Nanocomposite Synthesis of Nanodiamond and Molybdenum Disulfide

A chemically conjugated nanodiamond (ND)/MoS2 nanocomposite was synthesized with amine-functionalized MoS2 and acyl chloride-coordinated ND. The chemical structure and morphology of the nanocomposite were characterized to examine the dispersion of MoS2 on the ND platform. The results revealed that the degree of dispersion was enhanced with increasing ratio of MoS2 nanosheets to ND. Moreover, the nanosheets consisted of several molecular interlayers that were well-dispersed on the ND platform, thereby forming a nanophase. The efficient electrocapacity of the ND/MoS2 nanocomposite was considerably greater than that of the MoS2 electrode alone. Furthermore, the nanophase distribution of MoS2 on ND with a graphitic shell provided a large surface area and reduced the diffusion distance of ions and electrons. Therefore, the nanophase electrode showed higher electrochemical capacitance than that of the MoS2 electrode alone.


Introduction
Numerous novel materials and composites including two-dimensional (2D) materials have been exploited with the aim of enhancing electrocapacity, which can be applied for biosensing platforms and electrocatalytic performance. A 2D layered material can be defined as an unsupported crystalline solid with molecular layer thickness characterized by intralayer storage for heat, charge, and light transport [1][2][3][4]. This transport occurs in the presence of intralayer covalent bonds and intercalation-based interaction [5]. Specifically, MoS 2 as a 2D transition metal dichalcogenide (TMD) exhibits the unique characteristic of charge confinement in the 2D layer in the absence of interlayer interaction along the z-axis [6]. Functional features of 2D MoS 2 layers include high thermal and chemical stability for functionalization [7,8], large surface area [4], good mobility [9], and intercalation-based physical interaction [10,11]. For final functional performance, the construction of an electrode and nanophase distribution of an aggregation-free 2D MoS 2 molecular layer are critical to induce maximal functional features.
Carbon-based materials including fullerenes, graphene, and carbon nanotubes, are considered as fundamental platform materials of energy conversion and storage, owing to their thermal stability, conductivity, and mechanical properties [12][13][14][15]. Furthermore, the electron buffering capability of these materials stems from their high surface-to-volume ratio and the unsaturated carbon bonds. Physicochemical characteristics can reduce oxygen adsorption on a catalyst surface, thereby improving the material performance. In general, the porous structure of carbon materials is characterized by a multiscale nanocage and a high surface-to-volume ratio that provide electron transfer and electrocatalytic active sites. These advantages have often been applied to nanocomposites with metal to induce synergistic functional effects, including solar cells, batteries, and supercapacitors [16][17][18][19][20].
X-ray photoeletron spectroscopy (XPS; ThermoFisher Scientific Co., Waltham, MA, USA) measurements were also conducted with an Al Kα energy source. The spectra were analyzed using Avantage software (version 1.6, Thermo Fisher Scientific, Waltham, MA, USA). Transmission electron microscopy (TEM) images were obtained with a JEM-2100F electron microscope (JEOL, Tokyo, Japan). For sample preparation, 10 μL each of ND, MoS2, and ND/MoS2 nanocomposite was dropped on Formvar/Carbon on a 200 mesh grid (TED PELLA Co., Redding, CA, USA.) and dried for 10 min at 60 °C in an oven.

Cyclic Voltammetry (CV) Measurements
A dispersion of ND/MoS2 nanocomposite (1 mg/mL) was prepared in Nafion solution (0.5% in DI water) for electrode fabrication. Droplets of the dispersion (10 μL) were placed on the glassy carbon working electrode (diameter 3 mm) and dried at 80 °C for 1 h. Cyclic voltammetry (CV) measurements (potential: −0.8 to 0.2 V, scan rate: -0.2 to 1.0 V) were performed using a three-electrode system (reference electrode: Ag/AgCl, counter electrode: platinum wire (57 mm in length, OD 0.5 mm), working electrode: ND/MoS2 or MoS2 deposited glassy carbon electrode). Two sets of measurements using a potensiostat (DY2322, Digi-Ivy, Austin, TX, USA) were also performed at potentials ranging from 0.05 to 0.5 V·s −1 and scan rates of 0.05, 0.1, and 0.5 V/s. The corresponding CV plots were recorded in a 0.1 M KOH solution (15 ml). Five cycles in 2 sets were performed to obtain a voltamogram. After stabilization from repeated cycles, an oxidation and reduction curve in each second set of a working electrode was shown for data presentation.

