several times with hexane and filtered to remove unreacted n-butyl lithium. As-obtained black powder was dried at RT under vacuum for 24 h. For functionalization of MoS2 sheets with PD, LixMoS2 flakes were dispersed in Tris-buffer aqueous solution (10 mM and pH 8.5) and then sonicated in an ultrasonication bath (Branson, model 1510) for 30 min to exfoliate the MoS2 sheets. Following centrifugation, the precipitates were
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discarded and the supernatant was collected, after which dopamine hydrochloride (2 mg/mL, Sigma–Aldrich) was added and the mixture was stirred at RT for 2 h. The PD/MoS2 powder was washed using deionized (DI) water thoroughly, then collected by filtration. To deposit the NPs onto PD/MoS2, the PD/MoS2 (1 mg) was mixed with 1 10 3 M of H2PtCl6 (or HAuCl43H2O) in bisolvent of ethanol (2 mL) and DI water (2 mL). The resultant mixture was sonicated for 1 min, after which the synthesized NP/PD/MoS2 nanohybrids were filtered, washed with DI water, and dried at 60 °C under vacuum. Transmission electron microscopy (TEM) images were collected using a JEM-2100F HR-TEM at an acceleration voltage of 200 kV. Xray diffraction (XRD) data were obtained on a Rigaku D/MAX-2500 (40 kW) with a q/q goniometer equipped with a Cu Ka radiation generator. X-ray photoelectron spectroscopy (XPS) was carried out using a VG multilab 2000 (Thermo VG Scientific) with a monochromatic Mg Ka X-ray source (hv = 1253.6 eV) under 10 7 Torr in a vacuum analysis chamber. The atomic force microscopy (AFM) images were recorded in the noncontact mode using a Nanoman Digital Instruments 3100 AFM (VEECO) with an etched silicon aluminum coated tip. The UV–visible absorption spectra were collected by using a Mecasys Optizen 2120UV spectrophotometer. The zeta potentials of colloidal suspension were measured by a Nicomp 380 ZLS particle analyzer. The reduction of 4-nitrophenol to 4-aminophenol by NaBH4 was studied as a model reaction to probe the catalytic activity of as-prepared Au/PD/MoS2. The 0.04 mL solution of 4-nitrophenol (10 mM) was mixed with 2.5 mL water and 0.5 mL of aqueous solution of NaBH4 (80 mM) in a quartz cuvette. Subsequently, 10 mg of as-prepared Au/PD/MoS2 as a catalyst was added in the solution. Immediately after adding the catalyst, UV–Vis spectra were recorded for the real time reaction at every 1 min intervals at room temperature.
3. Results and discussion Fig. 1a shows a schematic image of an experimental procedure for designing highly stable NP/PD/MoS2 nanohybrids. As starting materials, LixMoS2 samples were prepared from commercial bulk MoS2 powder and passed through the intercalation of lithium ions [6]. The LixMoS2 sheets were well dispersed in DI water through gentle sonication, which resulted in a yellow solution (Fig. 1b). After coating of polydopamine conducted by the self-polymerization of dopamine, highly stable PD/MoS2 nanohybrids with a black
Fig. 1. (a) Schematic diagram of the modification of exfoliated MoS2 with PD and the preparation of NP/PD/MoS2 hybrids. (b) Photograph images of the exfoliated MoS2, PD/ MoS2, Pt/PD/MoS2, and Au/PD/MoS2.
