Controlling the Degree of Functionalization: In-Depth Quantification and Side-Product Analysis of Diazonium Chemistry on SWCNTs

Article abstract of DOI:10.1002/chem.201902330

We present an in-depth qualitative and quantitative analysis of a reaction between 4-iodobenzenediazonium tetrafluoroborate and single-walled carbon nanotubes (SWCNTs) via thermogravimetric analysis coupled with mass spectrometry (TG-MS) or a gas chromatography and mass spectrometry (TG-GC-MS) as well as Raman spectroscopy. We propose a method for precise determination of the degree of functionalization and quantification of physisorbed aromates, detaching around their boiling point, alongside covalently bonded ones (cleavage over 200 °C). While the presence of some side products like phenol- or biphenyl species could be excluded, residual surfactant and minor amounts of benzene could be identified. A concentration-dependent experiment shows that the degree of functionalization increases with the logarithm of the concentration of applied diazonium salt, which can be exploited to precisely adjust the amount of aryl addends on the nanotube sidewall, up to 1 moiety per 100 carbon atoms.

Full text of DOI:10.1002/chem.201902330

A Journal of
Accepted Article
Title: Controlling the Degree of Functionalization: In-Depth
Quantification and Side-Product Analysis of Diazonium Chemistry
on SWCNTs
Authors: Milan Schirowski, Andreas Hirsch, and Frank Hauke
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201902330
Link to VoR: http://dx.doi.org/10.1002/chem.201902330
Supported by

10.1002/chem.201902330
Chemistry - A European Journal
FULL PAPER
Controlling the Degree of Functionalization: In-Depth
Quantification and Side-Product Analysis of Diazonium
Chemistry on SWCNTs
Milan Schirowski,a,b Frank Hauke,a,b Andreas Hirscha,b,*
Abstract: We present an in-depth qualitative and quantitative
analysis of reaction between 4-iodobenzenediazonium
absorption-, and emission spectroscopy, thermogravimetric
a
analysis (TGA), X-ray photoelectron spectroscopy (XPS),
microscopy and rarely infrared (IR) spectroscopy are usually
applied to shed light on the chemical identity of the nanomaterial.
However, mentioned analytical methods are qualitative or semi-
quantitative at most. A precise analysis of the degree of
functionalization or identification of side-products including the
influence of reactant concentration has rarely been performed.
Oftentimes, SWCNTs functionalized with the desired functional
group is anticipated the only product with mere confirmation by
XPS or IR.
tetrafluoroborate and single-walled carbon nanotubes (SWCNTs) via
thermogravimetric analysis coupled with mass spectrometry (TG-
MS) or a gas chromatography and mass spectrometry (TG-GC-MS)
as well as Raman spectroscopy. We propose a method for precise
determination of the degree of functionalization and quantification of
physisorbed aromates, detaching around their boiling point,
alongside covalently bonded ones (cleavage over 200 °C). While the
presence of some side products like phenol- or biphenyl species
could be excluded, residual surfactant and minor amounts of
benzene could be identified. A concentration-dependent experiment
shows that the degree of functionalization increases with the
logarithm of the concentration of applied diazonium salt, which can
be exploited to precisely adjust the amount of aryl addends on the
nanotube sidewall, up to 1 moiety per 100 carbon atoms.
In this study, we intended to develop a method to precisely
determine the degree of functionalization (DoF) of a selected
diazonium salt-based reaction. Raman spectroscopy, which can
be considered standard analysis for many nanomaterials, and
forefront analytical tools, consisting of a TGA device coupled
with MS (TG-MS) or a gas chromatograph with MS detector (TG-
GC-MS) were applied to obtain a complete image of side-
products including aromates deriving from side-reactions and
residual surfactant. Suitable reference experiments were
performed to distinguish between non-covalently and covalently
bonded aromates as well as to precisely determine the cleavage
temperature of both species. Furthermore, we investigated the
influence of diazonium salt concentrations on the reaction, in
particular on the degree of functionalization, by applying a large
width of reactant concentrations. Moreover, the ID/IG ratio
obtained by Raman spectroscopy is commonly used to obtain an
impression on whether or not a reaction succeeded yet is known
to show down-turned hyperbolic curve behaviour as reported by
Cançado et. al. for graphene[11] which precludes simple
quantification. For SWCNTs this effect was observed as well but
is less thoroughly investigated.[10a, 12] Thus, we also analyzed the
correlation between the ID/IG ratio and the degree of
functionalization in order to elaborate if quantification or at least
semi-quantitative statements based solely on Raman
spectroscopy are reasonably feasible.
