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Table 1 XPS atomic percent and average oxidation state (AOS) data for
MnO2 nanorods
XPS narrow scans of the C 1s, N 1s, O 1s and Mn 2p regions
were analysed to investigate the bonding between modifying
groups and MnO2. All samples show the expected O 1s peaks for
Mn–O–Mn, M–OH and H–O–H23 at B530, 531 and 532 eV,
respectively (Fig. 3a–d). However NP-MnO2 and AP-MnO2 have
an additional peak at B533 eV. This binding energy is in the
region expected for O in metal–O–C bonds5,24 but also for O in
the NO2 group of NP.25 However, the presence of the peak in
AP-MnO2 which does not contain the NO2 group confirms that
aryl groups are covalently attached to the nanorod surface
through Mn–O–C bonds. The N 1s spectra for the modified samples
(Fig. 3e and g) have a peak at B400 eV assigned to amine (for
AP-MnO2) and azo groups (NP-MnO2 and AP-MnO2);18 NP-MnO2
also shows the expected peak at B406 eV assigned to the N in NO2
groups.18 (The absence of this peak in AP-MnO2 confirms that there
are no NO2 groups and hence that the O 1s peak at B533 eV for this
sample must be due to Mn–O–C bonding.) Adventitious carbon
complicates the C 1s scans (Fig. S5a–e, ESI†) preventing assignment
of peaks and similarly the large amount of multiplet splitting
present in the Mn 2p region (Fig. S5f–j, ESI†) prevents detection of
any small changes in the Mn signal after modification.
We have demonstrated covalent modification of MnO2 nanorods
using diazonium ions under two conditions: using an isolated
diazonium salt in 0.1 M NaOH and through in situ formation of
the diazonium ion in ACN solution. XPS studies give direct evidence
that aryl groups are attached to the nanorod surface via Mn–O–C
bonds however the possible involvement of Mn–C bonds cannot
be discounted. The stable attachment of aryl groups opens many
opportunities for enhancing the performance of MnO2 materials
through tuning the surface properties. Applications of these
modified materials are under investigation in ongoing work.
At%
MnO2
sample
AOS
error
Mn
O
C
N
K
Na C+:Mna AOS
NaOH blank 21.3 53.2 17.2
—
3.2 5.1 0.39
3.992b 0.005d
3.97c 0.07e
3.86b 0.04d
3.65c 0.07e
NP-MnO2
ACN blank
AP-MnO2
17.3 47.9 23.7 3.1 2.6 5.3 0.46
24.4 55.6 16.8
21.6 56.1 18.1 1.5 2.8
—
3.2
0.13
0.13
a
b
C+ is the number of charge-balancing cations (Na+ and K+). AOS
c
measurement from potentiometric titration. AOS measurement from
XPS splitting data. Standard error of mean (3 samples). Standard
error of the regression.
d
e
in Na content could be due to a simple replacement of charge-
compensating H+ by Na+ or alternatively could indicate a
decrease in the average oxidation state of MnO2 after exposure
to basic conditions. The latter possibility was investigated by
determining the average oxidation states (AOS) of the materials
(see ESI,† Experimental and Fig. S4).
The data in Table 1 show that NP-MnO2 and the NaOH blank
samples have AOS close to the expected value of 4 and there
is no evidence that modification with NP groups or treatment
in basic conditions affects the AOS. This suggests that the
observed increase in Na+ after immersion in NaOH is due to
replacement of H+ by Na+. On the other hand, the AP-MnO2
sample which was modified in ACN has a significantly lower
AOS. Although the mean AOS for the ACN blank sample is also
lower, it is not significantly different to the NP-MnO2 sample
and hence it is unclear whether it is the grafted AP groups or
the reaction solvent that leads to the decrease in AOS. This is a
question for further investigation.
Notes and references
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´
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Fig. 3 XPS narrow scans: O 1s (a–d) and N 1s (e–h) for NP-MnO2 (a, e),
NaOH blank (b, f), AP-MnO2 (c, g) and ACN Blank (d, h).
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Chem. Commun., 2014, 50, 13687--13690 | 13689