Results and Discussion
UV-vis absorption spectra were obtained for the MoS2 nanosheets, ND-COCl, and ND/MoS2 nanocomposite with ratios of 1:1, 1:2, 1:4, and 1:8 ( Figure 1). Typical MoS2 excitation absorption peaks, which occurred at 630 and 690 nm, were attributed to the direct gap transitions at the K point [30][31][32]. These indicate the lowest optical band gap of the MoS2 nanosheets (i.e., ~1.8 eV) that were changed, owing to the quantum confinement in the sheets. Changes in the UV absorbance were evaluated for the MoS2/ND nanocomposite with ratios of 1:1 to 1:8. The optical absorbance peaks of

Nanocomposite Formation of MoS 2 and Nanodiamonds
ND-COOHs (40 mg) gifted from Nanoresource (Seoul, Korea) were mixed with 100 mL of thionyl chloride (Samcheon Chemical Co., Seoul, Korea) and 0.5 mL of dimethylformamide anhydrous (DMF) in a 250 mL round-bottomed flask. The ND-COOHs were well-dispersed for 15 min of bath sonication under an ice bath. The ND dispersion for acylation was stirred for 24 h at 70 • C under a N 2 purge. After reaction, the dispersion was washed repeatedly five times with tetrahydrofuran anhydrous (THF) and the NDs, separated by centrifugation, were dried at 60 • C in an air-circulated oven.
The MoS 2 nanosheets (1 mg/mL) were dispersed in DMF. For chemical conjugation, 200 µL of cystamine (5 mg/mL) in DMF was added to 1 mL of the MoS 2 dispersion. This dispersion mixture was then sonicated for 1 h under an ice bath. After sonication, the mixture was left to stand at room temperature for 24 h. Then, functionalized MoS 2 was washed repeatedly three times with DMF, separated by centrifugation at 14,600 rpm, and completely dried at 60 • C in a vacuum oven.
Acylated NDs (1 mg/mL) and amine-functionalized MoS 2 nanosheets in DMF were dispersed with 5 min of bath sonication. The dispersion of acylated ND and amine-functionalized MoS 2 with desired ratios (1:1, 1:2, 1:4, and 1:8) was mixed by sonication under an ice bath for 30 min, and shaken with a vortex for 24 h. After the reaction, the composite was washed and completely dried at 60 • C in a vacuum oven.
X-ray photoeletron spectroscopy (XPS; ThermoFisher Scientific Co., Waltham, MA, USA) measurements were also conducted with an Al Kα energy source. The spectra were analyzed using Avantage software (version 1.6, Thermo Fisher Scientific, Waltham, MA, USA). Transmission electron microscopy (TEM) images were obtained with a JEM-2100F electron microscope (JEOL, Tokyo, Japan). For sample preparation, 10 µL each of ND, MoS 2 , and ND/MoS 2 nanocomposite was dropped on Formvar/Carbon on a 200 mesh grid (TED PELLA Co., Redding, CA, USA.) and dried for 10 min at 60 • C in an oven.

Cyclic Voltammetry (CV) Measurements
A dispersion of ND/MoS 2 nanocomposite (1 mg/mL) was prepared in Nafion solution (0.5% in DI water) for electrode fabrication. Droplets of the dispersion (10 µL) were placed on the glassy carbon working electrode (diameter 3 mm) and dried at 80 • C for 1 h. Cyclic voltammetry (CV) measurements (potential: −0.8 to 0.2 V, scan rate: −0.2 to 1.0 V) were performed using a three-electrode system (reference electrode: Ag/AgCl, counter electrode: platinum wire (57 mm in length, OD 0.5 mm), working electrode: ND/MoS 2 or MoS 2 deposited glassy carbon electrode). Two sets of measurements using a potensiostat (DY2322, Digi-Ivy, Austin, TX, USA) were also performed at potentials ranging from 0.05 to 0.5 V·s −1 and scan rates of 0.05, 0.1, and 0.5 V/s. The corresponding CV plots were recorded in a 0.1 M KOH solution (15 ml). Five cycles in 2 sets were performed to obtain a voltamogram. After stabilization from repeated cycles, an oxidation and reduction curve in each second set of a working electrode was shown for data presentation.

