Organometallic complex-assisted synthesis and characterization of Mo-carbides

2. BCN-shell Encaged Multiple Phases of Molybdenum Carbide for Effective Hydrogen

2.4 Results and Discussion

2.4.1 Organometallic complex-assisted synthesis and characterization of Mo-carbides

The generalized preparation procedure for B- and N-doped carbon (BCN) shell protected molybdenum carbides (named as Mo2C@BCN) is described in Figure 2.1a, and discussed in detail under Experimental Section. Firstly, as-prepared different imidazole (Im)-borate (Eqn. 2.1) monomers (Table 1.1) were used as an organic linker to make an organometallic complex of Mo cation at 130 ⁰C under nitrogen in glycerol and dilute aqueous acetic acid solution (Eqn. 2.2). The color change with synthesis time is also monitored as in Figure 2.1b.

𝑐𝑐(𝐼𝐼𝑠𝑠) +𝐶𝐶₃𝐵𝐵𝑂𝑂₄→^∆ (𝐼𝐼𝑠𝑠)_𝑛𝑛𝐵𝐵(𝑂𝑂𝐶𝐶)_3−𝑛𝑛+𝑐𝑐𝐶𝐶₂𝑂𝑂 (𝐸𝐸𝑎𝑎. 2.1) (𝐼𝐼𝑠𝑠)_𝑛𝑛𝐵𝐵(𝑂𝑂𝐶𝐶)3−𝑛𝑛+𝑀𝑀𝑐𝑐𝐶𝐶𝑉𝑉5𝐶𝐶𝑎𝑎.𝐶𝐶𝐻𝐻₃𝐶𝐶𝑂𝑂𝑂𝑂𝐻𝐻,𝐺𝐺𝐸𝐸𝐸𝐸

�⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯� 𝑀𝑀𝐼𝐼𝐵𝐵 − 𝑐𝑐𝑐𝑐𝑠𝑠𝑐𝑐𝑉𝑉𝑠𝑠𝑐𝑐+𝐶𝐶𝐶𝐶𝑉𝑉 (𝐸𝐸𝑎𝑎. 2.2)

The dark brown precipitate of Mo-Im-Borate complex (MIB) are collected which possess zeolitic- imidazolate framework-like (ZIF) topologies because of imidazole linkers can make Mo-Im-Mo bridge just like Si-O-Si in zeolite framworks.³¹ The Powder X-ray diffraction (XRD) pattern of MIB complex did not show any significant peak due to its amorphous nature as shown in Figure 2.2a. Then, the as- obtained MIB complex was calcined at 900 ⁰C in N2 atmosphere to synthesize BCN network encapsulated dark grey Mo2C nanoparticles (NPs). During the thermal decomposition at higher temperature, Mo reacts with the imidazole-borate linkers and converted to molybdenum carbides NPs

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covered by the few layers of BCN. Five different phases (hexagonal β, η, orthorhombic α, β and cubic α) are successfully formed by just manipulating imidazole-borate ligand structures by varying the imidazole-to-boric acid ratio by keeping the all other conditions same (Table 2.1). Due to the different structure, the imidazole-borate ligands may have tendency to coordinate with Mo atoms making different spatial atomic arrangements in MIB to yield distinct phases of molybdenum carbide.

Figure 2.1. (a) Schematic illustration for preparation of BCN-encaged diffrent phases of molybdenum carbide and evolution of color during the synthesis of Mo-Im-Borate complex.

The formation of highly crystalline different phases like orthorhombic (α-Mo²C, β-Mo²C), hexagonal (β- Mo2C, η-MoC) and cubic (α-MoC1-x) molybdenum carbides are confirmed by XRD (Figure 2.2b) along with simulated XRD patterns of all phases. Surprising, the multiple phases are easily synthesized by just slight tuning imidazole-borate ligand structure at the same annealing temperature of 900⁰C for 5 h in N2. Additionally, the cubic α-MoC1-x and hexagonal η-MoC phase could also be synthesized at 750 ⁰C (Figure 2.2c) and 800 ⁰C (Figure 2.2d), respectively, which are much lower temperature as for α-MoC1-x (800 ⁰C) and hexagonal η-MoC (1050 ⁰C) by annealing a mixture of Mo salts and organic compounds³². Additional, for the preparation of η-MoC the other metallic impurities like NiI² and Cu was also required as guest atoms to make metal organic framework, the removal of such impurities is difficult to get the pure product.^32,6 The particle size calculated by applying Scherrer formula (Figure 2.2e), the average crystallite sizes of o-α-Mo2C (14 nm), η-MoC (6.3 nm), h-β-Mo2C (11.6 nm), α- MoC1-x (4.2 nm) and o-β-Mo²C (8.9 nm), which are smaller or comparable with previously reported in literature.^{6, 21, 28} Mo-complex structure effectively prevent from agglomeration of NPs during the

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confined carburization reaction. The irregular spherical particles of grain size 30-200 nm were observed by Field emission scanning electron microscopy (FESEM) as shown in Figure 2.2f.