Introduction
Single-walled carbon nanotubes (SWCNTs) are nowadays
commonly functionalized in order to exploit their unique
properties. One of the most prominent functionalization routes is
the covalent functionalization, which has been applied to
improve the nanotubes‟ solubility in organic or aqueous
solvents,[1] to separate metallic from semiconducting species,[2]
and to eventually allow for application, for example in
5]
supercapacitors,[3] batteries,[4] biomedicine,[1d,
or material
reinforcement in polymers.[6] Among functionalization routes
which often require very aggressive chemicals like elemental
fluorine,[7] carbenes, nitrenes[1b, 8], or alkali metals under strictly
inert conditions,[9] one of the rather simple, low-hazardous, rapid,
and non-destructive pathways is the reaction between neutral
SWCNTs and diazonium salts. The reaction is known since
2001 and the underlying single electron transfer (SET)
mechanism has been studied well.[2b, 10] Nevertheless, analytic
methods utilized for nanomaterials like SWCNTs are limited
mainly due to their polydispersibility, making classic techniques
like nuclear magnetic resonance (NMR) and mass spectrometry
(MS) unfavorable. Hence, other techniques like Raman-,
Results and Discussion
For the present study, nine different concentrations of diazonium
salt were added to a SWCNT dispersion in 1 % SDBS (sodium
dodecylbenzenesulfonate, which is known to disperse SWCNTs
well and results in good spectroscopic features)[13] solution
yielding (4-iodophenyl)-functionalilzed nanotubes (SWCNT-PhI)
(see Scheme 1). Applied concentrations of diazonium salts were
within the range of 0.001 mol/L – 0.333 mol/L (for details see
Table S1). Lower concentrations were not accomplished, as
balancing with sufficient precision isn‟t facile. At higher
[a]
[b]
*
M. Schirowski, Dr. F. Hauke, and Prof. Dr. A. Hirsch
Joint Institute of Advanced Materials and Processes (ZMP)
Friedrich-Alexander University of Erlangen-Nürnberg
Dr.-Mack-Str. 81, 90762 Fürth, Germany
M. Schirowski, Dr. F. Hauke, and Prof. Dr. A. Hirsch
Chair of Organic Chemistry II
Friedrich-Alexander University of Erlangen-Nürnberg
Nikolaus-Fiebiger-Str. 10, 91054 Erlangen, Germany
andreas.hirsch@fau.de
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/chem.201902330
Chemistry - A European Journal
FULL PAPER
concentrations, solubility of the diazonium salt reaches its limit,
and stirring turns out to be challenging as the dispersion gets
highly viscous. In order to exclude the influence of the reaction
procedure (e.g. sonication, solvent and surfactant), a reference
sample (Ref 1a) was treated likewise, but without any diazonium
salt.
The obtained functionalized SWCNTs were analyzed by TG-MS,
which is a powerful analytical tool to determine the total mass
loss while simultaneously providing information about the
chemical identity of the detached molecules. The MS ion current
of each functionalized SWCNT-PhI sample shows the expected
+
mass fragments of iodobenzene (m/z 204 (C6H5I∙+); 77 (C6H5 )
+
and 51 (C4H3 )) at temperatures between 150 °C and 400 °C
(see Figure 1), which hence will be referred to as „main mass
loss region‟. While a mass loss is still observable at higher
temperatures, it does not derive from the desired sidewall-
bonded aryl moiety, as indicated by the missing MS ion currents.
It is likely originating from impurities, residual solvent and
surfactant, or lattice decomposition (vide infra).[14]
Figure 1. TG-MS spectrum of SWCNT-PhI (c(diazonium) = 0.083 M). The
colored lines indicate the MS traces of the three most prominent iodobenzene
masses (m/z 204 (C6H5I∙+); m/z 77 (C6H5+) and m/z 51 (C4H3+)). The sample
mass (TGA spectrum) is depicted as black line.
However, it is necessary to bear in mind that not 100 % of the
mass loss in the main mass loss region loss region originates
from the desired iodophenyl moieties. In the reference sample
(Ref 1a), consisting exclusively of SWCNTs sonicated in
surfactant solution, a mass loss of 5.9 % can be observed,
which might derive from physisorbed water, other non-covalently
bound molecules, or defects which are inherent to the pristine
material. Therefore, this considerable value needs to be taken
into account in order to calculate an accurate mass loss of
iodophenyl groups and thus a degree of functionalization (DoF).
As seen in Figure 2, the TGA spectra of three chosen
SWCNT-PhI samples are identical until 150 °C. Then, in the
main mass loss region (marked by dashed vertical lines), the
difference between high and low functionalization becomes
distinct, revealing higher mass losses for higher-functionalized
samples. At even higher temperatures (>400 °C), no indication
for a difference in the samples can be found, as the TGA curves
are parallel in that region. This corroborates the results of TG-
MS (see Figure 1) and proves that the cleavage of the 4-
iodophenyl group occurs in the main mass loss region between
150 °C and 400 °C. A TG-MS spectrum of pure SDBS (see
Figure S3) shows that the surfactant itself decomposes at
Figure 4) appear in the functionalized SWCNT-PhI samples, one
can anticipate that residual surfactant is the source of the
underlying mass loss. The SDBS molecule, being physisorbed
to the nanotubes, can surpass the washing procedure due to
strong van-der-Waals interactions with the aromatic ring system
of the SWCNTs which is common for molecules that can
15]
enclose the nanotubes and/or contain large π-systems.[13a,
Alternatively, the high mass loss temperature and the toluene
peak are an indicator for lattice decomposition, which has been
previously reported for graphene[14] and SWCNTs[16]
.
Eventually, the mass loss deriving exclusively from the desired
functional group ∆m(FG) can be calculated by
∆ ∆
∆
; (Eqn 1)
where ∆m400-150 is the mass loss of the sample in the main mass
loss region and ∆mRef,400-150 is the mass loss of the reference
sample (Ref 1a) in that temperature range. Subsequently, the
DoF can be calculated utilizing equation
∆
+
;
(Eqn 2)
around 480 °C, revealing MS ion peaks of m/z = 91 (C7H7 ), 77
+
+
(C6H5 ), and 51 (C4H3 ). Since, likewise, both a corresponding
mass loss and the corresponding MS peaks (see Figure 2 and
where DoF is the degree of functionalization, M(C) the molar
mass of carbon (12.01 g/mol), m(C) the mass of carbon in the
nanotube lattice (remaining sample mass at 650 °C) and M(FG)
the molar mass of the functional group (203.00 g/mol for
iodophenyl).