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color in water were observed (Fig. 1b). The MoS2 was coated with polydopamine via the mutual interactions between the catechol groups of PD and the MoS2 surface [11]. This PD coating method has been employed to give highly conformal layers with a thickness that can be controlled by varying the reaction time [12]. Pt and Au NPs were selected as the model for deposition of NPs because of wide interest in their use in many applications. Sonochemical in-situ synthesis was employed to deposit the NPs on the surface of PD/MoS2 owing to its simplicity, short reaction time, and accuracy [13]. Previous reports describe that the sonochemical method is useful for deposition of NPs on 2D nanosheet materials (e.g., graphene) [14–16]. NP precursors were chelated with catechol groups of PD, after which they were nucleated and grown on the surface of PD/MoS2 by ultrasound irradiation. Hence, the catechol groups in PD provided abundant nucleation and growth sites for a uniform deposition of NPs, while preventing the reaggregation of the MoS2 sheets in an aqueous solution (Fig. 1b). After deposition of Pt NPs on PD/MoS2, colloidal
suspension of the resulting Pt/PD/MoS2 remained well-dispersed in water (Fig. S1) and had 38.5 mV of zeta potential. It is wellknown that the zeta potential values below 30 mV indicate the formation of stable dispersion, from a general colloidal science perspective [17]. These foundings clearly demonstrated a highly stable dispersion of the Pt/PD/MoS2 in water. The morphology and structure of PD/MoS2 sheets were confirmed by the results of TEM, AFM, XPS and Raman analysis (Fig. 2). TEM images showed ultrathin and flat PD/MoS2 sheets, in which PD conformally coated the entire MoS2 sheets. As shown in Fig. 2b, HR-TEM revealed that the MoS2 surface was coated with the shell of the PD layer with a thickness of 5–7 nm after coating for 1 h. The average thickness of PD/MoS2 estimated from AFM image (Fig. S2) was 7.4 ± 0.35 nm, indicating that ultrathin MoS2 nanosheets were coated with PD. As the coating time increased from 1 to 2 h, the PD coating layer increased to a thickness of about 30 nm (Fig. 2d). These findings indicate that the PD coating thickness can be controlled by varying the reaction time. In addition,
Fig. 2. TEM images of PD/MoS2 samples with different coating time ((a) and (b) for 1 h and (c) and (d) for 2 h). (e) XPS survey spectrum of PD/MoS2. (f) Raman spectra of bulk MoS2 and PD/MoS2.
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the presence of the PD layer on the MoS2 surface was investigated by XPS (Fig. 2e). The significant C 1s, N 1s, and O 1s peaks indicate obvious formation of PDs on PD/MoS2 sheets. To understand the uniformity of the layer thickness of MoS2 sheets, we also conducted Raman spectroscopy using the E12g and A1g modes of the PD/MoS2 nanohybrids (Fig. 2f) [18]. The pristine bulk MoS2 exhibited two prominent E12g (383.5 cm 1) and A1g (407.2 cm 1) peaks corresponding to the in plane (2 S atoms in a direction opposite to the Mo atom) and out of plane (S atoms in opposite directions) vibrations for bulk MoS2 (Fig. 2f) [18]. When the layer number is less than 6, the energy gap between the A1g and E12g peak positions (D) can be used to determine the number of MoS2 layers. The pristine bulk MoS2 exhibited a D value of 26.1 cm 1 which matches well with the results of previous studies [18]. The D value for the PD/MoS2 sample was estimated to be 23.7 cm 1, indicating that the PD/MoS2 is less than 4 layers [18].
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Fig. 3 shows the TEM images for Pt/PD/MoS2 and Au/PD/MoS2 nanohybrids. As shown in Fig. 3a–c, TEM images revealed that Pt NPs were uniformly distributed on the surface of the PD/MoS2 sheet. The average diameter of 2.8 ± 0.52 nm for Pt NPs was calculated by measuring the diameters in the TEM image (Fig. 3c). From XPS elemental analysis, we calculated the loading amount of Pt and Au NPs on PD/MoS2 to be 6.78 at.% and 10.5 at.%, respectively. In addition, elemental mappings using energy dispersive X-ray spectroscopy was carried out following the TEM image of Fig. 3a. Notably, the molybdenum signal for MoS2, the nitrogen signal for PD, and the Pt signal overlapped uniformly along the surface of the Pt/PD/MoS2 sheet. These findings indicate conformal coating of PD and uniform distribution of Pt NPs on the MoS2 sheets. Au NPs were also synthesized and arrayed onto the surface of the PD/MoS2 sheet using the same method as for Pt/PD/MoS2 by changing only the NP precursor. The presence of Au NPs was
Fig. 3. TEM images of Pt/PD/MoS2 and Au/PD/MoS2 hybrids. (a), (b), and (c) show the Pt NPs on PD/MoS2 surface. (d) EDS elemental mapping of Pt/PD/MoS2 hybrid. (e) and (f) show Au NPs on PD/MoS2 surface.
