Fauzi Ahmad1,4, Faridah Sonsudin2,
Said3, Norliza Baharom4, Adlan Akram Mohamad Mazuki4,
1Chemistry Department, Faculty of Science
2Centre for Foundation Studies in Science
3 Infra Analysis Laboratory, IPPP
University of Malaya, 50603, Kuala Lumpur,
4Chemical Group, National Metrology Institute of
4803, Bandar Baru Salak Tinggi, 43900, Sepang, Selangor
Corresponding author: [email protected]
Multiwall Carbon Nanotubes (MWCNTs), Inductively Coupled Plasma Mass
Spectrometry (ICP-MS), Scanning Electron Microscope (SEM), Transmission Electron
nanotubes (MWCNTs) are a potential alternative to commonly used catalyst
support structures in Fischer Tropsch process to produce waxes. Unfortunately,
metal impurities in MWCNTs developed negative health impact and are undesirable
for use in many applications. Currently, the research on quantification of
metallic impurities in MWCNTs is still lacking. In this work, comparative
studies of two MWCNTs from different manufacturers were reported. The
purification and the metals residual of commercial multiwall carbon nanotubes
were investigated using Inductively Coupled Plasma Mass Spectroscopy (ICP-MS)
while Scanning Electron Microscope (SEM) and Transmission Electron Microscopy
(TEM) were used to analyze the morphological change in MWCNTs. The results
showed that the metal impurities and amorphous carbon were successful reduced.
Kata kunci: Nanotub Karbon Pelbagai
Dinding (MWCNTs), Spektrometri Jisim Induktif Gabungan Plasma (ICP-MS),
Mikroskop Pengimbasan Elektron (SEM), Mikroskop Elektron Penghantaran (TEM).
Multiwall (MWCNTs) adalah alternatif yang berpotensi untuk struktur sokongan
pemangkin yang sering digunakan dalam proses Fischer Tropsch untuk menghasilkan olefin dan lilin. Malangnya, bendasing logam dalam
MWCNT menghasilkan impak kesihatan negatif dan tidak dikehendaki untuk kegunaan
dalam banyak aplikasi. Pada masa ini, penyelidikan terhadap penentuan bendasing
logam dalam MWCNTs masih kurang. Dalam kajian
ini, perbandingan diantara dua MWCNT dari
pengeluar yang berbeza dilaporkan. Pembersihan dan sisa logam nanotub karbon pelbagai
dinding komersial telah dikaji menggunakan Spektrometri Jisim Induktif Gabungan
Plasma (ICP-MS) manakala mikroskop pengimbasan
dan mikroskop elektron penghantaran (TEM) digunakan untuk menganalisa perubahan morfologi dalam MWCNTs.
Keputusan menunjukkan bahawa bendasing logam dan karbon amorfus telah berjaya
Last two decades several
synthesis methods of carbon nanotubes (CNTs) have been investigated (T.W. Ebbesen et al; 1992) and the
greater challenges is to obtain powder with high purity degree (>95%).
As-obtained, CNT powder normally is not pure. It contains particles of
carbonaceous materials (amorphous carbon particles, fullerenes and
nanocrystalline polyaromatic shells) and metal catalysts (generally compounded
by Co, Ni or Fe). The lack of analytical methods has led to metallic impurities
in CNTs. Metal impurities in CNTs are undesirable for their uses in diverse
applications, for instance, they may potentially have a negative health impact
when using in biomedical fields. The growth duration (J.K. Radhakrishnan et al; 2009), feed gas
composition (T.H. Fang et al; 2007), carbon or catalyst precursor composition,
temperature, feed gas flow rate, and carrier gas (I. Kunadian, et al; 2009) is a few
parameters which affect the quality of CNT production. The metal content in the
CNT can be detected by using inductively coupled plasma mass spectrometry
(ICP-MS). The ICP-MS is a well-established multi-element analytical technique
used for fast, precise and accurate determinations of trace elements. ICP-MS
has many advantages over other elemental analysis techniques such as atomic
absorption and ICP atomic emission spectrometry (ICP-AES). The ability to
handle both simple and complex matrices with a minimum of matrix interferences
is due to the high temperature of the ICP source. ICP-MS also has high
sensitivity and superior detection capability. Many researchers reported that MWCNTs
undergo structural changes and decomposition when heated. The study of MWCNTs
on oxidized nanotubes and stopped mid-oxidation have been measured using Transmission
Electron Microscopy (TEM) and Scanning Electron Microscope (SEM) (John H.