Despite being a powerful tool to reveal information about volatile
compounds created during the heating of the sample, TG-MS is
a limited analytical method, especially when multiple molecules
are cleaved from the sample and detected simultaneously
causing an overlap of the MS signals. However, this drawback
can be avoided when a chromatographic step is inserted
between the heating (i.e. TGA) and detection (i.e. MS) step,
allowing to distinguish multiple volatile compounds.[17] Thus, a
TG-GC-MS system was applied to unambiguously identify the
Scheme 1. Reaction of SWCNTs with a 4-iodobenzenediazonium salt in
1 % SDBS solution resulting in the aryl-functionalized SWCNT-PhI.
This article is protected by copyright. All rights reserved.

10.1002/chem.201902330
Chemistry - A European Journal
FULL PAPER
molecules formed in the gas phase of the SWCNT-PhI sample.
In our case, the gas phase at 280 °C was injected into the GC-
MS system, where iodobenzene and, to a much lower extent,
benzene, could be identified as only molecules present in the
gas phase (see Figure 3). The injection was set to 280 °C by
reason that the highest mass loss and the highest MS ion
current were observed around this temperature (compare Figure
1). Benzene, with a retention time (tR) of 5.2 min, most likely
derives from phenyl groups on the SWCNT sidewall as a result
of side reactions. However, the marginal amount of this side
product is difficult to quantify but can be neglected compared to
the amount of iodobenzene detected at tR=10.8 min. To avoid
phenyl groups as a side product, diazonium salts with more
redox-stable moieties, like 4-t-butylbenzenediazonium, could be
used in future works. However, the chosen 4-iodophenyl group
can be of great interest as it allows for further coupling via
Sonogashira or Ullmann type reactions.[18] GC peaks at retention
times higher than 10.8 min are revealed in the chromatogram as
well, but can likewise be detected in reference runs and hence
do not derive from the SWCNT samples. Via MS, those peaks
can be identified as siloxanes and therefore attributed to
fragments from the GC column (so-called „column bleeding‟).
Having proven by TG-GC-MS that iodobenzene, alongside
negligible amounts of benzene, is the only molecule cleaved
from the SWCNT-PhI sample in the main mass loss region, the
question arises whether it is covalently bonded or only
physisorbed onto the SWCNTs. Temperature-dependent Raman
spectroscopy has been utilized in several publications to prove
Figure 2. TGA spectra of low-functionalized (c(diazonium) = 0.021 M), high-
functionalized (c(diazonium) = 0.333 M) and non-functionalized (Ref 1a)
SWCNTs. Dashed lines at 150 °C and 400 °C to highlight the main mass loss
region.
that
a
rehybridization from sp3 carbon atoms to sp2
(rearomatization) takes place, proving the covalent attachment
of the desired aryl moieties as well as the reversibility of the
19]
reaction.[14,
However, physisorbed aromates alongside the
C-C-bonded aryl moieties could not be excluded. Utilizing a
simple reference experiment in the present study, physically
mixing iodobenzene with a SWCNT dispersion, allows for
confirmation of the coexistence of physisorbed iodobenzene and
moreover quantification of both species.
Figure 4 shows the TG-MS spectrum of SWCNT samples with
non-covalently bound iodobenzene, applying the same
procedure as for the covalent approach (see scheme 1), utilizing
iodobenzene instead of the 4-iodobenzendiazonium salt (Ref 1b).
The highest mass loss and MS ion current appear at a lower
temperature (191 °C) than for the covalent SWCNT-PhI samples.
This temperature neatly fits to the boiling point of iodobenzene
(188 °C), which hints that only physisorption rather than covalent
bonding took place. This assumption can be corroborated by
Raman spectroscopy showing that ID/IG ratios of Ref 1b are
532
633
unchanged compared to pristine material (ID/IG = 0.10, ID/IG
= 0.06, ID/IG785 = 0.11, Figure S4), and further by the fact that no
reasonable activation step was applied which could potentially
lead to the formation of C-C-bonds under the chosen conditions.
Utilizing Eqn 1 and Eqn 2, the amount of non-covalently bound
iodobenzene on the SWCNT surface can be calculated to
account for ca. 0.13 % in Ref 1b. Considering that aryldiazonium
salts in aqueous dispersion decompose to the corresponding
arene and phenol[20] it is plausible that a certain amount of
iodobenzene is formed as a side product during the covalent
Figure 3. TG-GC-MS spectra. Top: total ion chromatogram (TIC) of three
SWCNT-PhI samples with low (blue, c(diazonium) = 0.003 M), medium (green,
c(diazonium) = 0.021 M) and high (black, c(diazonium) = 0.333 M) degree of
functionalization. Bottom: MS spectra of benzene (tR= 5.2 min) and
iodobenzene (tR= 10.8 min).
This article is protected by copyright. All rights reserved.

10.1002/chem.201902330
Chemistry - A European Journal
FULL PAPER
approach and thus gets physisorbed to the SWCNTs. Next to
iodobenzene, other molecules like 4-iodophenol or biphenyl
species like 4,4‟-dioodobiphenyl can be expected to be formed
as decomposition product of the diazonium salt and thus get
physisorbed by the SWCNT surface as well. However, TG-GC-
MS proved their absence in all samples (see Figure 3). This can
be explained by an enhancement of water solubility for the
phenol, and by a statistically very unlikely radical recombination
of two iodophenyl radicals.
