10.1002/anie.201805998
Angewandte Chemie International Edition
COMMUNICATION
play synergistic roles. In addition, the physical “crushing” and
“mixing” roles of milling force was verified by SEM in Fig. S21.
In summary, ferrate(VI) solid under solvent-free conditions
displays intrinsically strong and unexpected oxidizing power,
which can be readily harnessed and promoted by
mechanochemistry for green oxidative transformation of both
organic and inorganic substrates. These findings open up a new
chemistry of ferrate(VI), which circumvents the problems
associated with the current solvent-based route and offers the
opportunity to uncover many new knowledge of ferrate(VI) from
both fundamental research and application aspects.
Acknowledgements
The authors acknowledge the financial support from National
Natural Science Foundation of China (51673114), Shanghai
Science and Technology Committee (17ZR1447300) and
National Key Research and Development Program of China
(2017YFA0207500). We thank the Instrumental Analysis Center
of SJTU. We are grateful to Dr. Jin-Long Pan for providing the
GC-MS tests.
Figure 4. Raman spectra of (a) MWNTs and (b) SWNTs (inset: liner fitting of
ID/IG with reaction time). (c) Different oxidation processes on defective and
perfect surface (the yellow lines mark the defective sites).
The different reactivities of ferrate(VI) towards MWNTs and
SWNTs originated from their different surface structures. The
MWNTs used in this study was found highly defective evidenced
by its intense D peak in Raman spectrum (Fig. 4a), while the
surface of starting SWNTs was nearly perfect (tiny D band in Fig.
4b). K2FeO4 exhibited high reactivity towards the surface defects,
such as C-H/C-OH dangling bonds and strained C=C bonds,
and converted them rapidly into oxygenated groups like -COOH,
thus affording a high oxidation degree. Large upshifts in D (+ 5
cm-1) and G bands (+ 10 cm-1) were observed after MWNT
oxidation (Fig. 4a), arising from the “doping” effect by the high
density of oxygenated groups.[21] On the other hand, the defect
degree, measured by the relative intensity of D to G band (ID/IG),
remained substantially unchanged from 1.34 (raw MWNTs) to
1.35 (1 h) and 1.36 (2 h). Thus, ferrate(VI) oxidation occurred
primarily at the defects without affecting the overall graphitic
structure (Fig. 4c), which are advantageous over many classical
approaches where the oxidizing species are aggressive towards
CNT skeleton and thus high-level functionalization is at the
expense of structural damage.[8,18a,22] The nondestructive
oxidation was also supported by TEM images with almost
identical features before and after the oxidation (Fig. S17).
The surface oxidation of SWNTs involved breaking the
highly inert C=C bonds in lattice (Fig. 4c), as reflected by the
gradual intensification of D band (Fig. 4b). The ID/IG was found to
increase almost linearly with time (Fig. 4b inset), implying that,
although the reaction was not rapid, its mildness allowed the
oxidation degree to be controlled by adjusting the reaction time.
In this fashion, a good oxidation degree can be obtained before
destroying the electronic structure, which was verified by Vis-
NIR results in Fig. S18, and the TEM results of SWNTs
subjected to various reaction times (Fig. S19) also supported the
mildness of such oxidative treatment. Presumably, the
mechanical ferrate(VI) oxidation of SWNTs follows one of the
two mechanisms: 1) direct oxidation of C=C bonds by K2FeO4
activated by mechanical force or 2) generation of defects by
milling force with subsequent in situ oxidation by the oxidizer. No
matter which mechanism dominates, it is clear that, in either
case, mechanical force and solvent-free reactivity of ferrate(VI)
Keywords: K2FeO4 • green chemistry • ball milling • carbon
materials
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