07-11-2016, 03:31 PM
The influence of treatment temperature on the acidity of MWCNT oxidized by
HNO3 or a mixture of HNO3/H2SO4
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abstract
The nature of multi-walled carbon nanotubes (MWCNTs) varies with the change in oxidation conditions.
In this work, the effect of treatment temperatures on the acidity of MWCNTs was studied. Oxidation
was performed by refluxing the MWCNTs in nitric acid or mixtures of sulfuric acid and nitric acid at
different temperatures. After oxidative treatment, a quantitative characterization of o-MWCNTs has been
performed using acid–base titrations which show that the number of surface acidic functional groups
increased by increasing the treatment temperatures. Energy dispersive X-ray (EDX) measurements show
that the oxygen content increased with increasing treatment temperatures. Fourier transform infrared
absorption spectroscopy (FTIR) was used for qualitative characterization. It has been demonstrated that
the acidity is a function of the type of oxidizing agent used and the treatment temperatures. Due to the
importance in attachment strategies and functionalization, this study adds to the global discussion of the
possibility of controlling the MWCNTs’ surface chemistry which plays a crucial role in determining its reactivity.
Introduction
Since their discovery, the potential of multi-walled carbon
nanotubes (MWCNTs) has been investigated for many highly
demanding applications. This is due to their unique chemical and
physical properties. As an essential step for many applications,
MWCNTs are activated by oxidative process to insert functional
groups on their sidewall and thus to satisfy special needs. Wet
chemical oxidation process is recognized as an efficient process for
MWCNTs dispersion and surface activation. The oxidation of MWCNTs
is widely performed by refluxing with oxidizing agents, such as
H2SO4 [1], HNO3 [2], H2SO4/HNO3 [3], KMnO4 [4], KMnO4/H2SO4
[5], HNO3/H2O2 [6] and H2O2 [7]. It has been observed that under
the oxidation process a variety of functional groups such as carboxylic
(–COOH), carbonyl (–C O), and hydroxyl (-OH) groups are
formed on the surface of MWCNTs. These functional groups promote
MWCNTs’ chemical reactivity. This in turn, enhances the
dispersability of MWCNTs in aqueous solutions and organic solvents
and thus can be directly used for composite fabrication.
For the qualitative characterization of the functional groups, different
techniques have been reported, such as X-ray photoelectron
spectroscopy (XPS) [8], Fourier transform infrared spectroscopy
(FTIR) methods [9], and thermal analysis–mass spectroscopy
Recently, it has been reported [11] that MWCNTs treated with
(NH4)2S2O8, H2O2, or O3 yielded higher concentrations of carbonyl
and hydroxyl functional groups while that treated with KMnO4,
HNO3 or H2SO4/HNO3 led to higher fractional concentrations of
carboxyl groups. Also, the carboxyl groups increase linearly with
increasing the percent concentration of HNO3.
Thus, different oxidizing conditions affect the concentration of
oxygen atoms incorporated into MWCNTs and the distribution of
oxygen-containing functional groups. The extent of the oxidation
process is mainly dependent on the treatment duration and temperature.
Some studies have attempted to investigate the effect of
oxidation time with using H2SO4/HNO3 mixture [12], Nitric acid
[13], on structure and surface chemistry of MWCNTs.
Quantitatively using acid–base titration method, it has been
reported that carboxylic groups became the dominant moieties of
oxidized carbon nanotubes after 6 h of oxidation with 70% nitric
acid [13]. It is important to determine the extent of MWCNTs’ surface
oxidation since it affects on both the sorption property and
the colloidal stability [3,14]. Oxidation of MWCNTs is considered to
be the first step that allows further functionalization by molecular
structure or nanoparticles. Therefore, knowledge of the concentration
of different oxygen functional groups on the nanotube surface
could allow better controlling the ultimate structure and property
of MWCNT-based materials [11].
Literature screening shows that HNO3 and a mixture of H2SO4
and HNO3 are most extensively used to oxidize MWCNTs. However,
the effect of treatment temperature has not been well investigated.
Each study reported different treatment temperatures. In the present work, the effect of different treatment temperatures on
the acidity of MWCNTs was investigated using HNO3 or mixtures
of H2SO4/HNO3.