Having a closer look at the TG-MS spectrum of the diazonium
approach (see Figure 1), one can see a shoulder of the
iodobenzene peaks at ca. 190 °C which we can now, thanks to
Ref 1b, attribute to physisorbed iodobenzene. Utilizing a suitable
fitting function two Gaussian curves can be fit into the MS
signals (see Figure 5, here m/z 77 was used because it was the
most intense signal). One Gaussian fit curve has its peak at a
temperature of 183 °C, matching the cleavage temperature of
the non-covalent species (see Figure 4) as well as the boiling
point of iodobenzene. The peak of the other Gaussian fit curve,
which by exclusion can now be attributed to purely covalently
bonded 4-iodophenyl groups, is located at 286 °C. A Lorentzian
fit curve, which was also taken into consideration because it is
usually applied for spectroscopic features like Raman signals,[21]
did not provide a satisfactory fit curve. The area of the MS peaks
allows for the quantification of the covalent and non-covalent
amount of iodobenzene, revealing that roughly 87 % of the
iodobenzene is bonded covalently to the SWCNTs sidewall and
13 % non-covalently.
To shed light on the influence on the degree of functionalization,
nine different concentrations of diazonium reactant were applied
for the synthesis of SWCNT-PhI. One might expect a linear
increase of the grafting ratio with rising diazonium
concentrations up to a point where no free space on the SWCNT
sidewall is available for further aryl moieties, resulting into an
anticipated saturation curve. However, the results reveal a
logarithmic correlation when plotting the calculated degree of
Figure 5. Experimental MS curve from the TG-MS experiment (m/z 77, black
line) of functionalized SWCNT-PhI (c(diazonium)
= 0.083 mol/L) with
corresponding fit curve (red line) consisting of two single Gaussian curves
(dashed red lines). Black dashed vertical lines point out the cleavage
temperatures of the covalently and non-covalently bound aryl moieties.
functionalization (as determined by Eqn 1 and Eqn 2) against
the concentration of applied diazonium salt (see Figure 6). For
classic analytical chemistry correlation coefficients of R² > 0.99
are demanded for a satisfactory linear slope. However, the
obtained R² values higher than 0.90 are excellent considering
the quantification of functional addends on nanomaterials. Note
that the amount of phenyl groups as side product and the
amount of physisorbed iodobenzene were ignored for this
calculation, as including them would require a TG-MS evaluation
for each single sample. Further, it should be mentioned that the
highest yielded degrees of functionalization of ca. 1 % (e.g. 1 in
100 carbon atoms carries an aryl group) are far lower than the
theoretical limit, as already the sample with the lowest
diazonium concentration contains one diazonium cation for 64
nanotube carbon atoms. Analogous to the reported C60Br24
revealing
a 1,4-addition pattern, likewise conceivable for
aromates on SWCNT sidewalls, the grafting ratio could
potentially increase up to 40 %.[22] Hence, only a minor fraction
of applied diazonium salt molecules eventually covalently bond
to the SWCNT sidewalls at high concentrations. Therefore, side
reactions like the decomposition of the diazonium cation are
predominant.
Utilizing Raman spectroscopy, the ID/IG ratio is known to
increase with increasing defect density. Yet, Cançado et al. have
shown that the correlation is not linear in the case of graphene,
as the ID/IG ratios drops at very high amounts of defects.[11]
A
similar behaviour was observed for SWCNTs[10a, 12], but up to
date not investigated as thoroughly as for graphene. For
nanotubes, the ID/IG ratio also highly depends on the laser
Figure 4. TG-MS spectrum of the physical mixture of SWCNTs with
iodobenzene (Ref 1b). Colored lines indicate the three most prominent
masses of iodobenzene (m/z 204 (C6H5I∙+); m/z 77 (C6H5+) and m/z 51 (C4H3+))
and of toluene (m/z 51, m/z 77, m/z 91 (C7H7+)). The sample mass (TGA
spectrum) is depicted as black line. Dashed vertical lines with indicated
temperatures at the MS peaks point out the cleavage temperatures.
excitation energy,[19] which further complicates
a precise
quantification of defect density. To investigate the influence of
the degree of functionalization on the Raman spectroscopic
properties, statistical Raman spectroscopy (SRS) was
performed with > 2,000 single point spectra for each sample and
This article is protected by copyright. All rights reserved.

10.1002/chem.201902330
Chemistry - A European Journal
FULL PAPER
tool for precise quantification, in particular when the degree of
functionalization exceeds 0.9 %.
As a final confirmation that the increasing mass loss indeed
derives from higher amounts of aryl groups covalently bonded to
the SWCNT sidewall, suitable reference experiments were
performed (Ref 2 and Ref 3, see Table S1). They reveal that the
addition of iodobenzene (see Figure S1) or biphenyl species
(see Figure S2) lead to no significant difference in the TGA
spectra of the samples. Hence, relevant amounts of the
mentioned aromates are not physisorbed to the SWCNTs, and
the higher mass loss derives exclusively from higher quantities
of covalently bonded aromates.
Figure 6. Degree of functionalization of nine functionalized SWCNT-PhI
samples as calculated by formula (1) and (2) plotted as
a function of
log(c(diazonium)) (based on mol/L, black squares). Red line: fitting linear slope
with indicated equation and stability index R².
three different laser excitation wavelengths (532, 633, and
785 nm, see Figure 7). The 785 nm laser yields the highest ID/IG
ratios. However, SWCNTs with different chiralities, metallicities,
and diameters are probed by varying excitation lasers, leaving
this absolute value rather insignificant.[23] Despite, independently
of the utilized excitation wavelength, the highest ID/IG ratio is
observed at diazonium concentrations of 0.16 M corresponding
to a degree of functionalization of roughly 0.9 %. At higher
concentrations of applied diazonium salt, a drop of the ID/IG ratio
is observed. Another potential explanation for this phenomenon,
aside from the hyperbolic behavior researched by Cançado,
would be that excessive amounts of diazonium salt in fact lead
to poorer degrees of functionalization. However, this can be
disproven by TGA (see Figure 6) and GC-MS results (see Figure
8, vide infra), both clearly demonstrating an increase of the
degree
of
functionalization
upon
higher
diazonium
concentrations.