Lehman et.al; 2011).
The objective of this paper
is to present an overview of the metals residual in two sample sources of
MWCNTs and their subsequent determination by Inductively Coupled Plasma Mass
Spectrometry techniques while the morphology of MWCNT and degree of
purification were observed using SEM and TEM. The purification and
functionalization of MWCNT by TGA and Raman has been reported detail in the previous paper (M.F.Ahmad et al; 2013).
of MWCNTs (Baytubes; Skyspring) had been prepared by catalytic chemical vapor
deposition (CCVD) process with purity >95%. According to the supplier, the
diameter of Baytubes MWCNT inner
mean diameter was 3-5 nm, outer mean diameter about 13 nm, and length of
> 1?m while 4 nm of inner mean
diameter, outer mean diameter about 10-20 nm, and length of 5-30 ?m for
Purification procedure of MWCNTs
In the oil
bath at temperature 100°C, 5 g of MWCNT was refluxed with 0.250 L conc. HNO3
for 2 hours. Then, the mixture was cooled down for 1 hour before diluted in 1.5
L of water. By using Buchner Funnel, the mixture solution was filtered and
rinsed with distilled water for several times (~1 L water). The treated MWCNTs were
re-dispersed in 0.5 L of water and stirred overnight. The treated MWCNTs was
filtered and washed several times with water until pH value turned to neutral
(filtrate pH is > 5) by using Buchner funnel. Later, the treated MWCNTs were
transferred into a covered dry beaker and were dried in oven for overnight at
temperature 100 ±100C.
Characterization of MWCNTs
obtained from the proposed method for metals determination in MWCNTs were investigated
using ICP-MS while the surface morphology of MWCNTs was analyzed by FESEM and
TEM. Before the ICP-MS determination,
the samples must be prepared in the solution form. By using microwave
digestion, 0.25 g of sample was diluted with 6 mL of HNO3 and 2 mL
of H2O2. Microwave heating program was (i) 800 W for 15
min (15 min ramp); and (ii) 0 W for 15 min for cooling step. Then, the sample
was diluted with 30 mL deionized water before it was analyzed by ICP-MS. The
content of metals in the MWCNTs was measured using ICP-MS with a hexapole
collision cell (Agilent 7500cx). The nebulizer argon gas flow rate was 0.75 L/min
for the glass concentric nebulizer. The plasma and auxiliary argon flow rates
were 13 L/min and 0.75 L/min, respectively. The forward rf power was 1500W. The
dwell time was 200 ms. Although 70 elements were scanned, only few elements
were found in the measurable concentrations in the multiwall carbon nanotubes.
