Covalently Bonded MoS2âBorocarbonitride Nanocomposites

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Covalently Bonded MoS-Borocarbonitride Nanocomposites Generated by Using Surface Functionalities on the Nanosheets and Their Remarkable HER Activity Kuppe Pramoda, Mohd Monis Ayyub, Navin Kumar Singh, Manjeet Chhetri, Uttam Gupta, Amit Soni, and Chrintamani Nages Ramachandra Rao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10782 • Publication Date (Web): 13 Dec 2017 Downloaded from http://pubs.acs.org on December 14, 2017

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Covalently

Bonded

MoS2-Borocarbonitride

Nanocomposites

Generated by Using Surface Functionalities on the Nanosheets and their Remarkable HER Activity K. Pramoda, Mohd Monis Ayyub, Navin Kumar Singh, Manjeet Chhetri, Uttam Gupta, Amit Soni and C. N. R. Rao* New Chemistry Unit, Chemistry and Physics of Materials Unit, Sheikh Saqr Laboratory and International Centre for Materials Science, Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Jakkur P. O., Bangalore 560064, India *E-mail: [email protected]

ABSTRACT In the light of the recent discovery that carbon-rich borocarbonitrides show electrocatalytic activity for generating hydrogen from water, we have synthesized nanocomposites by covalently cross-linking BC7N with MoS2 sheets to explore whether the HER activity can be significantly enhanced. In order to cross-link BC7N and MoS2 sheets, we have exploited the presence of different functional groups on the surfaces of BN (NH2) and graphene (COOH) domains of the borocarbonitride, as quantitatively determined by FLOSS. We have thus obtained two nanocomposites differing in the location of the cross-linking and these are designated as BN/BCN-MoS2 and G/BCN-MoS2, depending on which domains in the borocarbonitride participate in cross-linking. These nanocomposites were characterized by various spectroscopic methods including fluorescence labelling and their electrochemical and photochemical HER 1 ACS Paragon Plus Environment

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activity investigated. The composite where the graphene domains are cross-linked to MoS2 nanosheets, G/BCN-MoS2 (1:2), exhibits outstanding electrochemical HER activity with an onset potential of -30 mV (vs. RHE) and a current density of 10 mA cm-2 at an overpotential of -35 mV. This performance is closely comparable to that of Pt. The composite where the BN domains were cross-linked show somewhat lower activity. The physical mixture of BCN and MoS2, on the other hand, does not display any notable HER activity. The BCN-MoS2 composites also exhibit good photochemical activity. It is noteworthy that 2H-MoS2 which does not exhibit significant catalytic activity can be rendered highly active by cross-linking with BCN.

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Introduction Generation of hydrogen by photochemical or electrochemical means has assumed great importance in the last few years. To date, no catalyst has shown better hydrogen evolution reaction (HER) performance than platinum which suffers from issues related to stability and high cost.1-5 In the last few years, there has been effort to replace platinum by inexpensive earthabundant materials, preferably those which are metal free.6,7 Some nonmetallic materials such as C3N4,8,9 MoS210,11 and reduced graphene oxide12 have been used for the purpose. Amongst the various nonmetallic materials, borocarbonitrides, BxCyNz, deserve a special mention in the light of the recent discovery that carbon-rich borocarbonitride compositions, compare reasonably with platinum in electrochemical performance.13 We considered it important to further investigate and possibly improve the performance of borocarbonitrides by covalently linking them with MoS2, since both these component materials are intrinsically HER active.14,15 What is of special interest is that BCN layers contain both graphene and boron nitride (BN) type domains possibly along with the BCN rings.15,16 These domains contains different surface functional groups. Thus, the surface of the BN domains would contain amino groups while that of graphene domains would contain surface carboxyl and other oxygen functionalities.16 Making use of the presence of different surface functionalities in the BN and graphene domains, we have cross-linked the borocarbonitride, BC7N, with MoS2 with specific bonding occurs between MoS2 and the graphene domains or with the BN domains, and examined the electrochemical HER activity of these composites in comparison with that of the physical mixture as well as with the individual components. Interestingly, we have found outstanding improvement in the electrochemical HER activity comparable to that of Pt in the case of the covalently cross-linked BC7N-MoS2 (1:2) composites where the graphene domains of BC7N were bonded to MoS2. It is noteworthy that the



composite with high electrochemical HER activity exhibits good photochemical HER activity as well. We have not found any significant HER activity in the case of covalently cross-linked BNMoS2 composites possibly due to the absence of conductive carbon domains. Neither do we observe any meaningful HER activity with physical mixture of MoS2 and BC7N. In the presentation that follows, we have designated the BCN-MoS2 composites where BN domains are cross-linked to MoS2 sheets as BN/BCN-MoS2 and the composites where the graphene domains are cross-linked with MoS2 sheets as G/BCN-MoS2.

