Topics in Current Chemistry (2019) 377:4
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By contrast, aberration-corrected transmission electron microscopy (TEM) is a
more popular technique for characterization of single-atom metal catalysts. Single
metal atoms can be clearly observed in high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images, since the atomic number Z
of a metal is much higher than that of carbon [58]. In addition, accessary electron
energy-loss spectroscopy (EELS) and energy-dispersive X-ray spectroscopy (EDS)
can be performed to detect the chemical composition of samples at the atomic level
[59, 60]. Figure 4c–e shows the HAADF-STEM images and corresponding EELS
data for a graphene-based single-atom Fe catalyst [25]. The bright spots are isolated
Fe atoms, as determined on the basis of Z-contrast and EELS results. However, during imaging and chemical analysis of carbon-based single-atom metal catalysts, the
electron beam irradiation can damage their structure. A comparison of HAADFSTEM images of metallofullerene (Er@C82) peapods acquired at 30 kV and 60 kV
is shown in Fig. 4f [61]. During the STEM experiment at 60 kV, the metallofullerenes were punctured, and single Er atoms escaped from their fullerene cage and
traveled along the single-wall carbon nanotube (SWCNT). By contrast, the metallofullerene molecules observed at 30 kV largely retained an intact structure with a relatively low resolution. Thus, the rational choice of an appropriate operating voltage is
critical for observing the real structure of single-atom metal catalysts.
TEM enables the direct observation of single metal atoms on carbon supports,
but the visual field with these techniques is too small (several to dozens of nanometers). As a result, only partial information can be revealed. Thus, other techniques
are needed that can provide average information for samples.
3.2 Spectroscopy Techniques
Spectroscopy techniques, including X-ray absorption spectroscopy [extended
X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure
(XANES)], X-ray photoelectron spectroscopy (XPS), and Mössbauer spectroscopy,
have been extensively applied for characterizing the chemical environments and content of metal atoms in carbon-based single-atom metal catalysts. Figure 5 shows a variety of spectroscopic techniques revealing the average information of Fe–N–C catalysts
(a typical carbon-based single-atom Fe catalyst) [62]. Compared to the reference samples of Fe foil and Fe 2 O 3 (Fig. 5a), Fe–N–C-600 and Fe–N–C-600 prepared at 600 °C
and 700 °C are lacking a prominent peak at the position of Fe–Fe coordination. This
suggests that all Fe species are atomically dispersed in these Fe–N–C catalysts, with no
aggregation. However, there is a small peak of Fe–Fe coordination clearly observed in
the sample synthesized at 800 °C, suggesting the formation of Fe aggregates. In addition, like the reference samples of FePc and Fe(phen) x , the Fe–N–C catalysts show
a main Fe–N coordination peak at 1.47 Å, indicating the presence of atomically dispersed Fe species. Importantly, the coordination numbers of Fe–N can be determined
by fitting the EXAFS data. Combining the electrochemical evaluation and the EXAFS
fitting results, correlations can be established between catalytic activity and the local
environment of the metal centers, namely their coordination numbers of single-atom
metal catalysts, providing a scientific basis for further study [50, 63]. The normalized
Reprinted from the journal
134
1 3
By contrast, aberration-corrected transmission electron microscopy (TEM) is a
more popular technique for characterization of single-atom metal catalysts. Single
metal atoms can be clearly observed in high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images, since the atomic number Z
of a metal is much higher than that of carbon [58]. In addition, accessary electron
energy-loss spectroscopy (EELS) and energy-dispersive X-ray spectroscopy (EDS)
can be performed to detect the chemical composition of samples at the atomic level
[59, 60]. Figure 4c–e shows the HAADF-STEM images and corresponding EELS
data for a graphene-based single-atom Fe catalyst [25]. The bright spots are isolated
Fe atoms, as determined on the basis of Z-contrast and EELS results. However, during imaging and chemical analysis of carbon-based single-atom metal catalysts, the
electron beam irradiation can damage their structure. A comparison of HAADFSTEM images of metallofullerene (Er@C82) peapods acquired at 30 kV and 60 kV
is shown in Fig. 4f [61]. During the STEM experiment at 60 kV, the metallofullerenes were punctured, and single Er atoms escaped from their fullerene cage and
traveled along the single-wall carbon nanotube (SWCNT). By contrast, the metallofullerene molecules observed at 30 kV largely retained an intact structure with a relatively low resolution. Thus, the rational choice of an appropriate operating voltage is
critical for observing the real structure of single-atom metal catalysts.
TEM enables the direct observation of single metal atoms on carbon supports,
but the visual field with these techniques is too small (several to dozens of nanometers). As a result, only partial information can be revealed. Thus, other techniques
are needed that can provide average information for samples.
3.2 Spectroscopy Techniques
Spectroscopy techniques, including X-ray absorption spectroscopy [extended
X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure
(XANES)], X-ray photoelectron spectroscopy (XPS), and Mössbauer spectroscopy,
have been extensively applied for characterizing the chemical environments and content of metal atoms in carbon-based single-atom metal catalysts. Figure 5 shows a variety of spectroscopic techniques revealing the average information of Fe–N–C catalysts
(a typical carbon-based single-atom Fe catalyst) [62]. Compared to the reference samples of Fe foil and Fe 2 O 3 (Fig. 5a), Fe–N–C-600 and Fe–N–C-600 prepared at 600 °C
and 700 °C are lacking a prominent peak at the position of Fe–Fe coordination. This
suggests that all Fe species are atomically dispersed in these Fe–N–C catalysts, with no
aggregation. However, there is a small peak of Fe–Fe coordination clearly observed in
the sample synthesized at 800 °C, suggesting the formation of Fe aggregates. In addition, like the reference samples of FePc and Fe(phen) x , the Fe–N–C catalysts show
a main Fe–N coordination peak at 1.47 Å, indicating the presence of atomically dispersed Fe species. Importantly, the coordination numbers of Fe–N can be determined
by fitting the EXAFS data. Combining the electrochemical evaluation and the EXAFS
fitting results, correlations can be established between catalytic activity and the local
environment of the metal centers, namely their coordination numbers of single-atom
metal catalysts, providing a scientific basis for further study [50, 63]. The normalized
Reprinted from the journal
134