Results and Discussion
UV-vis absorption spectra were obtained for the MoS 2 nanosheets, ND-COCl, and ND/MoS 2 nanocomposite with ratios of 1:1, 1:2, 1:4, and 1:8 ( Figure 1). Typical MoS 2 excitation absorption peaks, which occurred at 630 and 690 nm, were attributed to the direct gap transitions at the K point [30][31][32] . These indicate the lowest optical band gap of the MoS 2 nanosheets (i.e.,~1.8 eV) that were changed, owing to the quantum confinement in the sheets. Changes in the UV absorbance were evaluated for the MoS 2 /ND nanocomposite with ratios of 1:1 to 1:8. The optical absorbance peaks of MoS 2 /ND nanocomposite appeared in the same region as those of the MoS 2 nanosheets. It was indicated that chemically conjugated MoS 2 /ND nanocomposite maintained the optical characteristics of the nanosheets. The intensity of the bands in the spectra were augmented with the increasing ratio of the nanocomposite, as shown in Figure 1 [33]. MoS2/ND nanocomposite appeared in the same region as those of the MoS2 nanosheets. It was indicated that chemically conjugated MoS2/ND nanocomposite maintained the optical characteristics of the nanosheets. The intensity of the bands in the spectra were augmented with the increasing ratio of the nanocomposite, as shown in Figure 1 [33]. Chemical conjugation of the nanocomposite and functional groups of intermediate products was characterized via FT-IR spectroscopy to identify sequential chemical modification of the acyl chloride comprising ND (ND-COCl), MoS2-NH2, and MoS2/ND nanocomposites ( Figure 2). The oxidative treatment formed a carbonyl group (C=O) and a hydroxyl group (-OH), occurring as absorption peaks at 1718 and 3400 cm −1 on the surface of the carboxylated ND, respectively. The graphitic shell around the crystalline ND was attributed to the absorption bands associated with C=C bond bending at 1625 cm −1 . After acyl chlorination, ND with an acyl chloride bond (C-Cl) was formed, corresponding to the stretching peak at 593 cm −1 , whereas no peak for ND-COOH was observed [34]. The results indicated that the carboxylated ND surface was activated by the acyl chloride group for an amine reactive reaction. The functionalization of MoS2 with cysteamine hydrochloride was characterized with comparison of the cysteamine hydrochloride. For the amine-functionalized MoS2, an N−H deformation vibration peak and an NH2 stretching vibration peak, derived from the chemical conjugation with cysteamine hydrochloride, occurred at 1604 and 3390 cm −1 , respectively [35]. This indicated the successful modification of amine-functionalized MoS2, where amine groups were positioned on the surface of MoS2 nanosheets. The formation of an amide bond (NHCO) between the acyl chloride of ND and the functionalized MoS2 nanosheets was confirmed via FT-IR spectroscopy of the ND/MoS2 nanocomposites. The spectra obtained for the nanocomposites exhibited characteristics of both ND-COCl and the functionalized nanosheets. The absorption bands associated with the ND/MoS2 peak were attributed to hydroxyl bond stretching and an amide bond (NHCO) at 3400 cm −1 and 1632 cm −1 , respectively. Furthermore, the relatively strong absorbance band region resulted probably from the overlapping of bands associated with C=C vibration and C=O stretching [36,37].  (Figure 2). The oxidative treatment formed a carbonyl group (C=O) and a hydroxyl group (-OH), occurring as absorption peaks at 1718 and 3400 cm −1 on the surface of the carboxylated ND, respectively. The graphitic shell around the crystalline ND was attributed to the absorption bands associated with C=C bond bending at 1625 cm −1 . After acyl chlorination, ND with an acyl chloride bond (C-Cl) was formed, corresponding to the stretching peak at 593 cm −1 , whereas no peak for ND-COOH was observed [34]. The results indicated that the carboxylated ND surface was activated by the acyl chloride group for an amine reactive reaction. The functionalization of MoS 2 with cysteamine hydrochloride was characterized with comparison of the cysteamine hydrochloride. For the amine-functionalized MoS 2 , an N−H deformation vibration peak and an NH 2 stretching vibration peak, derived from the chemical conjugation with cysteamine hydrochloride, occurred at 1604 and 3390 cm −1 , respectively [35]. This indicated the successful modification of amine-functionalized MoS 2 , where amine groups were positioned on the surface of MoS 2 nanosheets. The formation of an amide bond (NHCO) between the acyl chloride of ND and the functionalized MoS 2 nanosheets was confirmed via FT-IR spectroscopy of the ND/MoS 2 nanocomposites. The spectra obtained for the nanocomposites exhibited characteristics of both ND-COCl and the functionalized nanosheets. The absorption bands associated with the ND/MoS 2 peak were attributed to hydroxyl bond stretching and an amide bond (NHCO) at 3400 cm −1 and 1632 cm −1 , respectively. Furthermore, the relatively strong absorbance band region resulted probably from the overlapping of bands associated with C=C vibration and C=O stretching [36,37]. Nanomaterials 2019, 9, x FOR PEER REVIEW 5 of 11 The chemical structure of the ND/MoS2 nanocomposite, compared with that of ND-COCl, was determined from the measured XPS spectra, as shown in Figure 3a [38,39]. The C=O peak at 287.2 eV for ND-COCl was generated from both acyl chloride and some portion of the carboxyl group. The C=O peak at 287.2 eV arose from the amide group after chemical conjugation of ND/ND/MoS2 nanocomposites. After chemical conjugation between ND-COCl and amine-functionalized MoS2, the intensity of the peak at 286.0 V (C-O) increased significantly relative to that of the peak at 287.2 V (C=O). The results indicate that unreacted acyl chloride would decompose into carboxyl groups during the washing process and from contact with moisture. The O1 peaks of ND-COCl and ND/MoS2 at 531.8 and 533.1 eV were attributed to C=O and COC/COH, respectively. The intensity of the ND/MoS2 peak increased relative to that of the ND-COCl peak, indicating that the unreacted acyl chloride group was converted to the carboxyl group after chemical conjugation with the MoS2 nanosheets (Figure 3b,e). The Cl 2p peaks of the ND-COCl were deconvoluted into two conventional binding energies of 200.3 eV (Cl 2p 3/2) and 201.9 eV (Cl 2p 1/2). This indicated that ND-COOH was functionalized by an acyl chloride group on the surface [36]. Deconvolution of the Cl 2p peaks, corresponding to ND/MoS2 generated peaks at 200.3 eV (Cl 2p 3/2) and 201.9 eV (Cl 2p 1/2), have resulted from the physical adsorption of remnant chloride ions onto the ND surface and nanocage [34,36].  [38,39]. The C=O peak at 287.2 eV for ND-COCl was generated from both acyl chloride and some portion of the carboxyl group. The C=O peak at 287.2 eV arose from the amide group after chemical conjugation of ND/ND/MoS 2 nanocomposites. After chemical conjugation between ND-COCl and amine-functionalized MoS 2 , the intensity of the peak at 286.0 V (C-O) increased significantly relative to that of the peak at 287.2 V (C=O) . The results indicate that unreacted acyl chloride would decompose into carboxyl groups during the washing process and from contact with moisture. The O1 peaks of ND-COCl and ND/MoS 2 at 531.8 and 533.1 eV were attributed to C=O and COC/COH, respectively. The intensity of the ND/MoS 2 peak increased relative to that of the ND-COCl peak, indicating that the unreacted acyl chloride group was converted to the carboxyl group after