Experimental section BC7N nanosheets were prepared by the method as reported earlier.13,15 Boric acid (60 mg), activated charcoal (500 mg) and urea (2.4 g) were heated in a tubular furnace at 900 o

C under N2 atmosphere for 10 h. The obtained black product was treated with NH3 at 900

o

C for 5 h. Exfoliated MoS2 (1T-MoS2) was obtained by the Li-intercalation of bulk MoS2

(LixMoS2) with n-butyllithium reagent and subsequent exfoliation using de-ionized water as reported elsewhere.14 Carboxylate functionalization of MoS2 (MoS2-CH2COOH) was achieved by reacting exfoliated 1T-MoS2 solution with 10-fold excess of 2-bromoacetic acid and allowed to stir for 5 days at room temperature.17 The precipitated sample was washed with water and 2-propanol to remove unreacted acid, and dried at 60 °C under vacuum. Amine functionalized MoS2 (MoS2-C6H4NH2) was obtained by reacting 1TMoS2 with a solution of the iodobenzene reagent (12 equiv) dissolved in N,N′dimethylformamide (DMF) (60 mL) and the resulting mixture was allowed to stir at room temperature for 72 h.18 The black precipitate obtained was separated by centrifugation and then washed with DMF and ethanol to remove unreacted reagents, organic byproducts.


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BN/BCN-MoS2 composites were prepared by mixing BC7N (50 mg) and MoS2CH2COOH (50 mg) samples in dry DMF (5 ml) in a Schlenck flask through bath sonication.

To

the

resultant

dispersion,

coupling

reagents

N-(3-

(dimethylamino)propyl)N′ethylcarbodiimidehydrochloride (EDC·HCl, 20 mg) and 1hydroxybenzotriazole (HOBt, 20 mg) were added under N2 atmosphere and allowed to stir for 48 h. The obtained product was collected by vacuum filtration and washed several times with copious amounts of DMF and water to remove by products. For the synthesis of G/BCN–MoS2 nanocomposites, BCN (50 mg) and MoS2–C6H4NH2 (50 mg) samples were dispersed in DMF, and the aforementioned procedure was repeated. For the preparation of the physical mixture of BCN and MoS2, BCN (50 mg) and MoS2 (50 mg) were dispersed in aqueous solution and then filtered to get a solid product. Surface concentrations of functional groups were determined by chemical tagging and photoluminescence spectroscopy. Tagging of amine groups using NHS-Cy5 was carried out as follows. To a 2 mg sample under study, 10 µL of 1.42 mM of NHS-Cy5 dye solution, 100 µL of triethylamine (TEA) and 6 mL of DMSO were added and the resulting solution was stirred in dark at room temperature for 12 h.16,19 After the reaction, the obtained solutions were centrifuged and supernatant was transferred to a flask. Fluorescence tagging of the carboxyl group using 1-bromoacetyl pyrene was carried out as follows. To a 2 mg sample, 2 mL 1.7 mM solution of 1-(bromoacetyl)pyrene in DMF, 1.0 mg of K2CO3 and 1.0 mg of KI were added. The resulting solution was stirred in dark at 50 °C for 12 h and the aforementioned procedure was repeated. The electrochemical HER studies were done in 0.5 M H2SO4 using a typical threeelectrode cell where, Ag/AgCl (3 M NaCl) and Pt coil were used as reference and counter



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electrodes, respectively. Working electrode was prepared by drop-casting the catalyst-ink (ink was prepared by dispersing the catalyst in a mixture of 4:1 water-isopropanol along with nafion solution (5 wt %); 4:1:0.5 (v/v)) on a pre-cleaned glassy carbon electrode (GCE) (see supporting information for more details). For photochemical HER, catalyst (3 mg) was dispersed in aqueous solution of triethanolamine (15% v/v, 48 mL) by sonication in a glass vessel and

Eosin Y (14

µmmol) was added. This mixture was illuminated with halogen lamp (100 W) under constant stirring. A 2 mL of evolved gas was manually collected from the headspace and analyzed with gas chromatograph (PerkinElmer ARNL 580C) using a thermal conductivity detector.