To further corroborate the fact that indeed a logarithmic
correlation between the concentration of the diazonium salt and
the degree of functionalization is underlying, the areas of the
TG-GC-MS spectra were investigated. Hof et al. have shown
that the amount of functional groups cleaved off SWCNTs is
directly proportional to the areas in the gas chromatograms.[17]
Figure 8 shows that the peak areas of the gas chromatograms
deriving from the TG-GC-MS experiments correlate
logarithmically with the applied concentration of diazonium salt,
just like the TGA mass loss (see Figure 6). This proves that the
higher mass released from the SWCNT-PhI samples indeed
derives from higher amounts of aryl functionalities on the
nanotube sidewall rather than from increasing amounts of side
products. Thus, it confirms the logarithmic correlation between
the concentration of applied diazonium salt and the DoF.
Furthermore, the result verifies that Raman spectroscopy shows
a non-linear “Cançado-like” behavior making it an unsatisfactory
Figure 7. a) Raman mean spectra of low-functionalized (c(diazonium) =
0.0013 M), medium-functionalized (c(diazonium)
=
0.010 M)
and high-
functionalized (c(diazonium) 0.083 M) SWCNT-PhI samples with an
=
excitation wavelength of 633 nm. Spectra normalized to the G-band. b) ID/IG
ratios of nine SWCNT-PhI samples with varying concentrations of applied
diazonium salt at 532 nm (black), 633 nm (red) and 785 nm (blue) excitation
wavelength.
This article is protected by copyright. All rights reserved.

10.1002/chem.201902330
Chemistry - A European Journal
FULL PAPER
The obtained results allow for an interesting novelty in tailor-
made functionalization of SWCNT. For the first time, a desired
degree of functionalization can be adjusted, at least in the range
of 0.2 – 1.0 %. For this purpose, three different concentrations of
diazonium salts need to be reacted with a nanotube dispersion.
Subsequently, the resulting functionalized SWCNT-PhI should
be analyzed by TG-MS and the degree of functionalization (DoF)
calculated via Eqn 1 and 2. From the obtained regression curve
in the semi-logarithmic plot (see Figure 6), the necessary
diluting the dispersion of SWCNTs in aqueous surfactant
(experimental details see Table S2). This way, it was possible to
expose the nanotube to a vast excess of diazonium salt up to a
ratio of 64:1 (n(diazonium):n(carbon)). In contrast, the first series
was limited to a ratio of 4:1. Figure 9a depicts the Raman
spectroscopic data of the excess diazonium experiment. The
ID/IG ratios at both probed laser excitation wavelengths (532 and
633 nm) are more or less constant for all present samples
except for the samples with the highest diazonium salt excess.
In the latter two samples (32:1 and 64:1), the ID/IG values are
significantly increased, indicating
a lower (!) degree of
functionalization, considering being on the high-concentration
side of the hyperbolic Cançado curve (compare Figure 7a).[11]
For sample 16:1, a slight increase can be observed as well.
To corroborate the Raman data, TG-GC-MS experiments were
performed and summarized in Figure 9b, depicting the specific
GC area of the most prominent iodobenzene masses (m/z 77
and 204) alongside the calculated DoF (see Eqn 1 and 2). No
Figure 8. Experimental GC areas of the ion tracks m/z 77 (C6H5+, red line) and
m/z 204 (C6H5I∙+, black line) in dependence of applied diazonium salt
concentrations. GCa is the specific GC area, meaning GC area normalized per
mg sample.
concentration of diazonium salt required for the desired degree
of functionalization can be interpolated. By the analytical
techniques described above, the purity of the obtained material
as well as the amount of physisorbed aromates can be
determined. Alternatively,
a
calibration utilizing Raman
spectroscopy can employ for a very quick, but equally raw
estimation of the DoF. When changing the type of diazonium salt
(varying in reactivity due to electron-withdrawing or –donating
moieties)[2b] or the type of SWCNT starting material (varying in
reactivity due to diameter, metallicity, inherent defects, and other
factors), mentioned calibration procedure will most likely require
modification. Whether this method can be adapted for the
investigation of other nanomaterials such as graphene,
MWCNTs, or non-carbon-based ones needs to be investigated
in future works.
Lastly, we desired to further enhance our understanding of the
underlying reaction. It was our aim to shed light on whether it is
possible to increase the DoF unrestrictedly by tremendously
increasing the amount of added diazonium salt, or if, eventually,
a threshold of sidewall functionalities will be reached upon
thermodynamic saturation. As mentioned earlier, however,
higher concentrations of diazonium salts were challenging to
accomplish in the reaction mixture under the chosen conditions.
Hence, another experimental series was conducted, highly
Figure 9. Raman, TGA and GC-MS data of of the excess diazonium samples.
a) ID/IG ratios obtained by SRS with 532 and 633 nm laser excitation
wavelength. b) Specific GC areas (GCa) of m/z 77 (blue) and 204 (red),
representative of iodobenzene. Degree of functionalization (DoF) as calculated
from TGA data by Eqn 1 and 2 depicted in black. GCa normalized per mg
sample. GC area of sample 1:2 not recorded due to machine failure.
This article is protected by copyright. All rights reserved.

