Controlled synthesis of Mo2C micron flowers via vapor–liquid–solid method as enhanced electrocatalyst for hydrogen evolution reaction

Mo2C demonstrates excellent performance in catalysis, and it has been found to possess excellent hydrogen evolution reaction (HER) catalytic activity and highly efficient nitrogen fixation. The catalytic activity of Mo2C is greatly influenced and restricted by the preparation method. Sintering and carbon deposition, which affect the catalytic activity of Mo2C, are inevitable in the traditional vapor–solid–solid (VSS) process. In this study, we report the controllable synthesis of α-Mo2C micron flowers by adjusting the growth temperature via a vapor–liquid–solid (VLS) process. The density of the Mo2C micron flowers is closely related to the concentration of Na2MoO4 aqueous solution. The as-grown Mo2C micron flowers dispersed with Pt are validated to be an enhanced collaborative electrocatalyst for HER against Pt/VSS-Mo2C.


Introduction
][3] TMCs are considered to be similar to precious metals in the aspect of electrochemistry and catalysis.1][12][13] However, many collaborative catalysis suffer from low mass-specic activity owing to the low metal loading. 14In order to optimize metal loading, the support Mo 2 C crystals should have a high specic surface area which can provide abundant surface sites to enhance the collaborative catalysis.
6][17][18] In earlier studies, the sintering of the as-grown Mo 2 C crystals was inevitable, 19,20 inuencing the structure and morphology of Mo 2 C crystals, which results in the reduction of the specic surface area and catalytic activity.Therefore, it is important to improve the preparation methods to reduce the sintering and thus increase the specic surface area of the Mo 2 C crystals.
Herein, we report the synthesis of a-Mo 2 C crystals via an atmospheric pressure vapor-liquid-solid (VLS) method with Na 2 MoO 4 as the Mo precursor.The morphology of the Mo 2 C crystals could be controlled by adjusting the growth temperature.Mo 2 C micron owers were obtained when the growth temperature was 780 °C.2][13][14][15][16][17][18][19][20][21][22][23][24][25] Thus, the as-grown Mo 2 C crystals can form sheet morphology at an appropriate temperature comparing with the block morphology formation at higher temperatures or via the VSS mode.The advantage of VLS over VSS mode can be further demonstrated by comparing the HER catalytic activity of the as-grown Mo 2 C dispersed with Pt.Pt/VLS-Mo 2 C has a lower overpotential than Pt/VSS-Mo 2 C at a current density of 10 mA cm −2 .Mo 2 C crystals grown using the VLS method is of great signicance to improve their catalytic activity and expand their application elds.

Results and discussion
The CVD growth process of Mo 2 C on Au substrate is illustrated in Fig. 1a.The upper panel shows a typical VSS mode for the growth where (NH 4 ) 6 Mo 7 O 24 aqueous solution is used as the Mo precursor.As the growth temperature reaches 780 °C, (NH 4 ) 6 Mo 7 O 24 decomposed to form the solid state of MoO 3 particles, which were then carbonized to produce Mo 2 C when C 2 H 4 was introduced into the reaction chamber.Fig. 1b shows the X-ray photoelectron spectroscopy (XPS) was conducted to evaluate the chemical composition and valence state of the Mo 2 C crystals.7][28][29][30] In addition, two weak peaks were observed at 233.4 and 229.8 eV, representing the intermediate oxidation states of Mo (MoO x ). 28,29The MoO x may have resulted either from the exposure of Mo 2 C to air or from the oxidization of Mo 2 C during the XPS measurement process.Fig. 2b shows the C 1s XPS spectrum, whereby the peak located at the lower binding energy of 283.3 eV was assigned to C-Mo, 26,27,30 and those peaks at higher binding energies of 284.8, 286.3, and 288.1 eV can be ascribed to the carbons in the non-oxygenated C-C, C]O, and O-C]O, respectively. 28,29The XPS signals conrmed the identity of the Mo 2 C crystals, as expected.Raman spectroscopy and XRD were conducted to evaluate the structure of the Mo 2 C crystals (Fig. 2c and d).Raman spectrum showed a well-dened characteristic peak at 652 cm −1 , corresponding to the A g mode of a-Mo 2 C crystal. 31,32The diffraction peaks of Mo 2 C in the X-ray diffraction (XRD) spectra were consistent with the standard XRD pattern of Mo 2 C (PDF#31-0871), demonstrating that the as-grown Mo 2 C crystals were a-Mo 2 C.