Results and Discussion A borocarbonitride of the composition BC7N was prepared by the method reported in the literature.13,15 X-ray photoelectron spectroscopy (XPS) showed the presence of B–C, B–N, C–C and C–N bonds with the C-N bonds mainly arising from pyridinic and pyrrolic nitrogens (Figure S1). Quantitative estimation of the surface functionalities of BC7N was made by the fluorescence labelling of surface species (FLOSS), employing NHS-Cy5 and 1-(bromoacetyl)pyrene dye as the probes for the amine and carboxyl functionalities in the BN and graphene domains respectively.16,19 The number of amine and carboxyl groups present on the BCN surface were estimated using calibration graph as shown in Figures 1a and c respectively. Figure S2 shows the deconvoluted emission spectrum of 1-(bromoacetyl)pyrene which consists of four well-defined peaks at ∼376, 388, 385 and 410 nm, respectively. These are ascribed to the π → π* transitions and are cumulatively termed as monomeric emission.20


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5

5

3.2x10

4

(b)

3 2

5

1 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Concentration (µM)

2.843 µΜ 1.421 µΜ 0.710 µΜ 0.355 µΜ 0.178 µΜ

PL intensity

6x10 5 5x10 5 4x10 5 3x10 5 2x10 5 1x10 0

6

(a)

PL intensity (×105)

PL Intensity

5

2.4x10

Control (NHS-CY5 dye BCN

5

1.6x10

4

8.0x10

0.0

600

650 700 750 Wavelength (nm)

800

600


800

6

2.5x10

6

2.0x10

6

1.5x10

(c)

6

6

2.0x10

2.0x10

6

(d)

1.5x10

6

1.0x10

0.008 0.016 0.024 Concentration (mM)

0.0246 mM 0.0208 mM 0.0159 mM 5.56 µΜ 1.34 µΜ

6

1.0x10

5

5.0x10 0.0

PL intensity

6

Intensity

2.5x10

PL intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6

1.5x10

6

1.0x10

Control (1-(bromoacetyl)pyrene BCN

5

5.0x10

0.0 400 450 500 Wavelength (nm)

550


550

Figure 1. (a) Emission spectra of NHS-Cy5 of different concentrations (excitation wavelength 530 nm). Inset shows the calibration curve of fluorescent intensity as a function of concentration at 680 nm, (b) Emission spectra of NHS-CY5 dye after reaction with amine groups of BCN, (c) Emission spectra of 1-(bromoacetyl)pyrene of different concentrations (excitation wavelength 320 nm). Inset shows the calibration curve of fluorescent intensity as a function of concentration at 410 nm (monomeric emission), (d) Emission spectra of 1-(bromoacetyl)pyrene dye after reaction with carboxyl groups of BCN. In addition to monomeric signals, broad bands appear at longer wavelengths (at 428 to 450 nm) are attributed to the excited state dimer or “excimer” emission. The calibration curve of 1(bromoacetyl)pyrene is obtained from the emission spectra by monitoring the intensity at 410 nm 7 ACS Paragon Plus Environment


and plotting the intensity as a function of concentration as shown in the inset of Figure 1c. The BC7N sample showed surface concentrations of the amino and carboxylic groups to be 1.7 × 1019 and 8× 1020 groups/g, respectively. Similarly, number of carboxylic groups in MoS2-CH2COOH determined by FLOSS method is found to be 1.34 ×1018 groups/g. Scheme 1 shows the schematic of the synthetic strategy to prepare cross-linked BN/BCNMoS2 and G/BCN-MoS2 nanocomposites by the carbodimide reaction. BCN was reacted with carboxyl functionalized MoS2 (MoS2-CH2COOH), prepared by the reaction of metallic 1T-MoS2 with bromoacetic acid,17 to prepare BN/BCN-MoS2 using EDC as the coupling reagent.14,15

Scheme 1 Synthetic strategy for covalently cross-linked BN/BCN-MoS2 and G/BCN–MoS2 nanocomposites (EDC = 1-ethyl-3-(3 dimethylaminopropyl)carbodiimide). 8 ACS Paragon Plus Environment

