MohamadFauzi Ahmad1,4, Faridah Sonsudin2,Rosiyah Yahya1, MardianaSaid3, Norliza Baharom4, Adlan Akram Mohamad Mazuki4,Osman Zakaria41Chemistry Department, Faculty of Science2Centre for Foundation Studies in Science3 Infra Analysis Laboratory, IPPPUniversity of Malaya, 50603, Kuala Lumpur,Malaysia.4Chemical Group, National Metrology Institute ofMalaysia (NMIM), Lot PT4803, Bandar Baru Salak Tinggi, 43900, Sepang, Selangor Corresponding author: [email protected]:Multiwall Carbon Nanotubes (MWCNTs), Inductively Coupled Plasma MassSpectrometry (ICP-MS), Scanning Electron Microscope (SEM), Transmission ElectronMicroscopy (TEM). Abstract. Multiwall carbonnanotubes (MWCNTs) are a potential alternative to commonly used catalystsupport structures in Fischer Tropsch process to produce waxes. Unfortunately,metal impurities in MWCNTs developed negative health impact and are undesirablefor use in many applications.
Currently, the research on quantification ofmetallic impurities in MWCNTs is still lacking. In this work, comparativestudies of two MWCNTs from different manufacturers were reported. Thepurification and the metals residual of commercial multiwall carbon nanotubeswere 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 resultsshowed that the metal impurities and amorphous carbon were successful reduced.Kata kunci: Nanotub Karbon PelbagaiDinding (MWCNTs), Spektrometri Jisim Induktif Gabungan Plasma (ICP-MS),Mikroskop Pengimbasan Elektron (SEM), Mikroskop Elektron Penghantaran (TEM).Abstrak. Nanotub karbonMultiwall (MWCNTs) adalah alternatif yang berpotensi untuk struktur sokonganpemangkin yang sering digunakan dalam proses Fischer Tropsch untuk menghasilkan olefin dan lilin.
Malangnya, bendasing logam dalamMWCNT menghasilkan impak kesihatan negatif dan tidak dikehendaki untuk kegunaandalam banyak aplikasi. Pada masa ini, penyelidikan terhadap penentuan bendasinglogam dalam MWCNTs masih kurang. Dalam kajianini, perbandingan diantara dua MWCNT daripengeluar yang berbeza dilaporkan. Pembersihan dan sisa logam nanotub karbon pelbagaidinding komersial telah dikaji menggunakan Spektrometri Jisim Induktif GabunganPlasma (ICP-MS) manakala mikroskop pengimbasanelektron (SEM)dan mikroskop elektron penghantaran (TEM) digunakan untuk menganalisa perubahan morfologi dalam MWCNTs.
Keputusan menunjukkan bahawa bendasing logam dan karbon amorfus telah berjayadikurangkan.Introduction Last two decades severalsynthesis methods of carbon nanotubes (CNTs) have been investigated (T.W. Ebbesen et al; 1992) and thegreater challenges is to obtain powder with high purity degree (>95%).
As-obtained, CNT powder normally is not pure. It contains particles ofcarbonaceous materials (amorphous carbon particles, fullerenes andnanocrystalline polyaromatic shells) and metal catalysts (generally compoundedby Co, Ni or Fe). The lack of analytical methods has led to metallic impuritiesin CNTs. Metal impurities in CNTs are undesirable for their uses in diverseapplications, for instance, they may potentially have a negative health impactwhen using in biomedical fields. The growth duration (J.
K. Radhakrishnan et al; 2009), feed gascomposition (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 fewparameters which affect the quality of CNT production. The metal content in theCNT can be detected by using inductively coupled plasma mass spectrometry(ICP-MS). The ICP-MS is a well-established multi-element analytical techniqueused for fast, precise and accurate determinations of trace elements. ICP-MShas many advantages over other elemental analysis techniques such as atomicabsorption and ICP atomic emission spectrometry (ICP-AES).
The ability tohandle both simple and complex matrices with a minimum of matrix interferencesis due to the high temperature of the ICP source. ICP-MS also has highsensitivity and superior detection capability. Many researchers reported that MWCNTsundergo structural changes and decomposition when heated. The study of MWCNTson oxidized nanotubes and stopped mid-oxidation have been measured using TransmissionElectron Microscopy (TEM) and Scanning Electron Microscope (SEM) (John H.
Lehman et.al; 2011).The objective of this paperis to present an overview of the metals residual in two sample sources ofMWCNTs and their subsequent determination by Inductively Coupled Plasma MassSpectrometry techniques while the morphology of MWCNT and degree ofpurification were observed using SEM and TEM. The purification andfunctionalization of MWCNT by TGA and Raman has been reported detail in the previous paper (M.F.Ahmad et al; 2013).
Experimental MaterialsTwo typesof MWCNTs (Baytubes; Skyspring) had been prepared by catalytic chemical vapordeposition (CCVD) process with purity >95%. According to the supplier, thediameter of Baytubes MWCNT innermean diameter was 3-5 nm, outer mean diameter about 13 nm, and length of> 1?m while 4 nm of inner meandiameter, outer mean diameter about 10-20 nm, and length of 5-30 ?m forSkyspring MWCNT. Purification procedure of MWCNTs In the oilbath at temperature 100°C, 5 g of MWCNT was refluxed with 0.