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(JCPDS, Card No. 04-0862) [13]. In addition, the XRD pattern for Au/ PD/MoS2 revealed the formation of the face-centered cubic crystal structure of Au NPs with the (1 1 1), (2 0 0), (2 2 0), and (3 1 1) planes, respectively (JCPDS Card No. 4-784) [10]. The reduction of 4-nitrophenol to 4-aminophenol by NaBH4 was chosen as a model system in order to evaluate the catalytic activity of Au/PD/MoS2. The overall reaction of 4-nitrophenol was shown in Fig. 5a. In presence of Au/PD/MoS2, the 4-nitrophenolate can bind to the Au (1 0 0) surface through the two oxygen of the nitro group, leading to the faciliated kinetics for reduction reaction of 4-nitrophenol to 4-aminophenol [19]. The reduction process was monitored by UV–Vis spectroscopy. As shown in Fig. 5b the decrease in intensity of the 400 nm peak for p-nitrophenol has been observed with the progress of the time in the presence of Au/PD/MoS2 nanohybrids. The reduction of 4-nitrophenol was complete just within 8 min, indicating good catalytic activity due to a large amount of active sites of Au NPs on PD/MoS2. Fig. 4. XRD patterns for bulk MoS2 and PD/MoS2 (Inset is XRD patterns of Pt/PD/ MoS2 and Au/PD/MoS2.)
4. Conclusion Inspired by the highly adhesive ability of the mussel protein, we developed a versatile method for fabrication of NP-deposited MoS2 using PD in an aqueous solution. The surface modification of MoS2 with PD effectively prevented re-stacking of MoS2 sheets while providing a large amount of uniform deposition sites for the NPs. As a result, high density deposition of NPs using PD modification was obtained at the surface of MoS2 sheets. Acknowledgments This research was financially supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF2012R1A1A1041991) and the Korean Government (MSIP) (No. 2014R1A5A2010008). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2015.03.062. References [1] [2] [3] [4]
Fig. 5. (a) Reaction scheme for the reduction of 4-nitrophenol to 4-aminophenol. (b) Time-dependent UV–Vis spectra of the catalytic reduction of 4-nitrophenol to 4aminophenol with Au/PD/MoS2.
clearly observed in the TEM image (Fig. 3e). Although the size and array density of Au NPs differed from those of the Pt NPs, the formation of nano-crystalline structures was verified by the lattice fringes found in the HR-TEM image (Fig. 3f). The XRD measurements of bulk MoS2, PD/MoS2, Pt/PD/MoS2, and Au/PD/MoS2 samples are shown in Fig. 4. Comparison of the bulk MoS2 before exfoliation and PD coating revealed a prominent and sharp intensity (0 0 2) peak corresponding to a d-spacing of 0.62 nm [6]. This (0 0 2) reflection of the PD/MoS2 nanosheets is smaller than that of the bulk MoS2, indicating that a large number of PD/MoS2 sheets were highly exfoliated. Following deposition of Pt NPs, the Pt/PD/MoS2 nanohybrids showed three characteristic peaks corresponding to (1 1 1), (2 0 0), and (2 2 0) planes, indicating the formation of the face-centered cubic crystal structure of Pt NPs
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Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Short Communication
High density decoration of noble metal nanoparticles on polydopamine-functionalized molybdenum disulphide Muhammad Asif Hussain a,b,1, MinHo Yang c,1, Tae Jae Lee d, Jung Won Kim a,⇑, Bong Gill Choi a,⇑ a
Department of Chemical Engineering, Kangwon National University, Samcheok 245-711, Republic of Korea Department of Advanced Materials Engineering, Kangwon National University, Samcheok 245-711, Republic of Korea c Graduate School of Nanoscience and Technology, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea d Department of Nano Bio Research, National NanoFab Center (NNFC), Daejeon 305-701, Republic of Korea b