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Coupling of the amine functionalized MoS2 (MoS2-C6H4NH2),18 prepared by the reaction of 1TMoS2 with 4-iodoaniline, with BCN yields G/BCN-MoS2 nanocomposites. Scanning electron microscopy (SEM) images of BN/BCN-MoS2 and G/BCN-MoS2 showed evidence for layer-by-layer assembly structure on cross-linking the constituent layers (Figures 2 and 3). Such layer-by-layer arrangement is facilitated by the formation of the amide bond between the amine groups of BCN and the carboxyl groups on MoS2 and vice versa. In contrast, starting BCN and MoS2 show only thin-layer characteristics as evident in the TEM images (Figures S3a and b). The HRTEM image of the G/BCN-MoS2 (1:2) shows inter-layer spacings (002 peaks) correspond to both BCN (0.40 nm) and MoS2 (0.65 nm) layers, suggesting successful assembly between the heterolayers (Figure S3d). Elemental mapping of BN/BCNMoS2 and G/BCN-MoS2 nanocomposites display uniform distribution of Mo, S, B, C and N, confirming the homogeneous nature of the nanoassemblies (Figures 2 and 3).

Figure 2. Elemental mapping images of BN/BCN-MoS2 nanocomposites (Mo, magenta; S, dark cyan; B, green; C, yellow; N, red). 9 ACS Paragon Plus Environment


Figure 3. Elemental mapping images of G/BCN-MoS2 nanocomposites (Mo, magenta; S, dark cyan; B, green; C, yellow; N, red). FLOSS measurements of the nanocomposites showed decreased number of amine and carboxylic groups as revealed by Figures 4a and b. On cross-linking with MoS2, surface concentrations of amine and carboxylic groups of BCN reduced to 1.15× 109 and 4.3 × 1012 groups/g respectively (Table 1).

(a)

5

2.4x10

6

2.0x10

Control (NHS-CY5 dye) BN/BCN-MoS2 (1:2)

PL intensity

5

3.2x10

PL intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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5

1.6x10

4

8.0x10

(b)

6

1.5x10

Control (1-(bromoacetyl)pyrene) G/BCN-MoS2 (1:2)

6

1.0x10

5

5.0x10

0.0 600


0.0

800

400 450 500 550 Wavelength (nm) Figure 4. Emission spectra of (a) NHS-CY5 dye after reaction with amine groups of BN/BCNMoS2 (1:2); (b) 1-(bromoacetyl)pyrene dye after reaction with carboxyl groups of G/BCN-MoS2 (1:2) composites. 10 ACS Paragon Plus Environment

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Table 1. FLOSS results on the surface functional groups on BC7N, BN/BCN-MoS2 and G/BCNMoS2 composites.

Sample

Carboxylic groups (groups/g)

Amine groups (groups/g)

BCN

1.7 × 1019 ± 0.02× 1019

8.0× 1020 ± 0.2× 1020

BN/BCN-MoS2 (1:2)

-----------

1.15× 109 ± 0.1× 109

G/BCN-MoS2 (1:2)

4.3× 1012 ± 0.07× 1012

-----------

Raman spectra of the BN/BCN-MoS2 and G/BCN-MoS2 (1:2) nanocomposites show characteristic D and G bands of BCN at 1354 and 1596 cm-1, respectively,15 along with the E12g and A1g bands due to the trigonal (2H) polytype MoS2 at 376 and 405 cm-1, respectively (Figure 5a).21 With the increase in MoS2 content, the BCN-MoS2 nanocomposites show progressive enhancement in the relative intensities of the MoS2 bands compared to those of BCN bands (Figure S4).

(a)

(4) G/BCN-MoS2 (1:2)

(b)

(4) G/BCN-MoS2 (1:2)

(3) BN/BCN-MoS2 (1:2) (3) BN/BCN -MoS2(1:2) (2) MoS2-CH2-COOH (2) MoS2

(1) BCN

(1) BCN

600

200 300 400 1200 -11500 1800 Raman shift (cm )

1200 1800 2400 -1 Wavenumber (cm )

3000

Figure 5. (a, b) Raman and IR spectra of (1) BCN (2) MoS2 (3) BN/BCN-MoS2 (1:2) and (4) G/BCN-MoS2 (1:2) composites. 11 ACS Paragon Plus Environment