250 L conc. HNO3for 2 hours. Then, the mixture was cooled down for 1 hour before diluted in 1.5L of water. By using Buchner Funnel, the mixture solution was filtered andrinsed with distilled water for several times (~1 L water). The treated MWCNTs werere-dispersed in 0.5 L of water and stirred overnight.
The treated MWCNTs wasfiltered and washed several times with water until pH value turned to neutral(filtrate pH is > 5) by using Buchner funnel. Later, the treated MWCNTs weretransferred into a covered dry beaker and were dried in oven for overnight attemperature 100 ±100C. Characterization of MWCNTs Resultsobtained from the proposed method for metals determination in MWCNTs were investigatedusing ICP-MS while the surface morphology of MWCNTs was analyzed by FESEM andTEM. Before the ICP-MS determination,the samples must be prepared in the solution form. By using microwavedigestion, 0.
25 g of sample was diluted with 6 mL of HNO3 and 2 mLof H2O2. Microwave heating program was (i) 800 W for 15min (15 min ramp); and (ii) 0 W for 15 min for cooling step. Then, the samplewas diluted with 30 mL deionized water before it was analyzed by ICP-MS. Thecontent of metals in the MWCNTs was measured using ICP-MS with a hexapolecollision cell (Agilent 7500cx). The nebulizer argon gas flow rate was 0.75 L/minfor the glass concentric nebulizer.
The plasma and auxiliary argon flow rateswere 13 L/min and 0.75 L/min, respectively. The forward rf power was 1500W. Thedwell time was 200 ms. Although 70 elements were scanned, only few elementswere 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 95Mowere monitored.
The collision cell technique(CCT) was employed for theelimination of the polyatomic interferences of 40Ar, 40Ar,ArO+, Na2O+, NaAr+ ,and others inthe 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 usingPlasma Chromatographic software. FieldEmission Scanning Electron Microscope (FESEM) was used to analyze the surfacesof MWCNTs. Field emission scanning electron microscopes, (FESEM, Zeiss Gemini)delivers ultra-high resolutions down to 1 nm for the most demanding electronmicroscope applications. The small amount of sample was adhered to the aluminumstub using carbon conductive tape. The stub was mounted on the stub holder andloaded into the chamber. Vacuum pump was used to create the vacuum inside theanalysis chamber.
The test is then initiated using the software provided by themanufacturer. The morphology of MWCNTs were investigated by high resolutionTransmission Electron Microscopy (HRTEM) by using “Zeiss 4BRA 200FE” ataccelerating voltage of 200 kv. Before characterization,samples were suspended in isopropyl alcohol and sonicated for one hour in orderto separate the finest particles of each sample.
After sonication, finest particles of samples were collected from on topof the isopropyl alcohol by using disposable pipette. Then, the sample wastransferred to carbon enhanced copper grids and dried in air. Results and Discussion Figure 1: The major elements from two different sources Skyspring andBaytubes From Figure1, 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, Niand Mn were detected. The impurities in the acid treated Baytube samples werehighly removed as compared to Skyspring samples.
The ICP-MS result shows thatby using HNO3, most of the metal particles were removed and give thebest result for acid treatment method. Summarizing of the MWCNTs impuritieswere shown in Table 1. Table1: The summarize of MWCNTs impurities by ICP-MS Bumpysurface c d Smooth surface Bumpy surface Figure 2: SEM imageof (a). pristine skyspring (b). purified skyspring (c). pristine baytube (d).purified baytube.
Figure 2 shows the morphology of outer surfaces ofMWCNT before and after purification process with difference sources. (a) and (c)shows that the pristine MWCNT of Skyspring and Baytube contributed to a smoothsurface while oxidation MWCNT of Skyspring and Baytubes; (b) and (d) showsbumpy surface that reduced the amorphous carbon in MWCNT. It can be seen thatthere is no MWCNTs structural damage occurred due to the tubes shortening byacid oxidation process. b.NE a.NE d.
NE C.NE Figure 3: HRTEMimage of (a) pristine skyspring, (c) purified skyspring, (b) pristine baytube,(d) purified baytube of MWCNTThe treatmentwith nitric acid of varying temperature and concentrations were the commonmethod to reduce amorphous carbon and remove metal oxide in MWCNT andsubsequently functionalize the surface (Ali Rinaldi et al., 2011). H.Kajiuraet.al reported three steps of purification process consisting of soft oxidationwith 2.
8 N HNO3 for 6-24 hours, air oxidation for 10 min at 550oCand high temperature vacuum treatment for 3 hours at 1600oC. Approximately 20% of the weight of theinitial 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 andpurified MWCNT samples were shows in Figure 3c, 3d. The pristine of MWCNT showedan amorphous carbon around the tubes. The images of purified MWCNT taken by TEM clearlydistinguish that an amorphous carbon had been reduced from MWCNT. Conclusion Thepurification process of MWCNTs can easily be achieved by oxidation using nitricacid. The ICP-MS shows that the metals decrease after purification for bothSkyspring and Baytubes samples. The acid treatment has strongly enhanced thepercentage of purity of Baytube sample as compared to Skyspring sample. Asshown in the SEM result, a bumpy surface appeared after acid treatment whileless impurity amorphous carbon was reduced in Baytubes and Skyspring MWCNTs,thus proved that the purification process was successful.
Acknowledgments Theauthors thank the University of Malaya, Kuala Lumpur, Malaysia for supportingthis 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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