chemical conjugation with the MoS 2 nanosheets (Figure 3b,e). The Cl 2p peaks of the ND-COCl were deconvoluted into two conventional binding energies of 200.3 eV (Cl 2p 3/2 ) and 201.9 eV (Cl 2p 1/2 ). This indicated that ND-COOH was functionalized by an acyl chloride group on the surface [36]. Deconvolution of the Cl 2p peaks, corresponding to ND/MoS 2 generated peaks at 200.3 eV (Cl 2p 3/2 ) and 201.9 eV (Cl 2p 1/2 ), have resulted from the physical adsorption of remnant chloride ions onto the ND surface and nanocage [34,36]. Figure 4 shows the XPS spectra of the MoS 2 and ND/MoS 2 nanocomposite. Two strong peaks for the Mo 3d peak of MoS 2 are observed at 229.85 for doublet Mo 3d 5/2 and 232.98 eV for and Mo 3d 3/2 (Figure 4a). The peaks, corresponding to the S 2p 1/2 and S 2p 3/2 orbital of divalent sulfide ions (S 2 −) occur at binding energies of 163.85 and 162.65 eV, respectively, as shown in Figure 4b. The results are consistent with the values reported for a MoS 2 crystal [35,[40][41][42]. The XPS spectra of ND/MoS 2 also revealed typical MoS 2 crystalline characteristics with chemically induced shifting. Furthermore, the peak position moved from 229.85 to 229.53 eV for Mo 3d 5/2 and from 232.98 to 232.64 eV for Mo 3d 3/2 , respectively. Chemically induced shifts were also observed for ND/MoS 2 S 2p, with the peaks shifting from 162.65 to 162.44 eV for S 2p 3/2 and 163.85 to 163.71 eV for S 2p 1/2 peaks [43]. This indicated that the chemical shift of the ND/MoS 2 nanocomposite to lower binding energy than that of MoS 2 resulted from chemical conjugation with ND.   [35,[40][41][42]. The XPS spectra of ND/MoS2 also revealed typical MoS2 crystalline characteristics with chemically induced shifting. Furthermore, the peak position moved from 229.85 to 229.53 eV for Mo 3d5/2 and from 232.98 to 232.64 eV for Mo 3d3/2, respectively. Chemically induced shifts were also observed for ND/MoS2 S 2p, with the peaks shifting from 162.65 to 162.44 eV for S 2p3/2 and 163.85 to 163.71 eV for S 2p1/2 peaks [43]. This indicated that the chemical shift of the ND/MoS2 nanocomposite to lower binding energy than that of MoS2 resulted from chemical conjugation with ND.  The morphologies of MoS2, ND, and ND/MoS2 nanocomposites ( Figure 5) were evaluated with TEM. The MoS2 nanosheets was featured as average size of 300-400 nm along the long axis ( Figure  5a,b). The MoS2 nanosheets had a crystalline structure, and several molecular layers were stacked or folded in a planar form [44]. Furthermore, the NDs were well-dispersed with small agglutinins ranging from 80 to 200 nm (Figure 5c,d). The ND/MoS2 nanocomposite with chemical conjugation showed morphological features that several molecular layers of thin MoS2 nanosheet enveloped ND agglutinins (Figure 5e-h). Amine-functionalized MoS2 nanosheets and NDCOCls were chemically reacted through surface contact, suggesting that the ND/MoS2 nanocomposite was successfully synthesized (Figure 5e-h). The morphologies of MoS 2 , ND, and ND/MoS 2 nanocomposites ( Figure 5) were evaluated with TEM. The MoS 2 nanosheets was featured as average size of 300-400 nm along the long axis (Figure 5a,b). The MoS 2 nanosheets had a crystalline structure, and several molecular layers were stacked or folded in a planar form [44]. Furthermore, the NDs were well-dispersed with small agglutinins ranging from 80 to 200 nm (Figure 5c,d). The ND/MoS 2 nanocomposite with chemical conjugation showed morphological features that several molecular layers of thin MoS 2 nanosheet enveloped ND agglutinins (Figure 5e-h). Amine-functionalized MoS 2 nanosheets and NDCOCls were chemically reacted through surface contact, suggesting that the ND/MoS 2 nanocomposite was successfully synthesized (Figure 5e-h).