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 28 November 2014 Accepted 31 March 2015 Available online 8 April 2015 Keywords: Composite materials Nanostructures Polymers Chemical synthesis Ultrasonic techniques
a b s t r a c t Here, we report a highly stable colloidal suspension of nanoparticles (i.e., Pt and Au)-deposited MoS2 sheets, in which polydopamine (PD) serves as surface functional groups. The adoption of polydopamine coating onto the MoS2 surface enables homogeneous deposition of nanoparticles in an aqueous solution. As-synthesized nanohybrids are thoroughly characterized by transmission electron microscopy (TEM), Raman spectroscopy, and X-ray diffraction (XRD) measurement. These intensive investigations reveal that noble metal nanocrystals are uniformly distributed on the surface of ultrathin MoS2 sheets (4 layers). Moreover, as-prepared Au/PD/MoS2 nanohybrids can be applied as a heterogeneous catalyst for reduction of 4-nitrophenol to 4-aminophenol, and they exhibit an excellent catalytic activity. Ó 2015 Elsevier Inc. All rights reserved.
1. Introduction Recent developments in nanostructured metal chalcogenides (e.g., MoS2, WS2, VS2, and TiS2) have shown promising electronic, ⇑ Corresponding authors. Fax: +82 33 570 6535. E-mail addresses: [email protected] (J.W. Kim), [email protected] (B.G. Choi). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.jcis.2015.03.062 0021-9797/Ó 2015 Elsevier Inc. All rights reserved.
optical, catalytic, and mechanical properties [1,2]. These investigations have triggered wide interest owing to the potential for their use in many applications, including electronics, light-emitting diodes, sensors, and energy conversion and storage devices [3–5]. Several methods have been developed to prepare exfoliated metal disulfide sheets (MDS), including mechanical exfoliation, chemical vapor deposition, electrochemical exfoliation, and liquid exfoliation methods [4]. Utilization of strong reducing agents such as n-butyl lithium enables a lithium ion-intercalation and exfoliation
M.A. Hussain et al. / Journal of Colloid and Interface Science 451 (2015) 216–220
process in two-dimensional (2D) nanosheets, leading to a simple, reliable, and scalable fabrication of single layer nanosheets [6]. Although single components of exfoliated MDS have shown interesting results in many applications, the intrinsic properties of MDS may not be limited to meet the strict requirements for further applications. One of the most promising strategies is integration of nanoparticles (NPs) and MDS into a nanohyrbid structure, which is a benchmarking process for graphene/NP nanohybrids [7–10]. The resulting nanohybrids can provide multiple functionalities and even novel properties, leading to enhanced device performance. Several approaches have been employed to fabricate MDS-based nanohybrids including a chemical reducing method, direct deposition, physical vapor deposition, and microwave treatment [7–10]. However, in most of these methods, the 2D nanosheets tend to agglomerate due to their van der Waals interactions, which are responsible for significant reduction of the surface area. Hence, it is still necessary to advance synthetic method for the production of stable colloidal MDS-based nanohybrids in an aqueous solution. Here, we report a simple, but versatile strategy for preparation of a stable dispersion of NP-deposited MDS nanohybrids in an aqueous solution. Polydopamine (PD), inspired by an adhesive protein in mussels [11], was adopted as a coating material to introduce functional groups onto the MDS surface. PD shells formed on individual MDSs effectively prevented re-stacking of MDSs while providing direct growth of NPs on 2D sheets under ultrasonic irradiation.
