1 Ligand-Free Sub-Nanometer Metal Clusters in Catalysis
9
lowest unoccupied molecular orbital), which is obtained from the UV–vis spectrum.
Thus, the absorption wavelength is directly correlated with atomicity and vice versa.
Complementary emission (fluorescence) analysis circumvents the possible mask or
interference by other absorbing species in UV–vis that could be present in the analyte,
such as organic molecules and other metal compounds, since the clusters have the
particularity of behaving as potent quantum dots. Thus, irradiation of the clusters
in their corresponding absorbing wavelengths gives clear emission bands, which
does not occur with most of organic compounds and metal precursors, including
nanoparticles (see Fig. 1.8).
1.2.3.2 Mass Spectrometry
Routine and high-resolution electrospray ionization–mass spectrometry with
quadrupole detectors are commonly employed to determine the empirical formula
of metal clusters with ligands in solution, and matrix-assisted laser desorption/ionization time-of-flight (MALDI–TOF) spectrometers are used for solid
samples. Following this, these mass spectrometry techniques can be employed not
only to determine the mass of ligand-free metal clusters but also for selection, separation, isolation and deposition of individual clusters on solid surfaces [69]. Most
metals show a unique isotopic pattern which unveils the atomicity of the cluster. Even
for monoisotopic metals, such as the case of Au with a practical single isotope at
197 Da., mass spectrometry is useful since metal clusters appear beyond the minimum
detectable mass of the instrumentation, typically 400 Da., which is a clear advantage
with respect to lighter metals (Fig. 1.9) [55].
[Cu5]
[Pt3] –
Fig. 1.9 Left: ESI–QTOF spectrum of small (Cu5) clusters in ethanol solution in negative ion
mode with the simulation of the relative intensity peaks. Electrospray ionization/mass spectrometry
with a quadrupole time-of-flight (ESI–QTOF) measurements of the samples. Right: ESI–QTOF
measurements of Pt samples taken at 60 °C with 0.005 mol% of Karstedt’s catalyst (reprinted with
permission from Refs. [11 and 16]. Copyright © 2015 by American Chemical Society and Copyright
© 2019 John Wiley & Sons, Inc., respectively)
9
lowest unoccupied molecular orbital), which is obtained from the UV–vis spectrum.
Thus, the absorption wavelength is directly correlated with atomicity and vice versa.
Complementary emission (fluorescence) analysis circumvents the possible mask or
interference by other absorbing species in UV–vis that could be present in the analyte,
such as organic molecules and other metal compounds, since the clusters have the
particularity of behaving as potent quantum dots. Thus, irradiation of the clusters
in their corresponding absorbing wavelengths gives clear emission bands, which
does not occur with most of organic compounds and metal precursors, including
nanoparticles (see Fig. 1.8).
1.2.3.2 Mass Spectrometry
Routine and high-resolution electrospray ionization–mass spectrometry with
quadrupole detectors are commonly employed to determine the empirical formula
of metal clusters with ligands in solution, and matrix-assisted laser desorption/ionization time-of-flight (MALDI–TOF) spectrometers are used for solid
samples. Following this, these mass spectrometry techniques can be employed not
only to determine the mass of ligand-free metal clusters but also for selection, separation, isolation and deposition of individual clusters on solid surfaces [69]. Most
metals show a unique isotopic pattern which unveils the atomicity of the cluster. Even
for monoisotopic metals, such as the case of Au with a practical single isotope at
197 Da., mass spectrometry is useful since metal clusters appear beyond the minimum
detectable mass of the instrumentation, typically 400 Da., which is a clear advantage
with respect to lighter metals (Fig. 1.9) [55].
[Cu5]
[Pt3] –
Fig. 1.9 Left: ESI–QTOF spectrum of small (Cu5) clusters in ethanol solution in negative ion
mode with the simulation of the relative intensity peaks. Electrospray ionization/mass spectrometry
with a quadrupole time-of-flight (ESI–QTOF) measurements of the samples. Right: ESI–QTOF
measurements of Pt samples taken at 60 °C with 0.005 mol% of Karstedt’s catalyst (reprinted with
permission from Refs. [11 and 16]. Copyright © 2015 by American Chemical Society and Copyright
© 2019 John Wiley & Sons, Inc., respectively)