RSC Advances
The morphology and density of Mo 2 C crystals can be tuned remarkably by changing the growth temperature and the concentration of Na 2 MoO 4 aqueous solution.Fig. 3a and   b present the SEM images of the Mo 2 C micron sheets grown with 30 and 75 mg per mL Na 2 MoO 4 aqueous solutions at 780 °C, respectively.When the Na 2 MoO 4 aqueous concentration was 30 mg mL −1 , it provided a low concentration of Mo species, resulting in few nucleation sites, and thus, only a low quantity of Mo 2 C micron sheets appeared, as shown in Fig. 3a.By increasing the Na 2 MoO 4 aqueous concentration to 75 mg mL −1 , the shape of the Mo 2 C micron sheets became more evident, whereby some micron sheets have begun to form ower-like shapes.The inset in Fig. 3b is the SEM image of an individual Mo 2 C micron ower.Energy dispersive X-ray spectroscopy (EDS) mapping were recorded for the spatial distribution of the Mo and C elements (Fig. 3c and d), and both of them were found to be distributed uniformly in the micron owers with sharp edges, exhibiting the uniformity of the Mo 2 C crystals.Subsequently, the inuence of growth temperature was investigated, and the 75 mg per mL Na 2 MoO 4 aqueous solution was used as  In order to further demonstrate the advantage of VLS in synthesizing Mo 2 C, the HER catalytic activities of VLS-Mo 2 C and VSS-Mo 2 C were compared.The samples of VLS-Mo 2 C (150 mg per mL Na 2 MoO 4 ) and VSS-Mo 2 C [150 mg per mL (NH 4 ) 6 Mo 7 O 24 ] were synthesized on Au substrates, and both the two kinds of Mo 2 C were loaded with 2 nm Pt for the electrochemical test.The HER catalysis was evaluated in 1.0 M KOH solution using a typical three-electrode system with the studied materials as the working electrodes, Hg/HgO as the reference electrode, and the Pt foil as the counter electrode.
Fig. 4a shows the linear sweep voltammetry (LSV) curves of Pt/VLS-Mo 2 C, Pt/VSS-Mo 2 C, and Pt/Au with a scan rate of 5 mV s −1 .Compared with the Pt/VSS-Mo 2 C and Pt/Au, the Pt/VLS-Mo 2 C has a lower overpotential of 52 mV versus the reversible hydrogen electrode (RHE) at a current density of 10 mA cm −2 , indicating that the VLS mode can substantially improve the collaborative catalytic performance of Pt/Mo 2 C toward HER in alkaline condition.The derived Tafel slope of Pt/VLS-Mo 2 C and Pt/VSS-Mo 2 C is around 166 and 222 mV dec −1 , respectively (Fig. 4b), indicating that the hydrogen evolution on both of them undergoes the Volmer mechanism, and water dissociation is the rate-determining step.Critically, a substantially decreased Tafel slope of Pt/VLS-Mo 2 C revealed that the sluggish water dissociation behavior had improved signicantly.In addition, electrochemical impedance spectroscopy (Fig. 4c) showed that Pt/VLS-Mo 2 C possessed a lower charge transfer resistance than Pt/VSS-Mo 2 C. The signicantly reduced impedance further suggest that Pt/VLS-Mo 2 C can substantially boost the interfacial electron-transfer kinetics between the Mo 2 C and Au foil, which promotes the HER dynamic process.The electrochemical surface areas of Pt/Mo 2 C crystals were further estimated by deriving the electrochemical double layer capacitance (C dl ) from the cyclic voltammetry studies, as shown in Fig. 4d-f.The Pt/VLS-Mo 2 C was found to have a larger C dl of 13.2 mF cm −2 than Pt/VSS-Mo 2 C (11.1 mF cm −2 ), indicating that the VLS mode can increase the electrochemical surface areas of the as-grown Mo 2 C crystals.
We compared the Pt/VLS-Mo 2 C over the state-of-the-art of electrocatalysts for HER, as shown in Table 1.We believe that the VLS method could offer new insights into the synthetic approaches for Mo 2 C and provide new strategies for constructing metal-loading catalysts with high HER catalytic activity.

