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Efficient gas-phase purification using chloroform for metal-free multi-walled carbon nanotubes
Goak, Jeung Choon,Lim, Chang Jin,Hyun, Yesub,Cho, Eunkyung,Seo, Yongho,Lee, Naesung Elsevier 2019 Carbon Vol.148 No.-
<P><B>Abstract</B></P> <P>Gas-phase treatments using chloroform (CHCl<SUB>3</SUB>) and water vapor were used to remove metal impurities (MIs) from as-prepared multi-walled carbon nanotubes (AP-MWCNTs), which is a non-destructive way to produce highly pure CNTs with extremely low MI content and with retention of CNT properties. After purification using CHCl<SUB>3</SUB>, the MIs in the AP-MWCNTs had significantly decreased to 12 ppm, representing a purification efficiency of 99.8%. This purification method, applicable to various AP-MWCNTs with high contents and different compositions of MIs, is more effective than liquid-phase purification using acids and does not damage the CNT structure and morphology. This moderate purification is due to a combination of factors. Chlorine radicals and hydrochloric acid from the thermal decomposition of CHCl<SUB>3</SUB> react with MIs encased by graphitic carbon layers to form metal chlorides that sublime at high temperature. Hydrogen, oxygen, and water vapor generated during the formation of the metal chlorides etch graphitic carbon layers and lead to the formation of new pores and cracks, which provide fast reaction and diffusion paths. During metal purification, the MWCNT surface becomes lightly coated with chlorine or chlorinated carbon, which are eliminated from the surface post-treatment using water vapor.</P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
Goak, Jeung Choon,Lee, Sung Ho,Han, Jong Hun,Jang, Se Hong,Kim, Ki Buem,Seo, Yongho,Seo, Young-Soo,Lee, Naesung Elsevier 2011 Carbon Vol.49 No.13
<P><B>Abstract</B></P><P>This study evaluates the effect of anionic and cationic surfactants on the dispersion of purified SWCNTs in water in terms of dispersibility and on electrical conductivity of TCFs and electronic band structures of SWCNTs. The dispersibility of surfactants in an aqueous SWCNT suspension is assessed with the amount of SWCNTs dispersed, the content of surfactants required to suspend SWCNTs, and the long-term stability of dispersion. Sodium dodecylbenzene sulfonate (SDBS) shows better dispersibility and electrical conductivity of SWCNTs than sodium dodecyl sulfate, sodium cholate, and cetyltrimethyl ammonium bromide. Electronic band structures of SWCNTs vary with surfactants and nitric acid treatment, investigated by using UV–Vis–NIR and Raman spectroscopy. Metallic and semiconducting SWCNTs and surfactants make electrostatic charge interactions between them, which occur in different manners according to the electronic types of tubes and the natures of surfactants. TCFs are fabricated by using the SWCNT suspension dispersed with SDBS, which reveal a low percolation threshold with the two dimensional percolation behavior. The highest ratio of dc to optical conductivity (<I>σ<SUB>dc</SUB></I>/<I>σ<SUB>op</SUB></I>) is observed to be ∼23.1, corresponding to sheet resistance of 69Ω/sq at the 550-nm optical transmission of 80%, upon nitric acid treatment of the SWCNT films.</P>
곽정춘 ( Jeung Choon Goak ),김태양 ( Tae Yang Kim ),정종완 ( Jong Wan Jung ),서영수 ( Young Soo Seo ),석중현 ( Jung Hyun Sok ),이내성 ( Nae Sung Lee ) 대한금속재료학회(구 대한금속학회) 2013 대한금속·재료학회지 Vol.51 No.2
This study evaluated the purity of single-walled carbon nanotubes (SWCNTs) in the arc-synthesized SWCNT samples by using thermogravimetric analysis (TGA). The as-produced SWCNT samples were heat-treated in air for 20 h at 275-475℃ and characterized by scanning and transmission electron microscopes and TGA to establish oxidation temperature ranges of SWCNTs and carbonaceous impurities comprising the samples. Based on these oxidation temperature ranges, derivative thermogravimetric curves were deconvoluted, and differentiated peaks were assigned to SWCNTs and carbonaceous impurities. The compositions and the SWCNT purities of the samples were obtained simply by calculating the areal ratios under the deconvoluted curves. TGA studies on purity evaluation and thermal stabilities of SWCNTs and carbonaceous impurities are likely to provide us with a simple route of thermal oxidation purification to acquire high-purity SWCNT samples.
곽정춘,이내성,Goak, Jeung-Choon,Lee, Nae-Sung 한국전기전자재료학회 2008 전기전자재료학회논문지 Vol.21 No.2
Single walled carbon nanotubes (SWCNTs) can be modified to produce a new concept of hybrid materials by introducing atoms or molecules inside their cylindrical empty space. Such an endohedral doping of the SWCNTs is expected to decisively alter their electronic transport and mechanical properties, In this study, we purified SWCNTs by three-step purification processes and formed the peapod structure by introducing $C_{60}$ fullerenes inside the SWCNTs. $C_{60}$ molecules were observed to be regularly arranged by transmission electron microscopy. In Raman spectra, the radial breathing mode (RBM) rather than the other modes was significantly affected by the endohedral injection of $C_{60}$. The RBM intensity was more greatly reduced in the large-diameter SWCNTs than the small-diameter ones, Raman spectroscopy is expected to be a key technique for analyzing $C_{60}$-encapsulated SWCNTs.