Furthermore, the out of plane A1g mode of BN/BCN-MoS2 and G/BCN-MoS2 (1:2) nanocomposite is red-shifted compared to 2H-MoS2 probably due to the intercalation of BCN between MoS2 nanosheets as a result of cross-linking (Figure 5a).22 The infrared spectrum of BCN exhibits a band at 1580 cm-1 due to the C=C stretching mode along with a broad band in the 2900-3100 cm-1 region corresponding to the residual amine groups (Figure 5b). MoS2CH2COOH exhibits bands at 1735 and 3156 cm-1 assigned to -C=O and –OH stretching modes of carboxylic groups along with a C-S stretching band at 700 cm-1 arising from covalent functionalization. The BN/BCN-MoS2 nanocomposite shows a carbonyl stretching band at ~1690 cm-1, confirming cross-linking of the BCN and MoS2 layers by the amide bond (Figure 5b).23 Similarly, G/BCN-MoS2 exhibits a C═O stretching band at 1672 cm–1 due to the amide bond between the carboxyl groups of graphene domains and amine functionalized MoS2 (MoS2C6H4NH2) layers along with other characteristic vibration frequencies from amino benzene. Surface area and porosity of nanocomposites obtained from N2 adsorption-desorption isotherms at 77 K show microporous type-I features (low pressure) along with a type-H4 hysteresis loop associated with narrow slit-like pores (high pressure) (Figure S5), according to IUPAC nomenclature.24,25 Slit-shaped pores are formed due to cross-linking and stacking of BCN and MoS2 layers. The Brunauer−Emmet−Teller (BET) surface area of BN/BCN-MoS2 and G/BCN-MoS2 (1:2) nanocomposites are 540 and 512 m2/g, respectively while the physical mixture of the same overall composition shows a lower surface area of 208 m2/g due to non porous nature. Electrocatalytic HER activity of cross-linked BN/BCN-MoS2 and G/BCN-MoS2 nanocomposites was investigated in N2 saturated 0.5 M H2SO4 solution using a conventional three-electrode cell. 12 ACS Paragon Plus Environment

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(a) (1)

2

Current Density i (mA/cm )

0

(2)

-10

(6) (5)

(4)

(3)

-20

(1) MoS2 (2) BCN (3) BCN-MoS2 (1:2, Mixture) (4) BN/BCN-MoS2 (1:2, Composite) (5) G/BCN-MoS2 (1:2, Composite) (6) Pt/C (40 wt %)

-30 -40 -50

-0.20

-0.15

-0.10

-0.05

0.00

Potential (V) vs RHE

-0.3 Overpotential (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(b)

(1) MoS2 (2) BCN (3) BCN-MoS2 (1:2, Mixture) (4) BN/BCN-MoS2 (1:2, Composite) (5) G/BCN-MoS2 (1:2, Composite) (6) Pt/C (40 wt %)

(1)

-0.2 (2)

-0.1

(3) (6)

(4) 0.0 -1.0 -0.5 0.0 0.5 1.0 1.5 2 Log[-i (mA/cm )]

(5)

2.0

Figure 6. HER electrocatalytic performance comparison. (a, b) Linear sweep voltammogram polarization curves and Tafel plots of (1) MoS2, (2) BCN, (3) BCN-MoS2 (1:2, Mixture), (4) BN/BCN-MoS2 (1:2, Composite) (5) G/BCN-MoS2 (1:2, Composite) samples including (5) Pt/C (40 wt %). 13 ACS Paragon Plus Environment


Polarization curves of cross-linked assemblies along with that of physical mixture of BCN and MoS2 are presented in Figure 6a, where data on a commercial platinum catalyst (Pt/C, 40 wt%, Sigma Aldrich) is also included for comparison. The onset potential (η), the measure of extra energy needed to attain a faradic process of H2 generation, values of all catalysts is estimated from semi logarithmic plot of Potential vs Current of polarization curves. It can be seen in Figure 6a that relatively lower onset potential value is obtained for the cross-linked BN/BCN-MoS2 and G/BCN-MoS2 catalysts compared to the physical mixture, suggesting better catalytic nature of the former. Initial BCN and MoS2 display onset potential (η) values of -0.08 and -0.18 V, respectively. The physical mixture of BCN and MoS2 shows a 17 mV improvement in η (-0.063 V) compared to BCN only (−0.08 V), whereas the covalently cross-linked nanocomposites show 30 and 45 mV shifts in the onset potential (-0.05 and -0.035 V) in the positive direction in BN/BCN-MoS2 (1:2) and G/BCN-MoS2 (1:2), respectively. The improved HER activity in crosslinked catalysts can be understood in terms of enhanced surface area and more exposed active sites. Although BET measurements give a qualitative comparison of the geometrical surface areas (Figure S5), in HER activity it would be better to compare electrochemically active surface area (ECSA). Hence we estimated ECSA of cross-linked catalysts by the double layer capacitance (Cdl) method and compared them with the physical mixture as Cdl is directly proportional to the ECSA for carbon based materials (Figure S6).13 The estimated ECSA for BN/BCN-MoS2 (1:2), G/BCN-MoS2 (1:2) and respective physical mixture were 0.012, 0.014 and 0.004 mF cm-2, respectively (Figure S6), suggesting higher ECSA in cross-linked catalysts. Electrochemical impedance spectroscopy (EIS) was also used to investigate the electrochemical behaviour of composite and physical mixture (Figures S7). The two distinct regions in Nyquist plot corresponding to BCN and MoS2 in case of physical mixture suggests that the two


