After our article was published
we became aware of the comprehensive and enlightening study by Liu
et al.,[1] which we would like to accentuate.
In their work, they detect the complete reconstruction of NiMoO4·xH2O nanorods into a highly
porous and loose γ-NiOOH structure by electrooxidation in 1
M KOH. By high-resolution transmission electron microscopy (HRTEM)
and electron tomography analysis, they could observe that molybdenum
leaching, before the oxidation of Ni2+ to Ni3+, causes an amorphous Ni–O layer. This agrees with our X-ray
diffraction (XRD) data after molybdenum etching, in which no crystalline
phase other than the one of flower-NiMoO4 could be detected.[2] By HRTEM, they could infer that the formed γ-NiOOH
nanorods are built up from nanosheets when the etching and oxidation
step occur subsequently and not simultaneously, which confirms our
observation of a roughened sheet-like morphology of our nanorods after
catalysis. As in our work, the removal of the vibration spectra of
the nanorods was detected by time-resolved in situ Raman spectroscopy measured without applied bias. However, in contrast
with us, they suggest that it is the vibration environment that is
responsible for the shift of the peak at 355 cm–1 to lower wavenumbers, whereas in our work, this lower wavenumber
is assigned to the presence of a flower-NiMoO4 sheet structure
between the NiMoO4·xH2O nanorod structure and nickel foam. This was confirmed in our study
by performing complementary XRD and Raman spectroscopy studies of
flower-NiMoO4 and NiMoO4 nanorods by both selective
etching and the additional synthesis of samples with domination of
one of the allotropes. We also observe a shift of the peak at 948
cm–1 to slightly lower wavenumbers in their spectra,
which, again, is consistent with presence of flower-NiMoO4 sheet structures between the rods and the foam. Interestingly, the
anhydrous form of NiMoO4·xH2O, which is also known as α-NiMoO4, shows a much
slower leaching rate in 1 M KOH compared with the nanorod-shaped NiMoO4·xH2O.[3] Eventually, with 30 wt % KOH or an increased temperature
to 51.9 °C in 1 M KOH (as shown in an adjacent study[3]), molybdenum leaching was achieved for α-NiMoO4. They attributed this to a very limited molybdenum leaching
rate that was accelerated by higher concentrated KOH or temperature.[2] With the same reasoning and instead considering
two different crystal structures, one with more dense/closer packed
Ni atoms, it would agree with our detected different molybdenum leaching
rates among the different nanostructures, which also possess different
crystal structures. This addendum is meant to highlight and acknowledge
some recent work we missed in our contribution, with the intention
that the additional comments and comparisons made here bring a more
complete understanding of the structures and processes present in
these systems.