10.1002/chem.201902330
Chemistry - A European Journal
FULL PAPER
clear trend is observable considering the diazonium
concentration and the corresponding DoF. Rather, the
calculated amount of grafted aryl addends seems to intensely
fluctuate, which was not the case for the first experimental series
(compare Figure 6). This is most likely due to an insufficient
precision of the TGA balance in relation to the small amounts of
samples. In the present experiment, highly diluted SWCNT
samples had to be utilized in order to allow for the required high
diazonium-to-carbon ratios, often yielding less than 1 mg of
functionalized nanotube sample. While this amount is too small
for most conventional TGA machines, it is sufficient for MS and
Raman experiments. Therefore, the specific GC areas elucidate
the matter, providing more stable and reliable information about
the samples. Both probed GCa values are predominantly
constant for samples 1:1, 2:1, 4:1, and 8:1, corroborating the
previous Raman results and indicating a stable degree of
functionalization. Merely the m/z 77 area is slightly increased for
sample 1:1, which can be anticipated a statistical outlier. On the
other hand, the three samples with the highest diazonium
constant amount of SWCNTs revealed a logarithmic correlation
between the degree of functionalization (DoF) and diazonium
concentration as corroborated by TGA mass loss and GC-MS
area analysis. This allows for a precise adjustment of the degree
of functionalization in future works after brief calibration. The
maximum possible DoF utilizing this reaction is around 1 %.
Tremendously increasing the diazonium reactant concentration
(n(diazonium):n(carbon) > 8:1) results in an opposed effect,
decreasing the amount of 4-iodophenyl moieties on the SWCNT
sidewall and promoting side reactions. Moreover, when Raman
spectroscopy was conducted,
a hyperbolic “Cançado-like”
behavior for ID/IG ratios was observed, with maximum ID/IG
values at a degree of functionalization of 0.9 %, decreasing at
higher DoFs. This establishes Raman spectroscopy as a decent
semi-quantitative method for low grafting ratios (e.g. < 0.9 %),
but futile if used exclusively and precise quantification is
necessary.
excess reveal
a drop (distinct for 64:1 and 32:1, less
Experimental Section
pronounced for 16:1) of the GCa, indicating a lower amount of
covalent 4-iodophenyl moieties. The TGA spectra of those high-
excess samples (see Figure S3) reveal an additional mass loss
step at 630 °C, which is not observable for SWCNT samples
treated with lower amounts of diazonium salt. The origin of this
mass loss is unclear, but it is definitely too high to derive from
covalent aryl moieties. Much more likely, it seems to be caused
by side reactions like polymerization of the diazonium reactant
or a damaging of the nanotubes‟ lattice.[17]
Summarizing, the experiment with high diazonium excess has
illustrated the restrictions of the examined reaction between
neutral SWCNTs and diazonium salts. Raman spectroscopy and
TG-GC-MS analysis have uniformly displayed that the degree of
functionalization (DoF) cannot be increased indefinitely by
addition of unconfined amounts of diazonium reactant. Rather,
saturation is reached at DoFs of around 1 %, and addition of
tremendous amounts of the highly reactive diazonium compound
results in a descent of aryl moieties accompanied by undesired
side reactions.
Used materials: HiPco SWCNTs (diameter 0.8 – 1.2 nm, length
100 – 1,000 nm) were purchased by Unidym (batch #P2772). All
other chemicals were purchased by Sigma-Aldrich and used
without further treatment if not stated otherwise.
Synthesis of 4-iodobenzenediazonium tetrafluoroborate:
Modified from a known procedure.[24] 63 mmol of 4-iodoaniline
was dispersed in 250 mL of water. Then, 25 mL HBF4 solution
(47 %) was added forming a neat purple solution. The solution
was cooled with an ice bath and 116 mmol of saturated NaNO2
solution was added dropwise via dropping funnel, not exceeding
5 °C. After complete addition, the formed yellowish precipitate
was collected, filtered, and washed with 50 mL of ice-cold water
and 50 mL diethyl ether. After finally drying in vacuo for 24 h, the
diazonium salt was yielded (78 %).
Synthesis of (4-iodophenyl)-SWCNT: Reaction equation in
scheme 1. 6.0 mg (0.50 mmol) of HiPco SWCNTs was
dispersed in 6.0 mL of 1 % SDBS-solution by sonicating for
30 min. The black dispersion was then cooled to room
temperature and 4-iodobenzenediazonium tetrafluoroborate
added at once. Initial applied concentration of the diazonium salt
was 0.333 mol/L (636 mg). For the series, the diazonium
concentration was diluted by factor 2 until reaching 1.3 mmol/L
(2.5 mg), corresponding to molar ratios between 4:1 and 1:64
(n(diazonium):n(carbon)). The dispersion was stirred for 24 h
Conclusions
We have thoroughly investigated the reaction between
nanotubes and diazonium salts by the basis of a model system
consisting of 4-iodobenzenediazonium tetrafluoroborate and
HiPco SWCNTs. We found that the 4-iodophenyl groups
covalently bonded to the SWCNTs as main product get cleaved
off the SWCNTs in the main mass loss region between 150 °C
and 400 °C. Negligible amounts of benzene could be detected
via TG-GC-MS, indicating partial removal of the iodine. For the
first time we were able to prove and quantify physisorbed
iodobenzene species in the functionalized samples, which
detaches from the nanotubes around its boiling point of 188 °C,
alongside the covalently bonded aryl group. In contrast, covalent
C-C-bonds of the aryl-SWCNT junctions are broken at 286 °C.
Applying varying concentrations of diazonium salt onto a
and subsequently filtered through
a
0.25 µm cellulose
membrane and washed with 100 mL of water and acetone each.
