34
reactivity in different structures.
Since the strength of the bonding depends on how the adsorbate states
hybridize with the adsorbent states, the position of the center of the p level
of the chalcogen atoms might give a clue about the strength of the bonding
65, 66. The center of the p band with respect to the Fermi level can be
calculated as
p =
R
1
−1
()d
R
1
−1
()d
(4.1)
The results for the sulphides and selenides of Mo, W, Ti and Zr in both the
2H and 1T structures are summarized in the Table 4.2. The table indicates
that in the case of Mo and W, the 1T structure has higher binding energy
(HH
ads) for the hydrogen whereas in the case of Ti and Zr hydrogen binds
strongly in the 2H structure. It can also be seen that the compounds which
have higher binding energy have the center of the p-level closer to the Fermi
level. For example, in the case of group-6, the center of the p-level in the
2H structure lies deeper with respect to the Fermi level as compared to the
1T structure whereas in the group-4 the trend is opposite. Thus, it can be
concluded that the position of the p band center is somewhat correlated to the
binding energy.
Table 4.2: Heat of adsorption of hydrogen HH
ads and the center of the p-band
p for sulphides and selenides of Mo, W (group-6) and Ti, Zr (group-4) in the
2H and 1T structures.
2H p HH
ads
1T p HH
ads
MoS2 -2.00 1.68 ± 0.07 MoS2 -1.23 0.10 ± 0.13
MoSe2 -1.74 1.82 ± 0.13 MoSe2 -1.46 0.64 ± 0.11
WS2 -2.32 1.95 ± 0.08 WS2 -1.37 0.23 ± 0.14
WSe2 -2.03 2.03 ± 0.14 WSe2 -1.29 0.78 ± 0.15
TiS2 -1.02 -0.05 ± 0.13 TiS2 -1.45 0.40 ± 0.09
TiSe2 -0.89 0.44 ± 0.12 TiSe2 -1.38 0.90 ± 0.10
ZrS2 -0.96 0.11 ± 0.10 ZrS2 -1.42 0.94 ± 0.07
ZrSe2 -0.80 0.51 ± 0.10 ZrSe2 -1.34 1.19 ± 0.09
However, as shown in the Figure 4.6 there is hardly any trend when the
difference of the adsorption energies is plotted against the difference of the
center of the p level for large number of compounds. The absence of any trend
can be attributed to the large variation in the nature of the metal atoms which
makes it harder to generalize the analysis above for all the groups.