2. Experimental Prior to preparation of NP (i.e., Pt and Au)/PD/MoS2 nanohybrids, ultrathin MoS2 samples were synthesized by lithium intercalation and exfoliation method [6]. In a typical experiment, MoS2 flakes (0.1 g, Sigma–Aldrich) were purged by Ar gas for 4 h, after which n-butyl lithium (1.6 M, 1 mL, Sigma–Aldrich) was added into the sealed tube and the mixture was stirred at room temperature (RT) for 48 h under Ar. The resultant LixMoS2 samples were then washed several times with hexane and filtered to remove unreacted n-butyl lithium. As-obtained black powder was dried at RT under vacuum for 24 h. For functionalization of MoS2 sheets with PD, LixMoS2 flakes were dispersed in Tris-buffer aqueous solution (10 mM and pH 8.5) and then sonicated in an ultrasonication bath (Branson, model 1510) for 30 min to exfoliate the MoS2 sheets. Following centrifugation, the precipitates were
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discarded and the supernatant was collected, after which dopamine hydrochloride (2 mg/mL, Sigma–Aldrich) was added and the mixture was stirred at RT for 2 h. The PD/MoS2 powder was washed using deionized (DI) water thoroughly, then collected by filtration. To deposit the NPs onto PD/MoS2, the PD/MoS2 (1 mg) was mixed with 1 10 3 M of H2PtCl6 (or HAuCl43H2O) in bisolvent of ethanol (2 mL) and DI water (2 mL). The resultant mixture was sonicated for 1 min, after which the synthesized NP/PD/MoS2 nanohybrids were filtered, washed with DI water, and dried at 60 °C under vacuum. Transmission electron microscopy (TEM) images were collected using a JEM-2100F HR-TEM at an acceleration voltage of 200 kV. Xray diffraction (XRD) data were obtained on a Rigaku D/MAX-2500 (40 kW) with a q/q goniometer equipped with a Cu Ka radiation generator. X-ray photoelectron spectroscopy (XPS) was carried out using a VG multilab 2000 (Thermo VG Scientific) with a monochromatic Mg Ka X-ray source (hv = 1253.6 eV) under 10 7 Torr in a vacuum analysis chamber. The atomic force microscopy (AFM) images were recorded in the noncontact mode using a Nanoman Digital Instruments 3100 AFM (VEECO) with an etched silicon aluminum coated tip. The UV–visible absorption spectra were collected by using a Mecasys Optizen 2120UV spectrophotometer. The zeta potentials of colloidal suspension were measured by a Nicomp 380 ZLS particle analyzer. The reduction of 4-nitrophenol to 4-aminophenol by NaBH4 was studied as a model reaction to probe the catalytic activity of as-prepared Au/PD/MoS2. The 0.04 mL solution of 4-nitrophenol (10 mM) was mixed with 2.5 mL water and 0.5 mL of aqueous solution of NaBH4 (80 mM) in a quartz cuvette. Subsequently, 10 mg of as-prepared Au/PD/MoS2 as a catalyst was added in the solution. Immediately after adding the catalyst, UV–Vis spectra were recorded for the real time reaction at every 1 min intervals at room temperature.
3. Results and discussion Fig. 1a shows a schematic image of an experimental procedure for designing highly stable NP/PD/MoS2 nanohybrids. As starting materials, LixMoS2 samples were prepared from commercial bulk MoS2 powder and passed through the intercalation of lithium ions [6]. The LixMoS2 sheets were well dispersed in DI water through gentle sonication, which resulted in a yellow solution (Fig. 1b). After coating of polydopamine conducted by the self-polymerization of dopamine, highly stable PD/MoS2 nanohybrids with a black
Fig. 1. (a) Schematic diagram of the modification of exfoliated MoS2 with PD and the preparation of NP/PD/MoS2 hybrids. (b) Photograph images of the exfoliated MoS2, PD/ MoS2, Pt/PD/MoS2, and Au/PD/MoS2.
