Figure 4. Characterization of MoSe2-I-36h-Pt after stability test. (a) Post-test SEM image of surface, (b) Post-test XRD, (c) Post-test Mo 3d, (d) Post-test Se 3d, and (e) Post-test Pt 4f.
It is important to investigate the electrocatalyst’s properties after the stability test. The post-test morphology of MoSe2-I-36h was well maintained, as displayed inFigure 4a . XRD analysis reveals an increase in intensity of a peak at 13.1°, indicating the possible phase transformation from 1T to 2H-MoSe2 during the stability test (Figure 4(b)). Moreover, such phase transition was confirmed by post-test Raman analysis where reduction in 1T phase MoSe2 peaks were observed compared to the MoSe2-I-36h-Pt sample before the test. (Figure S18 ) This could be attributed to differences in electrochemical stability between the 1T and 2H phase where, unstable 1T phase may undergo transformation to a more energetically favorable 2H phase of MoSe2 during the stability test which may have resulted in slight increase in overpotential shown in Figure 3f. The post stability test, Mo 3d and Se 3d peak analysis also reveals the relative increase in the 2H peaks (Figure 4 (c) and (d) compared to the sample before the stability test (Figure S11). These changes might have originated from the transformation of MoSe2 from 1T to 2H-MoSe2 phase during the stability test. However, the surface state of MoSe2-I-36h-Pt still maintained 1T/2H mixed phases of MoSe2 despite the phase conversion. With respect to Pt 4f, the binding energies were nearly unchanged (Figure 4(e)), suggesting the properties of Pt were rarely affected by the stability test.