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components behave independently in electrochemical process (Figure S7a). In contrast, crosslinked BN/BCN-MoS2 and G/BCN-MoS2 catalysts show single region with lesser chargetransfer resistance (Rct) compared to BCN or MoS2 combined (Figure S7b). The enhanced performance of cross-linked catalyst is attributed to the distinctive 3D heterolayer network,26,27 as suggested by electron microscopy, along with the presence of microporous channels created in the nanocomposites due to cross-linking,28-30 offering a high electrochemical area for the diffusion of electrolyte. To further investigate the optimum ratio BCN and MoS2 in nanocomposites, we compared activity of cross-linked assemblies with varying BCN and MoS2 ratios of (2:1), (1:1) and (1:2), it is interesting that the catalytic activity increases with MoS2 content probably due to increase in number of active sites (Figure S8a). Cross-linked 1:1 and 1:2 BN/BCN-MoS2 exhibit the lowest overpotential of about -0.06 V at 10 mA cm−2 (η10) with the cathodic current density rising swiftly under more negative potential signifying the superior HER activity. To gain further insight into the kinetics of HER on the BCN-MoS2 catalysts, Tafel slopes were explored. The Tafel slope values obtained from the Butler-Volmer equation13 for various catalysts are 62, 58, 46, 36, 33 and 29 mV dec−1 for BCN, BN/BCN-MoS2 (2:1), (1:1), (1:2), G/BCN-MoS2 (1:2) and Pt/C, respectively while physical mixture of BCN-MoS2 (1:2) shows 71 mV dec−1. A summary of the onset potential, overpotential of 10 mA cm−2 (η10) and Tafel slopes of all catalysts is given in Table 2. The Tafel slope value of cross-linked BN/BCN-MoS2 and G/BCN-MoS2 (1:2) (36 and 33 mV dec−1) catalysts are close to Pt/C (29 mV dec−1) which suggests the efficacy of covalently bonded layers in fast electron transport to the active sites, exhibiting Volmer-Tafel mechanism. 15 ACS Paragon Plus Environment


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Table 2: Electrochemical performance comparison of BN/BCN-MoS2, G/BCN-MoS2 nanocomposites and Pt/C.

Sample

-2

Onset (mV)

η@10 mAcm

Tafel slope

vs. RHE

(mV) vs. RHE

(mV/dec)

MoS

-180 ± 9

-260 ± 13

106

BCN

-80 ± 4

-130 ± 6

62

BCN-MoS (1:2, Mixture)

-60 ± 3

-110 ± 5

71

BN/BCN-MoS (2:1, Composite)

-60 ±3

-90 ± 4

58


-50 ± 2

-60 ± 3

46


-50 ± 3

-60 ± 3

36

G/BCN-MoS (1:2, Composite)

-30 ± 2

-35 ± 2

33

Pt/C (40 wt%)

-20 ± 1

-40 ± 2

29

2

2

2

2

2

2

The above results clearly demonstrate that the cross-linking with BCN is an effective way to further improve catalytic performance of MoS2. We have also synthesized cross-linked BN-MoS2 composites and compared their activity with BN and MoS2 as shown in Figure S9. Cross-linked BN-MoS2 showed a more positive onset potential (-0.14 V) compared to BN (-0.24 V), indicating better catalytic nature of former. However, the activity of crosslinked BN-MoS2 is negligible compared to BCN-MoS2 possibly due to the large band gap insulator nature of BN which does not have the conductive carbon networks as in BCN. The higher activity of cross-linked BCN-MoS2 catalyst is attributed to the excellent intrinsic conductivity of cross-linked BCN layers which can facilitate electron transfer to the active sites in 3D network along with the possible hydrogen adsorption sites (sp3-C, 16 ACS Paragon Plus Environment

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Stone-Wales defect). Secondly, enhanced charge-transfer rate between heterolayers and increased surface area due to cross-linking further improve HER activity by providing the larger electrode/electrolyte interface area. In Table 3, we have given a comparison of the HER activity of cross-linked assemblies with some of the MoS2-metal free carbon based materials reported in the literature.