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color in water were observed (Fig. 1b). The MoS2 was coated with polydopamine via the mutual interactions between the catechol groups of PD and the MoS2 surface [11]. This PD coating method has been employed to give highly conformal layers with a thickness that can be controlled by varying the reaction time [12]. Pt and Au NPs were selected as the model for deposition of NPs because of wide interest in their use in many applications. Sonochemical in-situ synthesis was employed to deposit the NPs on the surface of PD/MoS2 owing to its simplicity, short reaction time, and accuracy [13]. Previous reports describe that the sonochemical method is useful for deposition of NPs on 2D nanosheet materials (e.g., graphene) [14–16]. NP precursors were chelated with catechol groups of PD, after which they were nucleated and grown on the surface of PD/MoS2 by ultrasound irradiation. Hence, the catechol groups in PD provided abundant nucleation and growth sites for a uniform deposition of NPs, while preventing the reaggregation of the MoS2 sheets in an aqueous solution (Fig. 1b). After deposition of Pt NPs on PD/MoS2, colloidal
suspension of the resulting Pt/PD/MoS2 remained well-dispersed in water (Fig. S1) and had 38.5 mV of zeta potential. It is wellknown that the zeta potential values below 30 mV indicate the formation of stable dispersion, from a general colloidal science perspective [17]. These foundings clearly demonstrated a highly stable dispersion of the Pt/PD/MoS2 in water. The morphology and structure of PD/MoS2 sheets were confirmed by the results of TEM, AFM, XPS and Raman analysis (Fig. 2). TEM images showed ultrathin and flat PD/MoS2 sheets, in which PD conformally coated the entire MoS2 sheets. As shown in Fig. 2b, HR-TEM revealed that the MoS2 surface was coated with the shell of the PD layer with a thickness of 5–7 nm after coating for 1 h. The average thickness of PD/MoS2 estimated from AFM image (Fig. S2) was 7.4 ± 0.35 nm, indicating that ultrathin MoS2 nanosheets were coated with PD. As the coating time increased from 1 to 2 h, the PD coating layer increased to a thickness of about 30 nm (Fig. 2d). These findings indicate that the PD coating thickness can be controlled by varying the reaction time. In addition,
Fig. 2. TEM images of PD/MoS2 samples with different coating time ((a) and (b) for 1 h and (c) and (d) for 2 h). (e) XPS survey spectrum of PD/MoS2. (f) Raman spectra of bulk MoS2 and PD/MoS2.
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the presence of the PD layer on the MoS2 surface was investigated by XPS (Fig. 2e). The significant C 1s, N 1s, and O 1s peaks indicate obvious formation of PDs on PD/MoS2 sheets. To understand the uniformity of the layer thickness of MoS2 sheets, we also conducted Raman spectroscopy using the E12g and A1g modes of the PD/MoS2 nanohybrids (Fig. 2f) [18]. The pristine bulk MoS2 exhibited two prominent E12g (383.5 cm 1) and A1g (407.2 cm 1) peaks corresponding to the in plane (2 S atoms in a direction opposite to the Mo atom) and out of plane (S atoms in opposite directions) vibrations for bulk MoS2 (Fig. 2f) [18]. When the layer number is less than 6, the energy gap between the A1g and E12g peak positions (D) can be used to determine the number of MoS2 layers. The pristine bulk MoS2 exhibited a D value of 26.1 cm 1 which matches well with the results of previous studies [18]. The D value for the PD/MoS2 sample was estimated to be 23.7 cm 1, indicating that the PD/MoS2 is less than 4 layers [18].