2. Experimental
2.1. Materials
MWCNTs was purchased from Cheap Tubes Com. Their speci-
fications are as follows; purity, >95%; outer diameter, 30–50 nm;
inside diameter, 5–10 nm; length, 10–20 m; average specific
surface area, 60 m2/g; Electrical Conductivity: >100 S/cm; Bulk density:
0.28 g/cm3; True density: ∼2.1 g/cm3. The 70% nitric acid
and 95% Sulfuric acid of analytical grade were obtained from
Sigma–Aldrich. The solutions of 0.01 M base and 0.01 M acid
for titration process were prepared from sodium hydroxide and
hydrochloric acid.
2.2. Oxidation of MWCNTs
MWCNTs were initially dispersed for 1 h by sonication in HNO3.
Typically, 1 g of MWCNTs was dispersed in 100 mL of 70% nitric
acid. Then, the MWCNT–HNO3 mixture was refluxed while stirring
vigorously for 6 h at a specific temperature. After refluxing process,
the mixture was allowed to cool at room temperature. The same
process was repeated using mixtures of nitric acid and sulfuric acid.
To investigate the temperature effect, temperatures of the refluxing
solution was varied as 60, 80, 100, 120 and 140 ◦C for each oxidant.
The oxidized MWCNT (o-MWCNTs) was purified by extraction
from the residual acids by repeated cycles of dilution with distilled
water, centrifugation and decanting the solutions until the pH was
approximately 5. After the purification process, o-MWCNTs were
dried overnight in an oven at 100 ◦C. After that, the dry o-MWCNTs
were pulverized in a ball-mill. Then, characterization and acidity
determination was performed.
2.3. Characterization
X-ray powder diffraction (Shimadzu XRD 6000) was used for the
structure phases of the o-MWCNTs. Field emission scanning electron
microscope (FESEM, FEI Nova-Nano SEM-600, Netherlands)
was used for characterization.
IR spectra in the range of 400–4000 cm−1 were recorded in KBr
pellets using a Thermo Nicolet FT-IR spectrophotometer at room
temperature on. Samples were prepared by gently mixing 10 mg
of each sample with 300 mg of KBr powder and compressed into
discs at a force of 17 kN for 5 min using a manual tablet presser. pH
measurements were monitored with a Fisher model 25 pH meter
calibrated with three buffered (pH 4, 7, and 10) solutions.
2.4. Surface acidic group determination
After the various treatments, the acidity of the o-MWCNTs was
determined performing a typical acid–base or Boehm titration [15].
The o-MWCNT samples, each weighing 5 mg of the material were
immersed in 50 mL of the NaOH solution of which was selected
to neutralize the acidic sites. Then the mixture was dispersed by
sonication bath for 5 min in a closed flask.
Reference samples of 50 ml of each solution were prepared and
treated in the same way as the analyzed samples. Closed flasks containing
reference and analyzed samples were placed on a rotary
shaker at room temperature with continuous stirring for 24 h to
reach the equilibration. Then, the analyzed samples were carefully
filtered. 50 mL of acid solution (HCl) was added into the filtrate and
washings, and then boiled for 20 min to degas the carbon dioxide
from the solution. After cooling at room temperature, the excess
HCl was slowly titrated with NaOH solution up to the neutral point,
pH 7.0, and the pH was monitored using a pH meter. The blank sample
which contained no MWCNTs was also titrated as described.
Titrations were performed in triplicate.
It has been shown that the treatment of MWCNTs with nitric
acid or mixtures of nitric acid and sulfuric acid mainly leads to the
formation of carbonyl, lactone and phenol groups on the nanotubes’
surface [16–18]. Surface acidity of oxidized MWCNTs is contributed
by many types of surface oxygen-containing functional groups such
as carboxylic groups, lactonic groups and phenolic groups. It can
be titrated by NaOH which neutralizes carboxylic, phenolic and
lactonic groups [15].
The titration process includes the following steps:
Step (1): MWCNT–COOH + NaOH (excess)→MWCNT–COO− Na+
Step (2): Filtration process to separate the MWCNT–COO− Na+
from the excess NaOH
Step (3): Addition of HCl excess into the filtrate
Step (4): Determination of the HCl excess by titration with NaOH
of the same concentrations.