Table 3: Comparison of electrochemical HER performance of BN/BCN-MoS2 and G/BCN-MoS2 nanocomposites of MoS2 with other carbon based materials.

Catalyst

Onset (mV) vs. RHE

η@10 mA cm (mV) vs. RHE

(mVdec )

-30 ± 2

-35 ± 2

33

-50 ± 3

-60 ± 3

36

-105

-150

41

-75

-104

63

(33)

-100

-150

101

(34)

-80

-85

40

-100

-165

50

Cross-linked G/BCN-MoS2 (1:2)

(a)

Cross-linked BN/BCN-MoS2 (1:2) RGO supported MoS2 MoS2-RGO

(a)

(31)

(32)

MoS2 Quantum dots-RGO MoS2 -Amorphous Carbon

Nanostructured MoS2-Carbon cloth MoS2-N doped graphene aerogel MoS2-Carbon Aerogel

(35)

(36)

-236

(37)

MoS2-C3N4 (14) (a)

-2

-261

(b)

Tafel slope -1

230

-140

-190

59

-236

-320

95

Present work, (b) η@20 mA cm-2 (mV)

For practical applications of the electrocatalyst we have studied the HER activity of G/BCN-MoS2 at different pH values of the electrolyte (0.5 M H2SO4 (1.3), 0.1 M NaOH (13.0)



and pH = 7.2 buffer solutions). The activity of catalyst in terms of the onset potential remains nearly same in this pH range as shown in Figure S10a. However, the current density is greater in the acidic medium (pH = 1.3). A comparative study of LSV and the corresponding Tafel slopes at different pH values of the electrolyte is shown in Figure S10. Encouraged by the superior electrochemical HER activity of cross-linked BCN-MoS2 nanocomposites, we have examined their performance in photochemical HER. The hydrogen evolution experiments were carried in aqueous solution of triethanolamine (TEOA, 15% (v/v)) under visible light in the presence of Eosin Y dye as a photosensitizer. Eosin Y (EY) on photoexcitation undergoes series of changes to form a highly reductive species, EY-, which subsequently transfer electrons to the catalyst for hydrogen evolution.12,38-40 In Figure 7a, we present the yield of H2 evolved by using cross-linked BN/BCN-MoS2 and G/BCN-MoS2 (1:2) and respective physical mixture as well as by using BCN and MoS2 alone. As can be seen in Figure 7b, BCN exhibits low activity of 136 µmol h–1 g–1 while few-layer MoS2 has an activity of 1663 µmol h–1 g–1 which suggests latter would be the preferred sites for HER in composites. In the case of a physical mixture of BCN and MoS2, improvement in photochemical HER activity with respect to MoS2 is ∼2 times, whereas the cross-linked BN/BCN-MoS2 (1:2) and G/BCNMoS2 (1:2) composites show an increase by ∼5 times (Figures 7a and b). This result clearly suggests that cross-linked BCN and MoS2 layers provides greater interaction between the two, thereby showing higher HER activity compared to the physical mixture where layers are randomly oriented. In Figure S11a, we present the yield of H2 evolved by using different BN/BCN-MoS2 (1:1, 1:2 and 2:1) assemblies.


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(a) 18000 (5) G/BCN-MoS2 (1:2, Composite)

(5)

9000

(3)

6000

(2)

H2 evolved (µmole/g)

12000

(4) BN/BCN-MoS2 (1:2, Composite) (4) (3) BCN-MoS2 (1:2, Mixture) (2) MoS2 (1) BCN

15000

3000 (1)

0 0.0

0.5 1.0 1.5 2.0 Time (h)

2.5 3.0

(b) 6800 -1 -1

Activity (µmoles h g )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


BN/BCN-MoS2 G/BCN-MoS2 MoS2

5100 3400 1700 0

1:0

2:1

1:1 1:2 1:2 BCN:MoS2

0:1

Figure 7. Comparison of the photochemical HER activity of (a) (1) BCN, (2) MoS2, (3) BCNMoS2 (1:2, mixture), (4) BN/BCN-MoS2 (1:2) and (5) G/BCN-MoS2; (b) Bar diagram showing the comparison of the HER activity of BN/BCN-MoS2 with the increase in MoS2 content along with the G/BCN-MoS2 (1:2) composite. 19 ACS Paragon Plus Environment