The morphologies of MoS2, ND, and ND/MoS2 nanocomposites ( Figure 5) were evaluated with TEM. The MoS2 nanosheets was featured as average size of 300-400 nm along the long axis ( Figure  5a,b). The MoS2 nanosheets had a crystalline structure, and several molecular layers were stacked or folded in a planar form [44]. Furthermore, the NDs were well-dispersed with small agglutinins ranging from 80 to 200 nm (Figure 5c,d). The ND/MoS2 nanocomposite with chemical conjugation showed morphological features that several molecular layers of thin MoS2 nanosheet enveloped ND agglutinins (Figure 5e-h). Amine-functionalized MoS2 nanosheets and NDCOCls were chemically reacted through surface contact, suggesting that the ND/MoS2 nanocomposite was successfully synthesized (Figure 5e-h).  On the same material set, scan rates of 0.05, 0.1, and 0.5 V/s, respectively, and range of potential voltage was differently applied to −0.8 to −0.2 V and −0.2 to 1.0 V. The shape and magnitude of the voltamogram was transitioned from a peak-like shape (Figure 6a-c) to quasi-rectangular shape (Figure 6d-f) depending on the scan rate and potential voltage. The quasi-rectangular shape typically indicates constant and time dependent concentration gradient of an electroactive surface where the electrode radius was typically smaller than diffusion layer [47]. Each CV graph is characterized by a quasi-rectangular shape, consistent with dual behavior, such as the electrical double layer capacitance [48]. MoS 2 nanosheets constitute the minimum area of the CV plot, whereas 1:8 and 1:6 ND/MoS 2 constitutes the maximum area, which corresponds to the enhanced capacitance. The CV curve of the ND/MoS 2 nanocomposites reveals the higher current response and large working area of these composites, compared with those of the MoS 2 nanosheet only ( Figure 6). The results suggest that the addition of nanodiamonds enhances the electrochemical activity and increases the specific capacitance of MoS 2 electrode alone [29,35].
The superior electrical performance of the ND/MoS 2 nanocomposite electrode, compared with that of the MoS 2 electrode alone, resulted from the unique nanostructure of the composite electrode. Moreover, the large surface area and the nanosized MoS 2 phase of the ND/MoS 2 composites may have resulted in significant reduction of the diffusion length associated with ion and electron transfer during the oxidation/reduction process. The electrode nanoscale phase makes these composites promising for various applications. Specifically, the NDs acted as nanoscale supports to functionalize synergistically the MoS 2 sheets, which served as a three-dimensional highly conductive current collector. The featured architecture of the ND/MoS 2 nanocomposites possessing a large specific surface of the electrode enables rapid and simultaneous electron and ion transport, thereby leading to excellent electrochemical capacitive performance [42].
typically indicates constant and time dependent concentration gradient of an electroactive surface where the electrode radius was typically smaller than diffusion layer [47]. Each CV graph is characterized by a quasi-rectangular shape, consistent with dual behavior, such as the electrical double layer capacitance [48]. MoS2 nanosheets constitute the minimum area of the CV plot, whereas 1:8 and 1:6 ND/MoS2 constitutes the maximum area, which corresponds to the enhanced capacitance. The CV curve of the ND/MoS2 nanocomposites reveals the higher current response and large working area of these composites, compared with those of the MoS2 nanosheet only ( Figure 6). The results suggest that the addition of nanodiamonds enhances the electrochemical activity and increases the specific capacitance of MoS2 electrode alone [29,35]. The superior electrical performance of the ND/MoS2 nanocomposite electrode, compared with that of the MoS2 electrode alone, resulted from the unique nanostructure of the composite electrode. Moreover, the large surface area and the nanosized MoS2 phase of the ND/MoS2 composites may have resulted in significant reduction of the diffusion length associated with ion and electron

Conclusions
A chemically conjugated ND/MoS 2 nanocomposite was synthesized with amine-functionalized MoS 2 and acyl chloride-coordinated NDs. The chemical structure and morphology of the nanocomposite were characterized, and the results revealed that the MoS 2 nanosheets were well-distributed on the ND platform, thereby forming a nanophase. Nanophase distribution of MoS 2 on ND with a graphitic shell may provide a large surface area and reduce the diffusion distance of ions and electrons. Therefore, the augmented electrochemical capacitance of the nanophase electrode was induced, compared to that of the MoS 2 electrode alone.
Author Contributions: For research articles was prepared and supported by conceptualization. S.Y.K.; data curation, D.L.; writing-original draft preparation, Y.K.; and supervision, E.K. and C.K.K.