The black solid was scraped off the buckypaper and dried in
vacuo for 24 h. Details see Table S1.
For the excess diazonium samples, a similar reaction procedure
was applied, utilizing 3.0 mg (0.25 mmol) of SWCNTs dispersed
in 500 ml of SDBS solution instead. This allowed for addition of
higher amounts of diazonium salt, ranging from 39.7 mg to 5.1 g,
corresponding
to
molar
ratios
of
1:2
–
64:1
(n(diazonium):n(carbon)). Details see Table S2.
Synthesis of reference samples: The same procedure as for (4-
iodophenyl)-SWCNT was applied (sonication, stirring, filtering
This article is protected by copyright. All rights reserved.

10.1002/chem.201902330
Chemistry - A European Journal
FULL PAPER
Hauke, A. Hirsch, J. Am. Chem. Soc. 2013, 135, 18385-18395; c) D.
Voiry, O. Roubeau, A. Pénicaud, J. Mater. Chem. 2010, 20, 4385-4391;
d) F. Liang, J. M. Beach, K. Kobashi, A. K. Sadana, Y. I. Vega-Cantu, J.
M. Tour, W. Billups, Chem. Mater. 2006, 18, 4764-4767; e) F. Liang, A.
K. Sadana, A. Peera, J. Chattopadhyay, Z. Gu, R. H. Hauge, W. Billups,
Nano Lett. 2004, 4, 1257-1260.
and washing), adding to the reactive diazonium compound
iodobenzene, biphenyl, 4,4‟-diiodobiphenyl, or no reactant at all
for the non-functionalized sample. Details see Table S1.
Raman spectroscopy: Measurements were performed on a
LabRam Aramis machine. Maps of 100 x 100 µm were recorded
with a step size of 2 µm, giving a total of 2601 single-point
spectra with acquisition times of 0.1 s. Utilized laser excitation
wavelengths were 532 nm, 633 nm and 785 nm, laser power
was <1 W to avoid defunctionalization.
TGA measurements: TG-MS (online) was performed on a
Netzsch Skimmer STA 409 CD. TG-GC-MS for performed on a
Pyris 1 TGA from Perkin-Elmer coupled with a Clarus 680 gas
chromatograph with Elite-5MS column and an SQ 8 quadrupole
mass spectrometer. Samples were heated with a constant rate
of 20 K/min from room temperature to 700 °C.
[10] a) M. S. Strano, C. A. Dyke, M. L. Usrey, P. W. Barone, M. J. Allen, H.
Shan, C. Kittrell, R. H. Hauge, J. M. Tour, R. E. Smalley, Science 2003,
301, 1519-1522; b) J. L. Bahr, J. M. Tour, Chem. Mater. 2001, 13,
3823-3824; c) J. L. Bahr, J. M. Tour, J. Mater. Chem. 2002, 12, 1952-
1958; d) G. Schmidt, S. Gallon, S. Esnouf, J. P. Bourgoin, P. Chenevier,
Chem. Eur. J. 2009, 15, 2101-2110; e) C. A. Dyke, M. P. Stewart, F.
Maya, J. M. Tour, Synlett 2004, 2004, 155-160.
[11] L. G. Cançado, A. Jorio, E. Martins Ferreira, F. Stavale, C. Achete, R.
Capaz, M. Moutinho, A. Lombardo, T. Kulmala, A. Ferrari, Nano Lett.
2011, 11, 3190-3196.
[12] R. Graupner, J. Raman Spectrosc. 2007, 38, 673-683.
[13] a) V. C. Moore, M. S. Strano, E. H. Haroz, R. H. Hauge, R. E. Smalley,
J. Schmidt, Y. Talmon, Nano Lett. 2003, 3, 1379-1382; b) M. Islam, E.
Rojas, D. Bergey, A. Johnson, A. Yodh, Nano Lett. 2003, 3, 269-273.
[14] G. Abellán, M. Schirowski, K. F. Edelthalhammer, M. Fickert, K.
Werbach, H. Peterlik, F. Hauke, A. Hirsch, J. Am. Chem. Soc. 2017,
139, 5175-5182.
Acknowledgements
The authors thank the Deutsche Forschungsgemeinschaft (SFB
953 “Synthetic Carbon Allotropes”).
[15] a) N. Nakashima, Sci. Technol. Adv. Mater. 2006, 7, 609-616; b) D.
Tasis, N. Tagmatarchis, V. Georgakilas, M. Prato, Chem. Eur. J. 2003,
9, 4000-4008; c) M. Zheng, A. Jagota, E. D. Semke, B. A. Diner, R. S.
Mclean, S. R. Lustig, R. E. Richardson, N. G. Tassi, Nat. Mater. 2003, 2,
338-342.
Keywords: Diazo compounds • Nanotubes • Surfactants •
Thermogravimetry • Quantification
[16] M. Schirowski, G. Abellán, E. Nuin, J. Pampel, C. Dolle, V. Wedler, T.-P.
Fellinger, E. Spiecker, F. Hauke, A. Hirsch, J. Am. Chem. Soc. 2018,
140, 3352-3360.
[1]
a) K. Kelly, I. Chiang, E. Mickelson, R. Hauge, J. Margrave, X. Wang, G.
Scuseria, C. Radloff, N. Halas, Chem. Phys. Lett. 1999, 313, 445-450;
b) M. Holzinger, O. Vostrowsky, A. Hirsch, F. Hennrich, M. Kappes, R.
Weiss, F. Jellen, Angew. Chem. Int. Ed. 2001, 40, 4002-4005; c) V.
Georgakilas, K. Kordatos, M. Prato, D. M. Guldi, M. Holzinger, A.