Conclusion
In summary, we demonstrated the VLS growth of a-Mo 2 C micron owers, which were realized by using liquid precursor for the rst time.The morphology and density of the Mo 2 C crystals could be controlled by tuning the growth temperature and concentration of Na 2 MoO 4 aqueous solution.The unique ower-like structure produces a high specic surface area and abundant surface sites on the surface, increasing the Pt loading and enhancing the collaborative catalysis.The comparison between Pt/VLS-Mo 2 C and Pt/VSS-Mo 2 C in terms of HER catalytic activities further demonstrated the advantage of VLS in synthesizing Mo 2 C crystals.Our study not only offers new insights into the synthetic approaches for Mo 2 C but also provides a new strategy for constructing metal-loading catalysts with high catalytic activity.

PAPER
SEM image of the as-grown Mo 2 C with 150 mg per mL (NH 4 ) 6 Mo 7 O 24 as the Mo precursor.The Mo 2 C demonstrated block morphology with size inconsistency.Mo 2 C micron owers with high specic surface area were synthesized via the VLS mode, and the schemes are shown in the bottom panel of Fig. 1a.150 mg per mL Na 2 MoO 4 aqueous solution replaces (NH 4 ) 6 Mo 7 O 24 aqueous solution as the Mo precursor.It is worth noting that the melting point of Na 2 MoO 4 is 687 °C, it melts into liquid state and forms a liquid-solid interface with Au substrate at the growth temperature (780 °C).Importantly, liquid has the advantage of a lower migration barrier, which is more benecial to the unrestricted diffusion and homogeneous distribution of the precursors on the Au substrate.Thus, uniform Mo 2 C micron sheets can be synthesized via the VLS mode.Moreover, the liquid-solid interface is conducive to the lateral growth of Mo 2 C micron sheets.As the size and density increases, the Mo 2 C micron sheets gradually form the Mo 2 C micron ower morphology, as shown in Fig. 1c.

Fig. 1
Fig. 1 (a) Schematic illustration of the VSS and VLS growth process of Mo 2 C. (b) and (c) Typical SEM images of the Mo 2 C crystals grown with VSS and VLS mode, respectively.

Fig. 2 Fig. 3
Fig. 2 (a) and (b) XPS spectra acquired at the Mo 3d and C 1s regions.Raman spectrum (c) and XRD pattern (d) of the as-grown Mo 2 C nanocrystals.

Fig. 4
Fig. 4 (a) The LSV curves of Pt/VLS-Mo 2 C, Pt/VSS-Mo 2 C, and Pt/Au with IR correction.(b) The corresponding Tafel slopes.(c) Nyquist plots of Pt/VLS-Mo 2 C and Pt/VSS-Mo 2 C collected at the open-circuit voltage.CV curves at different scan rates from 20 to 100 mV s −1 of (d) Pt/VLS-Mo 2 C and (e) Pt/VSS-Mo 2 C. (f) The plots of DJ versus scan rates for the Pt/VLS-Mo 2 C and Pt/VSS-Mo 2 C, respectively.

Table 1
Comparison of some molybdenum carbide based electrocatalysts for HER