The catalytic activities of 1:1 and 1:2 BN/BCN-MoS2 assemblies are much higher compared to few-layer MoS2 with the 1:2 nanocomposite demonstrating the highest activity of 6965 µmol h– 1g–1. The 2:1 BN/BCN-MoS2 nanocomposite with more BCN content shows lower activity of 656 µmol h−1 g–1 (Figure 7b). A similar trend in HER activity was encountered in electrochemical studies as well. The above results suggest that cross-linking of BCN layers to MoS2 clearly enhances the catalytic activity of the latter wherein the carbon-rich BCN act as channels for transferring electrons to the MoS2 active sites across the network. Our hypothesis is further substantiated by the results on BN-MoS2 nanocomposites, prepared by cross-linking BN with MoS2, which show negligible activity (Figure S11b). The high HER activity of BN/BCN-MoS2 and G/BCNMoS2 can be attributed to the enhanced charge-transfer rates as well as the more exposed active edge sites of MoS2. Cycling studies on BN/BCN-MoS2 and G/BCN-MoS2 showed stable H2 evolution over long periods (Figure S12). In Table S1, we compare the photochemical HER activity of cross-linked BCN-MoS2 assemblies with that of carbon based MoS2 composites.

Conclusions In conclusion, we have been able to synthesize two differently cross-linked nanocomposites with distinctive bonding between MoS2 sheets and graphene domains and BN domains in BC7N. The cross-linked assemblies show excellent hydrogen evolution activity relative to the physical mixtures. Electrochemical HER activity of the graphene domain cross-linked G/BCN–MoS2 (1:2) assembly is noteworthy with a Tafel slope of 33 mV dec−1 close to that of Pt (29 mV dec−1). The performance of this catalyst is best under acidic conditions (pH = 1.3). The performance of BN/BCN-MoS2 is generally poorer than that of G/BCN-MoS2. The present study


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demonstrates the advantages of cross-linking of catalytically active heterolayers BCN and MoS2 for the HER reaction. The enhanced HER activity in cross-linked composites arises from the increased charge-transfer rates as well as from the more exposed catalytically active sites. Photochemical HER activity of the BN/BCN-MoS2 (1:2) and G/BCN–MoS2 (1:2) assemblies is also superior, implying that the covalent cross-linking is a valuable strategy for HER and associated catalytic reactions. Noting that pristine 2H-MoS2 itself exhibits very poor photochemical HER activity, the present results show that the activity of 2H-MoS2 can be vastly improved by cross-linking with BC7N.

Associated content Supporting information Detailed information about electrochemical characterizations of BN/BCN-MoS2 and G/BCNMoS2 nanocomposites. XPS of BC7N (Figure S1). TEM (HRTEM) images of BCN, MoS2 and G/BCN-MoS2 (Figures S3a, b and d). Surface area curves of BN/BCN-MoS2, G/BCN-MoS2 (Figures S5). Double layer capacitance curves of BN/BCN-MoS2, G/BCN-MoS2 and BCNMoS2 mixture (Figure S6). Nyquist plots of BN/BCN-MoS2, G/BCN-MoS2 and BCN-MoS2 (Figure S7). Linear sweep voltammogram polarization curves (LSV) and Tafel plots of BCNMoS2 (2:1), BCN-MoS2 (1:1) and BCN-MoS2 (1:2) composites (Figures S8a and b). LSV of GCE, BN, BN-MoS2 (1:2) nanocomposites and MoS2 (Figures S9). Comparision of photochemical HER activity of BCN-MoS2 (2:1), BCN-MoS2 (1:1) and BCN-MoS2 (1:2) composites (Figure S11). Cycling study of H2 evolution on BN/BCN-MoS2 and G/BCN-MoS2 nanocomposites ((Figure S12).

Corresponding author 21 ACS Paragon Plus Environment


*E-mail: [email protected]

Acknowledgements K. Pramoda thanks JNCASR and CSIR for a fellowship.


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Covalently

Bonded

MoS2-Borocarbonitride

Nanocomposites

Generated by Using Surface Functionalities on the Nanosheets and their Remarkable HER Activity K. Pramoda, Mohd Monis Ayyub, Navin Kumar Singh, Manjeet Chhetri, Uttam Gupta, Amit Soni and C. N. R. Rao* TOC Graphic:


Covalently Bonded MoS2âBorocarbonitride Nanocomposites

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