Chemistry Reference
In-Depth Information
5.2 MS-Selected Deposition of Gold Cluster Ions
Preparative mass spectrometry, whereby a species is ionised, mass selected and
collected as a bulk sample for subsequent use, has a history dating back to the use of
calutrons during World War II for separation and isolation of
235
U[
333
]. The
low-energy collisions of ions with surfaces have been comprehensively reviewed
and, depending on the conditions used, can be exploited for the deposition of mass-
selected clusters onto surfaces [
334
]. This is an active area of research since it holds
great promise for the modification of surfaces and the synthesis of novel structures.
Soft landing (SL) experiments allow intact, mass-selected clusters to be applied to
surfaces. The ability to control the size, density and in some cases the morphology
of the deposited particles allows unprecedented flexibility in the creation of new
types of nanostructures and as such holds considerable promise in the development
of new catalysts. This dynamic field has been reviewed [
335
], including a consi-
deration of the types of instruments that have been developed, which are essentially
all 'homebuilt' [
334
]. The challenges of characterising the resultant surface covered
nanoclusters have also been discussed [
336
].
As noted by Johnson et al., mass spectrometry offers several unique capabilities for
deposition experiments, including (1) multiple ways of forming gas-phase ions such as
laser ablation and ESI of intact clusters; (2) the ability to mass select ions from a complex
mixture - this allows deposition of a monodisperse cluster from a polydisperse mixture;
and (3) since ions are used, it is possible to focus and pattern the ion beam, thereby
controlling the landing process. The surface onto which the clusters are deposited can
also be varied to examine how the support influences the reactivity of the landed cluster.
The next two sections consider the role of the surface on the structure and
properties of surface landed gold nanoclusters and the reactions of surface landed
gold nanoclusters with substrates.
5.2.1 Determining the Role of Surface on the Structure and Properties
of Surface Landed Gold Nanoclusters
Table
11
highlights the range of different surfaces that have been used in soft
landing of mass-selected gold NC ions, which include: graphite, highly ordered
pyrolytic graphite (HOPG); silica and alumina; titania; TiO
2
(110); and MgO. Some
key questions that have been addressed when assessing the outcomes of these
experiments include: Have the AuNCs remained intact, or have they fragmented
[
345
,
346
] Are the AuNCs mobile [
341
] Has agglomeration (or sintering) occurred
[
337
,
341
] What happens to the charge [
345
,
346
] Has the shape of the cluster
changed [
341
] What role do surface defects play on the structure, reactivity and
properties of soft landed AuNCs [
348
] In order to address the questions, techniques
have been developed to examine the structure and properties of surface landed
AuNCs. Interested readers in the details of these techniques are directed to key
reviews [
334
-
336
,
359
]. Table
11
also highlights some of the key findings for the
growing literature of soft landed AuNPs.