Functionalization of Multi-Walled Carbon Nanotubes with Poly(2-ethyl-2-oxazoline)
Tan, Soon Huat,Goak, Jeung Choon,Lee, Naesung,Kim, Jeong-Yeol,Hong, Sung Chul WILEY-VCH Verlag 2007 Macromolecular symposia Vol.249 No.1
<P>Multi-walled carbon nanotubes (MWCNTs) were successfully functionalized with water-soluble poly(2-ethyl-2-oxazoline) (PEOX) through a polymer wrapping process. Pristine MWCNTs were first treated with nitric acid (HNO<SUB>3</SUB>) at 120 °C for 24 hours. The resulting carboxylated MWCNTs were then mixed with 10 wt.% PEOX in ethanol, followed by sonication using a horn-type sonicator for 30 min and stirring at room temperature for 24 hours. Thereafter, unreacted free polymer was washed out repeatedly by centrifugation with an excess amount of ethanol. Scanning electron microscopy exhibited that the diameter of MWCNTs increased approximately by 10 nm after the functionalization of MWCNT with PEOX. Thermogravimetric analysis showed that approximately 15 wt.% polymer was incorporated in the functionalized MWCNTs. The functionalized MWCNTs gave excellent and stable dispersion in water as well as other organic solvents such as ethanol and DMF.</P>
기능화에 의한 단일벽 탄소나노튜브 정제 및 페이퍼 제조와 전계방출 특성 연구
곽정춘,이승환,이한성,이내성,Goak, Jeung-Choon,Lee, Seung-Hwan,Lee, Han-Sung,Lee, Nae-Sung 한국전기전자재료학회 2008 전기전자재료학회논문지 Vol.21 No.5
Single-walled carbon nanotubes (SWCNTs) were currently produced together with some contaminants such as a metallic catalyst, amorphous carbon, and graphitic nanoparticles, which should be sometimes purified for their applications. This study aimed to develop efficient, scalable purification processes but less harmful to SWCNTs. We designed three-step purification processes: acidic treatment, surface functionalization and soxhlet extraction, and heat treatment. During the soxhlet extraction using tetrahydrofuran, specifically, carbon impurities could be easily expelled through a glass thimble filter without any significant loss of CNTs. Finally, SWCNTs were left as a bulky paper on the filter through membrane filtration. Vertically aligned SWCNTs on one side of bulky paper were well developed in a speparation from the filter paper, which were formed by being sucked through the filter pores during the pressurized filtration. The bucky paper showed a very high peak current density of field emission up to $200\;mA/cm^2$ and uniform field emission images on phosphor, which seems very promising to be applied to vacuum microelectronics such as microwave power amplifiers and x-ray sources.
알루미늄 하부층이 탄소나노튜브의 성장 및 전계방출 특성에 미치는 영향
이승환,곽정춘,이한성,이내성,Lee, Seung-Hwan,Goak, Jeung-Choon,Lee, Han-Sung,Lee, Nae-Sung 한국전기전자재료학회 2008 전기전자재료학회논문지 Vol.21 No.2
We studied the effect of an Al underlayer on the growth of carbon nanotubes (CNTs) and their field emission characteristics, First of all, CNTs were grown on the Invar catalyst layers with different thickness of 1 to 10 nm, showing that the CNT length was saturated for the catalyst 5 nm or thicker. The CNTs grown on the 5-nm-thick catalyst were ${\sim}10{\mu}m$ long and ${\sim}30nm$ in diameter. Second, an Al underlayer was applied between the catalyst layer and the Ti diffusion barrier to reduce the diameters of CNTs for better field emission properties by forming spherical Al oxide particles on which smaller catalyst nanoparticles would occur. The optimal thickness of an Al underlayer underneath the 5-nm-thick catalyst was ${\sim}15nm$, producing the CNTs with the length of ${\sim}15{\mu}m$ and the diameter of ${\sim}15nm$. The field emission measurements, following the tape activation, showed that the thinner and longer CNTs gave rise to better field emission performance with the lower turn-on and threshold electric fields.
김영래,전홍준,이한성,곽정춘,황호수,공병윤,이내성,Kim, Young-Rae,Jeon, Hong-Jun,Lee, Han-Sung,Goak, Jeung-Choon,Hwang, Ho-Soo,Kong, Byung-Yun,Lee, Nae-Sung 한국전기전자재료학회 2009 전기전자재료학회논문지 Vol.22 No.4
This study investigated the growth characteristics of carbon nanotubes (CNTs) by changing a period of annealing time and a $C_{2}H_{2}/H_2$ flow ratio at temperature as low as $450^{\circ}C$ with inductively coupled plasma chemical vapor deposition. The 1-nm-thick Fe-Ni-Co alloy thin film served as a catalyst layer for the growth of CNTs, which was thermally evaporated on the 15-nm-thick Al underlayer deposited on the 50-nm-thick Ti diffusion barrier. The annealing at low temperature of $450^{\circ}C$ brought about almost no granulation of the catalyst layer, and the CNT growth was not affected by a period of annealing time. A study of changing the flow rate of $C_{2}H_{2}$ and $H_2$ showed that as the ratio of the $C_{2}H_{2}$ flow rate to the $H_2$ flow rate was lowered, the CNTs were grown to be longer With further decreasing the flow ratio, the length of CNTs reached the maximum and then became shorter. Under the optimized gas flow rates, we successfully synthesized CNTs with a uniform length over a 4-inch Si wafer at $450^{\circ}C$.