Figure 2.2. XRD pattern of (a) Mo-Im-borate complex, (b) as-synthesized multiple phase of molybdenum carbide at 900 ⁰C, (c) cubic α-MoC1-x at 750 to 900 ⁰C, (d) hexagonal η-MoC at 800-900

⁰C, (e) particle size distribution obtained from XRD pattern in (b) by Scherrer equation and (f) SEM image sample annealed at 900 ⁰C.

The transmission electron microscopy (TEM) image at low magnification (Figure 2.3a) display irregular shaped porous morphology as in SEM images. The small nanoparticles (5-14 nm) of molybdenum carbide are caged in BCN network as encircled by dots in good agreement with XRD.

High resolution TEM images in Figure 2.3b-d confirm that molybdenum carbide NPs are well protected in uniform BCN shells of 3.0±0.5 nm thick having interplanar distances of 0.377 and 0.345 nm of (012) planes of boron carbide (JCPDS 01-086-1120) and (002) planes of graphitic carbon, respectively.³³ The lattice fringes Figure 2.3b-f of hexagonal η-MoC (0.243), hexagonal β-Mo2C (0.229), orthogonal β-Mo2C (0.28), orthogonal α-Mo2C (0.246) and cubic α-MoC1-x (0.24) nm are belonging to the (006), (101), (211), (002) and (111) crystal planes of each phase, respectively. EDX elemental maps (Figure 2.3g-l) confirms the uniform distribution of Mo, B, N and C atoms throughout the particle which is another evidence of successful encapsulation of molybdenum carbides NPs by BCN.

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Figure 2.3. TEM image (a) at low magnification; scale bar: 100 nm, HR-TEM image of (b) hexagonal η-MoC, (c) hexagonal β-Mo²C, (d) orthogonal β-Mo²C, (e) orthogonal α-Mo²C, (f) cubic α-MoC^1-x and corresponding fast Fourier transform (FFT) pattern (inset); scale bar: 5nm. (g-l) EDS-STEM elemental maps for Mo (red), C (green), N (purple) and B (light blue).

Three oxidation states for molybdenum Mo⁺² (Mo-carbide), Mo⁺⁴ (MoO2) and Mo⁺⁶ (MoO3) are identified by X-ray photoelectron spectroscopy (XPS) of Mo 3d spectra as displayed in Figure 2.4a.

The minor surface oxides are observed in XPS because of exposure of NPs in air,^{22, 32} as confirmed by quantitative analysis of XPS in Table 2.2. Five peaks of B-C, B-N, B, B-O (B2O3) and MoB are identified in B1s spectra (Figure 2.4b). The signals at 398.2, 399 and 401 eV in N1s XPS (Figure 2.4c)can be assigned to pyridinic/B-N, pyrrolic and graphitic (N-C3) nitrogen,^33-34 along with Mo-N and Mo-Mo peaks at 394.2 eV and 395.5 respectively.^{32, 35} The C1s (Figure 2.4d) spectra display a Mo-C peak at 283 eV,³⁶ and near 284.5, 285.2 and 288 eV corresponds to C bonding with N, O, and B, accordingly.³³ The pore size in the range of 3-10 nm and BET surface areas 87.1 and 48.36 m²/g are found to be for h-β-Mo2C@BCN and c-α-MoC1-x@BCN by N2 adsorption−desorption isotherms as shown in Figure 1.4e-f.

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Figure 2.4. XPS spectra of (a) Mo 3d, (b) B 1s, (c) N 1s) and (d) C 1s. BET isotherms for (e) h-β- Mo2C@BCN, (f) c-α-MoC1-x@BCN and pore size distributions (inset).

Table 2.2. Surface composition of each phase of molybdenum carbides determined by XPS

Sample Mo

[At. %]

C [At. %]

N [At. %]

B [At. %]

O [At. %]

o-α-Mo2C@BCN 10.44 60.81 17.37 1.04 10.34 h-η-MoC@BCN 10.39 55.93 24.27 1.23 8.18 h-β-Mo2C@BCN 11.03 59.09 21.17 1.60 7.11 c-α-Mo2C@BCN 11.3 60.02 18.7 3.15 6.83 o-β-Mo2C@BCN 13.50 60.49 12.84 4.05 9.12

2.4.2 Hydrogen evolution reaction performances of BCN-protected molybdenum carbides