3. Results and discussion
The results and discussion are organized as follows: In the first
section, we investigate the influence of treatment temperatures
on the acidity of o-MWCNTs after different oxidative treatments
(HNO3 or a mixture of H2SO4/HNO3). In the second section, we
present the characterization of o-MWCNT that has been optimized
under the optimum experimental conditions.
Influence of treatment temperatures
Boehm acid–base titration method was used to determine the
acidity of the MWCNTs treated by either HNO3 or a mixture
of H2SO4/HNO3 for 6 h under different temperatures. In Fig. 1,
the mmol of acidic oxygen-containing surface groups per mg of
MWCNT material are given as a function of treatment temperature.
It can be seen form Fig. 1a that the total amounts of acidic groups
on o-MWCNTs gradually increased, under nitric acid treatment,
with increasing the temperature between 60 ◦C and 120 ◦C. Further
increase in the temperature results in no significant increase
of the acidity of MWCNTs under nitric acid treatment for 6 h. Thus,
optimum treatment temperature that can be selected is 120 ◦C, at
which 0.026 mmol/mg amount of acidity was obtained. Based on
mentioned experimental conditions, the most favorable reaction
temperature for an efficient oxidation of MWCNTs with nitric acid
treatment has been quantitatively determined by titration method
to be 120 ◦C.
On the other hand, Fig. 1b depicts the effect of temperature
on the oxidation process by a mixture of H2SO4/HNO3. It can
be observed that the acidity increase from 0.011 mmol/mg under
treatment temperature of 60 ◦C to 0.04 mmol/mg at 100 ◦C. The
acidity increased sharply to 0.08 and 0.15 mmol/mg when the
treatment temperature increased to 120 and 140 ◦C, respectively.
However, a mechanical lost was noticed at treatment conditions of 120 or 140 ◦C for 6 h. It is likely that the nanotube lattice became
severely destroyed at this stage [19], creating new defect sites and
facilitating formation of functional groups. Therefore, a treatment
temperature of 100 ◦C, with 6 h oxidation duration, was selected to
be the optimum when H2SO4/HNO3 mixture was used for MWCNTs
oxidation.
Last but not least, it is worthwhile mentioning that Boehm titration
results for unfunctionalized, purified MWCNTs (as a reference)
showed 0.0005 mmol/mg acidity.
3.2. Characterization of o-MWCNT
The change in acidity was convincingly confirmed by energy
dispersive X-ray spectrometry. Energy dispersive X-ray spectroscope
(EDX) measurements are obtained and shown for the
purpose of the quantitative representation of oxygen presence
in the samples. Fig. 2 shows the results of EDX measurement.
Fig. 2a represents the results of untreated MWCNT as a reference
which shows 2.61 at.%. The percentage increases up to
4.50 (Fig. 2b) and 9.49 (Fig. 2c) when MWCNT was treated with
HNO3 at 80 and 120 ◦C, respectively. However, when MWCNT
was treated with mixtures of H2SO4/HNO3 at 80 and 100 ◦C, oxygen
percentage increased to 8.33 and 14.33, respectively (Fig. 2d
and e). A further increase in the percentage was also observed
under H2SO4/HNO3 mixture treatment. Unfortunately, though
the acidity is high in the treatment with H2SO4/HNO3 mixture,
at temperatures of 120 and 140 ◦C, a mechanical lost was
noticed.
FTIR spectra of untreated and treated MWCNTs are shown in
Fig. 3. IR spectrum of untreated MWCNTs, Fig. 3a, shows a characteristic
peak at 1582 cm−1 assigns C C bond in MWCNTs. The band
at about 1160 cm−1 is assigned to C–C bonds. Fig. 3b depicts the IR
spectrum of MWCNTs treated with HNO3 for 6 h at 80 ◦C. The spectrum
shows the carbonyl characteristic peak at 1638 cm−1, which
is assigned to the carbonyl group from quinine or ring structure.
The carboxylic characteristic peak is not intense enough to be clear
in the spectrum. This may indicate that the oxidation conditions
are not strong. By increasing the temperature to 120 ◦C, the carboxylic
characteristic peak at 1710 cm−1 became clear as presented