Chapter 2. Activating MoS 2 Basal Plane with Ni 2 P Nanoparticles for Pt-Like Hydrogen Evolution
2.4 Results
2.4.1 Structure and Properties of Ni 2 P/MoS 2 Heterostructures
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To promote the strong interaction between Ni2P and MoS2 phases, their heterostructure was synthesized from a single precursor NiMoS4. The precursor was synthesized via a cation displacement reaction between (NH4)2MoS4 and Ni(CH3CO2)2·4H2O as described in the Experimental Section. The NiMoS4 salt reacted with in situ produced phosphine (PH3) gas from NaH2PO2·H2O decomposition at a preoptimized temperature of 500 °C for 2 h to produce the Ni2P/MoS2 heterostructure. To make the Ni2P/MoS2/N:RGO and Ni2P/MoS2/N:CNT hybrid catalysts, an exfoliated N-doped reduced graphene (N:RGO) or N:CNT was directly introduced in the NiMoS4 formation step before the in situ phosphidation reaction as illustrated in Figure 2.1.
Figure 2.1. Schematic illustration of Ni2P nanoparticles anchored on MoS2 sheets supported on N- doped carbon nanotubes (N:CNT). The NiMoS4 anchored on N-doped CNT reacts with in-situ produced PH3 gas to form Ni2P/MoS2/N:CNT heterostructure.
Thermogravimetric analysis (TGA) curves in Figure 2.2a reveal the decomposition process of (NH4)2MoS4. Following the initial evaporation of water up to 100 °C, MoS3 is formed by the loss of NH3 and H2S molecules until 240 °C. The phase transformation from MoS3 to MoS2 is observed up to 450 °C with further loss of H2S gas.13 Similarly, phosphine gas is generated by the thermal decomposition of NaH2PO2·H2O at ≈250 °C.40 Therefore, 450–550 °C is the optimized temperature window for proper phosphidation of the NiMoS4 precursor to form a Ni2P/MoS2 heterostructure. The Raman spectra in Figure 2.2b give further information about the structural evolution of MoS2 upon forming heterostructure and hybrid. Thus, the peaks at 379.5 and 407.3 cm−1 are attributed to E12g and A1g vibrational modes of MoS2, respectively. The redshift of E12g (378.5 cm−1) and A1g (401.4 cm−1) of Ni2P/MoS2 can be ascribed to the decreased number of MoS2 layers due to the presence of Ni and P species in the heterostructure.41 Some weak signals between 250 and 300 cm−1 in Ni2P/MoS2 display
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the defective features in MoS2.13, 42 Additionally, the high D (1345 cm−1) and G (1596 cm−1) band intensities ratio (ID/IG) in both Ni2P/MoS2/N:RGO (1.03) and Ni2P/MoS2/N:CNT (1.14) reveal the defective nature of graphitic carbon due to the presence of N and S dopants.43 The X-ray diffraction (XRD) pattern of Ni2P/MoS2/N:CNT hybrid catalyst in Figure 2.2c confirms the formation of the two phases, MoS2 and hexagonal Ni2P (space group: P-62m). Interestingly, the (002) peak of MoS2 displays an enlarged interlayer spacing (0.68 nm) than the standard (0.615 nm), indicating insertion of some heteroatoms (P, or Ni) between two MoS2 layers of the Ni2P/MoS2 heterostructure. The XRD patterns of Ni2P/MoS2-X (X = 450–550 °C) heterostructures synthesized at different annealing temperatures in Figure 2.2d indicate that a bimetallic phosphide (MoNiP2) phase is dominating when the temperature rises above 500 °C, instead of the heterostructure.
Figure 2.2. (a) Thermogravimetric analysis (TGA) of (NH4)2MoS4 in air and nitrogen. (b) Raman spectra for MoS2, Ni2P/MoS2, Ni2P/MoS2/N:RGO, and Ni2P/MoS2/N:CNT. (c) XRD pattern of MoS2, Ni2P, and Ni2P/MoS2/N:CNT. (d) XRD pattern of Ni2P/MoS2 heterostructures synthesized at different reduction temperatures of 450 °C (black), 500 °C (red) and 550 °C (blue).
