60
4 Halogenation of Metal Clusters
Al
−
n + HX → Al n−1 X
−
+ AlH
(4.6)
These two equations explained how the etching effect proceeds within the reactions of “HX + Al n
−
→ ” in forming Al n I
− species. For a certain Al cluster, these
two reaction pathways could refer to successive and circulatory reactions. For closedshell clusters such as Al 13
– , there could be significantly populated mass abundance
due to both initial population and the subsequent adsorption and dissociation of HI.
The eenergy diagram of a complete set of reaction steps for the Al 13
− has led to a
better understanding on the reactive mechanism, demonstrated as,
Al
−
13 + HX ↔ Al 13 HX
−
→
(4.7)
Al 13 HX
−
+ HX → Al 13 X
−
2 + H 2
(4.8)
Al 13 HX
−
+ HX → Al 13 X
−
+ H 2 + X
(4.9)
Here the reactions are favoured due to the weakly bonded Al 13 HX
− system which
will be unstable and allow for further reactions with HI. This is the essential difference
for the Al n
− reacting with HI compared with CH 3 I as mentioned in the above section.
It is notable that Eq. (4.8) (forming Al 13 I 2
− ) is more energetically favourable due to
the extreme exothermicity of H 2 generation [4, 51].
4.3 Reactivity and Stability of Al n I x
− Clusters
4.3.1 Selective Al n I x
− Surviving Oxygen Etching
As having demonstrated in the above investigation of Al n
− reacting with CH 3 I and
HX, several Al n I m
− clusters have been found to exhibit reasonable stability. In order
to examine selectively stable and unstable species in Al n I m
− clusters, researchers
have given an further insight by conducting gas-phase reactions of Al n I m
− clusters
with oxygen [51], which is known as an effective method to explore magic cluster
species. Two classes of gas-phase Al n I m
− clusters (i.e., Al 13 I x
− and Al 14 I y
− ) were
identified with magic stability selectively showing resistance to oxygen-etching. In
specific, the Al 13 I x
– clusters exhibit pronounced stability for those of even-numbered
iodine atoms; while interestingly, Al 14 I x
– series exhibit enhanced stability for those
with odd numbers of I atoms, as shown in Fig. 4.3.
First-principle calculations suggested that the reaction “Al 13
−
+ I 2 → Al 13 I 2
− ”
is energetically favorable by 3.63 eV; in comparison, the reaction “Al 14
−
+ I 2 →
Al 13 I
−
+ AlI” was also described to be energetically favorable by 2.25 eV, which
indicates an example of I 2 etching towards Al n
− clusters. Also the reaction “Al 13
−
+ I 2 → Al 13 I
−
+ I” is energetically allowed by 0.31 eV [51], and there are other
4 Halogenation of Metal Clusters
Al
−
n + HX → Al n−1 X
−
+ AlH
(4.6)
These two equations explained how the etching effect proceeds within the reactions of “HX + Al n
−
→ ” in forming Al n I
− species. For a certain Al cluster, these
two reaction pathways could refer to successive and circulatory reactions. For closedshell clusters such as Al 13
– , there could be significantly populated mass abundance
due to both initial population and the subsequent adsorption and dissociation of HI.
The eenergy diagram of a complete set of reaction steps for the Al 13
− has led to a
better understanding on the reactive mechanism, demonstrated as,
Al
−
13 + HX ↔ Al 13 HX
−
→
(4.7)
Al 13 HX
−
+ HX → Al 13 X
−
2 + H 2
(4.8)
Al 13 HX
−
+ HX → Al 13 X
−
+ H 2 + X
(4.9)
Here the reactions are favoured due to the weakly bonded Al 13 HX
− system which
will be unstable and allow for further reactions with HI. This is the essential difference
for the Al n
− reacting with HI compared with CH 3 I as mentioned in the above section.
It is notable that Eq. (4.8) (forming Al 13 I 2
− ) is more energetically favourable due to
the extreme exothermicity of H 2 generation [4, 51].
4.3 Reactivity and Stability of Al n I x
− Clusters
4.3.1 Selective Al n I x
− Surviving Oxygen Etching
As having demonstrated in the above investigation of Al n
− reacting with CH 3 I and
HX, several Al n I m
− clusters have been found to exhibit reasonable stability. In order
to examine selectively stable and unstable species in Al n I m
− clusters, researchers
have given an further insight by conducting gas-phase reactions of Al n I m
− clusters
with oxygen [51], which is known as an effective method to explore magic cluster
species. Two classes of gas-phase Al n I m
− clusters (i.e., Al 13 I x
− and Al 14 I y
− ) were
identified with magic stability selectively showing resistance to oxygen-etching. In
specific, the Al 13 I x
– clusters exhibit pronounced stability for those of even-numbered
iodine atoms; while interestingly, Al 14 I x
– series exhibit enhanced stability for those
with odd numbers of I atoms, as shown in Fig. 4.3.
First-principle calculations suggested that the reaction “Al 13
−
+ I 2 → Al 13 I 2
− ”
is energetically favorable by 3.63 eV; in comparison, the reaction “Al 14
−
+ I 2 →
Al 13 I
−
+ AlI” was also described to be energetically favorable by 2.25 eV, which
indicates an example of I 2 etching towards Al n
− clusters. Also the reaction “Al 13
−
+ I 2 → Al 13 I
−
+ I” is energetically allowed by 0.31 eV [51], and there are other