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Fig. 3 shows the TEM images for Pt/PD/MoS2 and Au/PD/MoS2 nanohybrids. As shown in Fig. 3a–c, TEM images revealed that Pt NPs were uniformly distributed on the surface of the PD/MoS2 sheet. The average diameter of 2.8 ± 0.52 nm for Pt NPs was calculated by measuring the diameters in the TEM image (Fig. 3c). From XPS elemental analysis, we calculated the loading amount of Pt and Au NPs on PD/MoS2 to be 6.78 at.% and 10.5 at.%, respectively. In addition, elemental mappings using energy dispersive X-ray spectroscopy was carried out following the TEM image of Fig. 3a. Notably, the molybdenum signal for MoS2, the nitrogen signal for PD, and the Pt signal overlapped uniformly along the surface of the Pt/PD/MoS2 sheet. These findings indicate conformal coating of PD and uniform distribution of Pt NPs on the MoS2 sheets. Au NPs were also synthesized and arrayed onto the surface of the PD/MoS2 sheet using the same method as for Pt/PD/MoS2 by changing only the NP precursor. The presence of Au NPs was
Fig. 3. TEM images of Pt/PD/MoS2 and Au/PD/MoS2 hybrids. (a), (b), and (c) show the Pt NPs on PD/MoS2 surface. (d) EDS elemental mapping of Pt/PD/MoS2 hybrid. (e) and (f) show Au NPs on PD/MoS2 surface.
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(JCPDS, Card No. 04-0862) [13]. In addition, the XRD pattern for Au/ PD/MoS2 revealed the formation of the face-centered cubic crystal structure of Au NPs with the (1 1 1), (2 0 0), (2 2 0), and (3 1 1) planes, respectively (JCPDS Card No. 4-784) [10]. The reduction of 4-nitrophenol to 4-aminophenol by NaBH4 was chosen as a model system in order to evaluate the catalytic activity of Au/PD/MoS2. The overall reaction of 4-nitrophenol was shown in Fig. 5a. In presence of Au/PD/MoS2, the 4-nitrophenolate can bind to the Au (1 0 0) surface through the two oxygen of the nitro group, leading to the faciliated kinetics for reduction reaction of 4-nitrophenol to 4-aminophenol [19]. The reduction process was monitored by UV–Vis spectroscopy. As shown in Fig. 5b the decrease in intensity of the 400 nm peak for p-nitrophenol has been observed with the progress of the time in the presence of Au/PD/MoS2 nanohybrids. The reduction of 4-nitrophenol was complete just within 8 min, indicating good catalytic activity due to a large amount of active sites of Au NPs on PD/MoS2. Fig. 4. XRD patterns for bulk MoS2 and PD/MoS2 (Inset is XRD patterns of Pt/PD/ MoS2 and Au/PD/MoS2.)
4. Conclusion Inspired by the highly adhesive ability of the mussel protein, we developed a versatile method for fabrication of NP-deposited MoS2 using PD in an aqueous solution. The surface modification of MoS2 with PD effectively prevented re-stacking of MoS2 sheets while providing a large amount of uniform deposition sites for the NPs. As a result, high density deposition of NPs using PD modification was obtained at the surface of MoS2 sheets. Acknowledgments This research was financially supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF2012R1A1A1041991) and the Korean Government (MSIP) (No. 2014R1A5A2010008). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2015.03.062. References [1] [2] [3] [4]
Fig. 5. (a) Reaction scheme for the reduction of 4-nitrophenol to 4-aminophenol. (b) Time-dependent UV–Vis spectra of the catalytic reduction of 4-nitrophenol to 4aminophenol with Au/PD/MoS2.
clearly observed in the TEM image (Fig. 3e). Although the size and array density of Au NPs differed from those of the Pt NPs, the formation of nano-crystalline structures was verified by the lattice fringes found in the HR-TEM image (Fig. 3f). The XRD measurements of bulk MoS2, PD/MoS2, Pt/PD/MoS2, and Au/PD/MoS2 samples are shown in Fig. 4. Comparison of the bulk MoS2 before exfoliation and PD coating revealed a prominent and sharp intensity (0 0 2) peak corresponding to a d-spacing of 0.62 nm [6]. This (0 0 2) reflection of the PD/MoS2 nanosheets is smaller than that of the bulk MoS2, indicating that a large number of PD/MoS2 sheets were highly exfoliated. Following deposition of Pt NPs, the Pt/PD/MoS2 nanohybrids showed three characteristic peaks corresponding to (1 1 1), (2 0 0), and (2 2 0) planes, indicating the formation of the face-centered cubic crystal structure of Pt NPs
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