The nature of chemical bonding in Ni2P/MoS2/N:CNT was further investigated by the high-resolution X-ray photoelectron spectroscopy (XPS). A doublet at 228.5 and 231.4 eV in the Mo 3d spectrum in
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Figure 2.3a is assigned to a low oxidation state of Moδ+ (0 < δ ≤ 4) attached to S and/or P to make S- Mo-P type bonding in Ni2P/MoS2/N:CNT.32 The peaks are shifted higher binding energy compared to Ni2P/MoS2 hybrid (227.9 and 231.2 eV) due to the interaction with N:CNT. Another doublet at 229.4 and 232.3 eV for Mo4+ 3d5/2 and Mo4+ 3d3/2 demonstrates the characteristics of MoS2.31 Two weak signals at 232.3 and 235.0 eV represent the surface oxidation of Mo species to form MoO3.32, 44 Similarly, a doublet in Ni 2p XPS spectra at binding energies of 853.3 and 870.5 eV in Figure 2.3b is ascribed to Ni3+ oxidation states in Ni2P and is shifted to higher binding energy than that of Ni2P/MoS2 hybrid (853.0 and 870.3 eV) due to the interaction with N:CNT. The peaks at 854.4 and 856.5 eV along with satellites (861.2 and 874.0 eV) in Ni 2p spectrum are attributed to Ni bonded to O atoms in the form of surface oxides/hydroxides.8, 45 Two doublets in S 2p (161.9, 163.0 eV) and P 2p (129.6, 130.8 eV) further confirm the formation of metal phosphosulfides in the material as displayed in Figure 2.3c, d. The characteristic signals of -C=S/-NS, -SO4 in S 2p,32 -P-N/-PON, -P-S, -P-C in P 2p,32 pyridinic, pyrrolic, graphitic nitrogen in N 1s, and -C-N/=N in C 1s spectra confirm that all elements are chemically interconnected with each other in both Ni2P/MoS2/N:CNT as shown in Figure 2.3c-f. In Ni2P, doping of a more electronegative element like S stabilizes the least stable P3− state in phosphides by withdrawing the electron density from it and then donates back to vacant d orbitals of Ni.32 Additionally, the doping of heteroatoms (N, S, P, and O) in the heterostructure is known to increase the number of proton (H+) adsorption sites to improve the HER performance.32, 46
Figure 2.3. (a) High resolution scans of (a) Mo 3d, (b) Ni 2p, (c) S 2p, (d) P 2p, (e) N 1s and (f) C 1s for the Ni2P/MoS2/N:CNT hybrid catalyst.
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The surface morphology of Ni2P/MoS2/N:CNT was determined by scanning electron microscopy (SEM) as shown in Figure 2.4a, b, which confirms the successful hybridization of all these three components into a porous hybrid structure. The SEM images of Ni2P/MoS2/N:RGO, Ni2P/MoS2, Ni2P, and MoS2 are also displayed in Figure 2.4c-h. High-resolution transmission electron microscopy (HRTEM) image of Ni2P/MoS2/N:CNT heterostructure in Figure 2.5 shows that Ni2P NPs deposited on MoS2 sheets are successfully grown on N:CNT. The HR-TEM images clearly show the co-existence of Ni2P, MoS2, and CNT. The particle size of Ni2P lies in the range of 8–15 nm as confirmed by TEM images of Ni2P/MoS2/N:CNT hybrid. Interestingly, stacking of MoS2 sheets is restricted to only three to seven layers with enhanced interlayer spacing (≈0.68 nm) due to the presence of Ni and P species as suggested by XRD and Raman analyses. Thus, Ni2P NPs residing on the flat basal plane of MoS2
suppress the further growth of the MoS2 layers. The lattice fringes 0.22, 0.20, and 0.25 nm correspond to (111), (201), and (200) planes of Ni2P (Figure 2.5c) and enlarged d-spacing (0.68 nm) is for MoS2
(002) plane (Figure 2.5d). The energy-dispersive X-ray spectroscopy (EDS) TEM elemental mappings confirm the uniform elemental distribution throughout the particles in Ni2P/MoS2/N:CNT (Figure 2.6).
Figure 2.4. Scanning electron microscopy (SEM) images of (a, b) Ni2P/MoS2/N:CNT, (c, d) Ni2P/MoS2/N:RGO, (e, f) Ni2P/MoS2, (g) Ni2P, (h) MoS2.
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Figure 2.5. High-resolution transmission electron (HR-TEM) images of Ni2P/MoS2/N:CNT heterostructure. (a) TEM image showing the Ni2P NPs distributed on MoS2 and CNT, and magnified image (inset) displaying MoS2, Ni2P and CNTs together. (b) HR-TEM image, magnified image of (c) Ni2P NPs, (d) a few layered MoS2 with enhanced interlayer distance and corresponding Fast Fourier transforms (FFTs).
Figure 2.6. (a) High-angle annular dark-field scanning transmission electron microscopy (HAADF STEM) image and (b) Energy-dispersive X-ray spectroscopy (EDS) TEM elemental mappings for Ni2P/MoS2/N:CNT hybrid catalyst.