The shape-dependent surface oxidation of 2D ultrathin Mo2C crystals

2D atomic crystals have been widely explored, usually owing to their numerous shapes, of which the typical hexagon has drawn the most interest. However, the relationship between shape and properties has not been fully probed, owing to the lack of a proper system. Here, we demonstrate for the first time the shape-dependent surface oxidation of 2D Mo2C crystals, where the elongated flakes are preferentially oxidized under ambient conditions when compared with regular ones, showing higher chemical activity. The gradual surface oxidation of elongated Mo2C crystals as a function of time is clearly observable. Structural determinations reveal that a discrepancy in the arrangement of Mo and C atoms between elongated and regular crystals accounts for the selective oxidation behavior. The identification of the shape-dependent surface oxidization of Mo2C crystals provides significant possibilities for tuning the properties of 2D materials via shape-control.


Experimental
Elemental quantification of Mo3d component populations was obtained using XPS combined with Scofield relative sensitivity factors corrected for an electron escape depth.Lorentzian asymmetric peak shape with tail dumping was used in peak fitting.Dumping parameters were set as derived by Baltrusaitis et al. 1 .XPS data processing was performed using a CasaXPS program (suite version 2.3.20).As described by Baltrusaitis et al. 1,2 , fitting complex XPS spectral envelopes without prior knowledge of the lineshapes involves certain degree of arbitrariness.It was minimized my constraining Mo3d peak area ratios and their splitting according to the fundamental parameters.

Results
Mo3d5/2 peak can be seen of MoC previously reported at 227.9 eV 3 which is very close to that of metallic Mo 4 due to the existence of Mo-Mo bonds 5 .MoC species comprised about 80% of the total Mo with minor oxygenated MoOx species with varying oxidation states.Oxidized sample has significantly reduced Mo-Mo bond content and increased MoOx species with ~67% of the surface exposed to the oxidized molybdenum compounds.In particular, peaks due to Mo4+, Mo5+ and Mo6+ were detected with the latter particularly significant in the oxidized sample.
Electronic Supplementary Material (ESI) for Nanoscale Advances.This journal is © The Royal Society of Chemistry 2019

Figure S1 .
Figure S1.(001) surfaces of eclipse-Mo 2 C, Mo 4 C and MoC; colum (a) is the cell structures of Mo 2 C, Mo 4 C based on eclipse-Mo 2 C with C in bulk center and ridge center deleted, MoC in P63/MMC space group; colum (b) is the side views of the (001) surfaces; colum (c) is the truncated (001) surfaces for direct comparison with the Figure 5c and 5f in the paper.

Figure S2 .
Figure S2.X-ray diffraction (XRD) patterns and standard PDF data of hexagonal β-Mo 2 C and η-MoC from experiments.

Figure S3 .
Figure S3.AFM thickness determination of the elongated M 2 C.

Figure S4 .
Figure S4.CH 4 -controlled morphology evolution of the M 2 C crystal on liquid Cu surface by CVD.It is observed that the number of elongated Mo 2 C will increase with decrease of the CH 4 gas flow rate.

Figure S5 .
Figure S5.Surface changing of distorted shaped Mo 2 C crystal, suggesting the very common phenomena among those distorted shaped flakes.All the scale bars are 5 m.

Figure S6 .
Figure S6.Surface changing of distorted hexagonal Mo 2 C crystal, suggesting the very common phenomena among those distorted shaped flakes.

Figure S7 .
Figure S7.Raman spectrum of the as-oxidized Mo 2 C samples, whereas the typical peaks for the MoO x was labeled.

Figure S8 .
Figure S8.Thickness effect of the oxidation behavior.Noted that the surface oxidation behavior can be both detected onto the surface of elongated flakes with different thickness, indicating a common behavior.

Figure S9 .
Figure S9.XPS survey spectra of the Mo 2 C covered with oxides in Figure 4a.

Figure S10 .
Figure S10.The XPS spectrum of before and after oxidization Mo 2 C crystals.

Figure S11 .
Figure S11.The reduction of the elongated Mo 2 C flake on the Cu surface.All the scale bars are 10 m.

Figure S12 .
Figure S12.The regular Mo2C crystals and the corresponding elongated ones.All the scale bars are 1 m.

Figure S13 .
Figure S13.Schematic showing shape evolution from regular to elongated ones.

Figure S14 .
Figure S14.The as-grown Mo 2 C flakes with various shapes.It is noted that the shape is sensitive to gas flow rate of CH 4 .The fractal and triangular Mo 2 C were obtained under certain growth conditions.All the scale bars are 5 m.

Figure S15 .
Figure S15.The TEM images and diffraction patterns of the hexagonal and elongated Mo 2 C crystals, noted that the diffraction data is collected from the middle of the samples.

Figure S16 .
Figure S16.The side view of atomic model of regular and elongated Mo 2 C crystals, respectively.

Table S1 .
Calculated d-spacing of different diffraction spots of Mo 2 C with and without periodic carbon vacancies.