Hirsch, J. Am. Chem. Soc. 2002, 124, 760-761; d) Z. Liu, X. Sun, N.
Nakayama-Ratchford, H. Dai, ACS Nano 2007, 1, 50-56; e) C. A. Dyke,
J. M. Tour, J. Phys. Chem. A 2004, 108, 11151-11159.
[17] F. Hof, R. A. Schäfer, C. Weiss, F. Hauke, A. Hirsch, Chem. Eur. J.
2014, 20, 16644-16651.
[18] a) R. Kumar, C. Rao, J. Mater. Chem. A 2015, 3, 6747-6750; b) A. S.
Jombert, M. K. Bayazit, C. R. Herron, K. S. Coleman, D. A. Zeze, Sci.
Adv. Mater. 2013, 5, 1967-1973.
[19] F. Hof, S. Bosch, J. M. Englert, F. Hauke, A. Hirsch, Angew. Chem. Int.
Ed. 2012, 51, 11727-11730.
[2]
[3]
a) B. Gebhardt, F. Hof, C. Backes, M. Müller, T. Plocke, J. Maultzsch, C.
Thomsen, F. Hauke, A. Hirsch, J. Am. Chem. Soc. 2011, 133, 19459-
19473; b) Y.-J. Do, J.-H. Lee, H. Choi, J.-H. Han, C.-H. Chung, M.-G.
Jeong, M. S. Strano, W.-J. Kim, Chem. Mater. 2012, 24, 4146-4151; c)
W.-J. Kim, N. Nair, C. Y. Lee, M. S. Strano, J. Phys. Chem. C 2008,
112, 7326-7331; d) W.-J. Kim, M. L. Usrey, M. S. Strano, Chem. Mater.
2007, 19, 1571-1576.
[20] a) W. A. Waters, J. Chem. Soc. 1942, 266-270; b) T. Cohen, A. G.
Dietz Jr, J. R. Miser, J. Org. Chem. 1977, 42, 2053-2058.
[21] A. C. Ferrari, J. Robertson, Phys. Rev. B 2000, 61, 14095.
[22] a) F. N. Tebbe, R. L. Harlow, D. B. Chase, D. L. Thorn, G. C. Campbell,
J. C. Calabrese, N. Herron, R. J. Young, E. Wasserman, Science 1992,
256, 822-825; b) D. A. Dixon, N. Matsuzawa, T. Fukunaga, F. N. Tebbe,
J. Phys. Chem. 1992, 96, 6107-6110.
M. De Marco, F. Markoulidis, R. Menzel, S. M. Bawaked, M. Mokhtar, S.
A. Al-Thabaiti, S. N. Basahel, M. S. Shaffer, J. Mater. Chem. A 2016, 4,
5385-5389.
[23] H. Telg, J. Maultzsch, S. Reich, F. Hennrich, C. Thomsen, Phys. Rev.
Lett. 2004, 93, 177401.
[4]
[5]
S. Zeng, H. Chen, H. Wang, X. Tong, M. Chen, J. Di, Q. Li, Small 2017.
A. Bianco, K. Kostarelos, M. Prato, Curr. Opin. Chem. Biol. 2005, 9,
674-679.
[24] E. B. Starkey, Org. Synth. 1939, 19, 40.
[6]
a) C. A. Mitchell, J. L. Bahr, S. Arepalli, J. M. Tour, R. Krishnamoorti,
Macromolecules 2002, 35, 8825-8830; b) J. N. Coleman, U. Khan, Y. K.
Gun'ko, Adv. Mater. 2006, 18, 689-706; c) O. Breuer, U. Sundararaj,
Polym. Compos. 2004, 25, 630-645; d) S. Ahir, Y. Huang, E. Terentjev,
Polymer 2008, 49, 3841-3854.
[7]
[8]
[9]
a) K. Bushimata, S.-i. Ogino, K. Hirano, T. Yabune, K. Sato, T. Itoh, K.
Motomiya, K. Yokoyama, D. Mabuchi, H. Nishizaka, J. Phys. Chem. C
2014, 118, 14948-14956; b) E. Mickelson, C. Huffman, A. Rinzler, R.
Smalley, R. Hauge, J. Margrave, Chem. Phys. Lett. 1998, 296, 188-194.
a) M. Holzinger, J. Abraham, P. Whelan, R. Graupner, L. Ley, F.
Hennrich, M. Kappes, A. Hirsch, J. Am. Chem. Soc. 2003, 125, 8566-
8580; b) C. Gao, H. He, L. Zhou, X. Zheng, Y. Zhang, Chem. Mater.
2008, 21, 360-370.
a) J. Gebhardt, S. Bosch, F. Hof, F. Hauke, A. Hirsch, A. Görling, J.
Mater. Chem. C 2017, 5, 3937-3947; b) F. Hof, S. Bosch, S. Eigler, F.
This article is protected by copyright. All rights reserved.

10.1002/chem.201902330
Chemistry - A European Journal
FULL PAPER
FULL PAPER
Thermogravimetric and Raman
analysis of aryl-functionalized
SWCNTs via diazonium compounds
allows for precise quantification of
product and side products, even
physisorbed ones. Logarithmic
concentration-dependence of the
diazonium reactant enables precise
adjustment of the degree of
Milan Schirowski, Frank Hauke, Andreas
Hirsch*
Page No. – Page No.
Controlling the Degree of
Functionalization: In-Depth
Quantification and Side-Product
Analysis of Diazonium Chemistry on
SWCNTs
functionalization up to 1.0 %.
This article is protected by copyright. All rights reserved.