Quantitative analysis for these metals was then conducted. Isotopes such as 53Cr,
55Mn, 54Fe, 57Fe, 59Ni, 60Co,
63Cu, 65Cu, 66Zn, 68Zn, and 95Mo
were monitored. The collision cell technique(CCT) was employed for the
elimination of the polyatomic interferences of 40Ar, 40Ar,
ArO+, Na2O+, NaAr+ ,and others in
the detection of 54Fe, 59Ni, 63Cu, 52Cr,
and others. Optimization was carried out daily with a normal tuning solution
(1ng/mL, Be, Co, In, U). Raw data were collected in the computer by using
Plasma Chromatographic software. Field
Emission Scanning Electron Microscope (FESEM) was used to analyze the surfaces
of MWCNTs. Field emission scanning electron microscopes, (FESEM, Zeiss Gemini)
delivers ultra-high resolutions down to 1 nm for the most demanding electron
microscope applications. The small amount of sample was adhered to the aluminum
stub using carbon conductive tape. The stub was mounted on the stub holder and
loaded into the chamber. Vacuum pump was used to create the vacuum inside the
analysis chamber. The test is then initiated using the software provided by the
manufacturer. The morphology of MWCNTs were investigated by high resolution
Transmission Electron Microscopy (HRTEM) by using “Zeiss 4BRA 200FE” at
accelerating voltage of 200 kv. Before characterization,
samples were suspended in isopropyl alcohol and sonicated for one hour in order
to separate the finest particles of each sample. After sonication, finest particles of samples were collected from on top
of the isopropyl alcohol by using disposable pipette. Then, the sample was
transferred to carbon enhanced copper grids and dried in air.
Results and Discussion
Figure 1: The major elements from two different sources Skyspring and
1, the Skyspring pristine samples showed that the major elements; Cr, Fe, Mg,
Al, Mo and Cu have been detected while for Baytube pristine samples only Co, Ni
and Mn were detected. The impurities in the acid treated Baytube samples were
highly removed as compared to Skyspring samples. The ICP-MS result shows that
by using HNO3, most of the metal particles were removed and give the
best result for acid treatment method. Summarizing of the MWCNTs impurities
were shown in Table 1.
1: The summarize of MWCNTs impurities by ICP-MS
Figure 2: SEM image
of (a). pristine skyspring (b). purified skyspring (c). pristine baytube (d).
Figure 2 shows the morphology of outer surfaces of
MWCNT before and after purification process with difference sources. (a) and (c)
shows that the pristine MWCNT of Skyspring and Baytube contributed to a smooth
surface while oxidation MWCNT of Skyspring and Baytubes; (b) and (d) shows
bumpy surface that reduced the amorphous carbon in MWCNT. It can be seen that
there is no MWCNTs structural damage occurred due to the tubes shortening by
acid oxidation process.
Figure 3: HRTEM
image of (a) pristine skyspring, (c) purified skyspring, (b) pristine baytube,
(d) purified baytube of MWCNT
with nitric acid of varying temperature and concentrations were the common
method to reduce amorphous carbon and remove metal oxide in MWCNT and
subsequently functionalize the surface (Ali Rinaldi et al., 2011). H.Kajiura
et.al reported three steps of purification process consisting of soft oxidation
with 2.8 N HNO3 for 6-24 hours, air oxidation for 10 min at 550oC
and high temperature vacuum treatment for 3 hours at 1600oC. Approximately 20% of the weight of the
initial raw soot remained and the final product contained metal less than 1%
(H. Kaijura et al. 2002). The morphologies of typical pristine Figure 3a,3b and
purified MWCNT samples were shows in Figure 3c, 3d. The pristine of MWCNT showed
an amorphous carbon around the tubes. The images of purified MWCNT taken by TEM clearly
distinguish that an amorphous carbon had been reduced from MWCNT.
purification process of MWCNTs can easily be achieved by oxidation using nitric
acid. The ICP-MS shows that the metals decrease after purification for both
Skyspring and Baytubes samples. The acid treatment has strongly enhanced the
percentage of purity of Baytube sample as compared to Skyspring sample. As
shown in the SEM result, a bumpy surface appeared after acid treatment while
less impurity amorphous carbon was reduced in Baytubes and Skyspring MWCNTs,
thus proved that the purification process was successful.
authors thank the University of Malaya, Kuala Lumpur, Malaysia for supporting
this research under funding No. PS358/2010A Postgraduate Research Fund (PPP).
Technical support from the Department of Chemistry, Faculty of Science,
University of Malaya, 50603, Kuala Lumpur, Malaysia and Chemical Group,
National Metrology Institute of Malaysia, SIRIM Berhad, are also acknowledged.
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