Investigating Nano-structured Catalysts at the Atomic scale by Field Ion Microscopy and Atom Probe Tomography - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 250

d n 9 r 4 n g | 8

Figure 10.1 (a) Transmission electron micrograph of a carbon supported platinum

catalyst (Adapted with permission from ref. 14. Copyright (2014) Ameri-
can Chemical Society). (b) Typical tip shape used for field emission/ion
microscopies. (c) Field ion micrograph of a clean rhodium tip sample
imaged in the presence of neon gas at B 50 K. (d) Ball model depicting
the structure of a curved face-centered cubic metal crystal, built with an
end radius similar to the sample imaged in (c).

surface reconstruction, catalysis or surface enrichment occurs. The tip sur-
faces can be prepared to high reproducibility while characterizing them with
atomic resolution by FIM imaging.
Studies using FIM and FEM on model systems mimicking the catalytic
behavior of individual metal nanoparticles provide data that are not ac-
cessible to conventional studies of supported catalysts. The temporal and
spatial resolution of field emission techniques are such that the local and
transient features of the individual nanoparticles can be imaged and even—
as will be described in detail—chemically probed. This makes possible e.g.,
the detection of fluctuation-induced effects. 15 The fluctuations are confined
to one single particle (a few nanometers in size) and can cause noise-induced
kinetic transitions in the nanosized reaction systems. These lead to severe
deviations from the kinetics of the same reaction studied on the mesoscopic
or macroscopic scale.
The application of FIM and FEM can provide a number of kinetic features
over a single nanosized catalyst particle; however, the determination of the
local chemical composition remains an issue that is still the subject of
current developments. 16,17 This identification is achieved by removing sur-
face material from the tip extremity as ions by field desorption (if ionization
is restricted to the adsorbed species), or by field evaporation (if the atoms of
the tip material are ionized). The fields required to remove adsorbates are
usually appreciably lower than those required to remove surface atoms. Once
formed, the ions are accelerated and collected by an ion detector. To
measure the time elapsed between the formation of the ion and its
detection—allowing then the determination of its mass to charge (m/z) ratio
by time-of-flight mass spectrometry—pulses are necessary to trigger ion-
ization within a very narrow time interval. These pulses are imposed
by nanosecond voltage pulses or thermal pulses using picosecond or fem-
tosecond laser sources. The combination of field emission methods with
time-of-flight mass spectrometry bears the generic term of 'atom probe
microscopy' (APM). 18,19

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