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Adv Na Sci N.noicl.Nanoi=chnol 4(2013l035013(6ppi Joi;10.l088(2043^i262;W/tl.l50U
Fabrication of carbon nanostructures from polymeric precursor by using anodic aluminum oxide (AAO) nanotemplates
Xuan T^ing Hoang'-^ Dae Thanh Nguyen", Bien Che Dong- and Huu Nieu Nguyen-
' Depanmenl of Polymer Materials, Faculty of Maienals Technology. University of Technology. Vietnam National University in Ho Chi Mmh City, 268 Ly Thuong Kiel Streel, District 10. Ho Chi Mmh City.
Vieinam
- Nalional Key Lab for Polyn^ er and Composiie Matenals. Universiiy of Technology. Vieinam National University in Ho Chi Mmh City. 268 Ly Thuong Kiel Street, Distnci 10, Ho Chi Minh City. Vietnam E-mail [email protected]
Received 25 January 2013 Accepted for publication 5 June 2013 Published 25 June 2013
Online al slacks.iop.org/ANSN/4/035013 Abstract
Anodic aluminum oxide (AAO) nano-templates are u.sed in many fields of nanotechnology, particularly for use in creation of nanowires and nanotubes. In ihis research, the meihod for fabricating AAO nano-templates in two different electrolyte solutions (sulfuric acid and oxalic acid) via Iwo step anodization procedure is presented The influence of paramelers related to bolh anodization steps such as the electrolyte, solution temperature, voltage and time on the pore size, porous distance and pore density was investigated. Scanning electron microscopy iSLVI) images ofthe nano-templates also pointed out ihe effectiveness of this anodization meihod. The synthesis of carbon nanostructures from a polymeric precursor such as epoxy via fully filling Ihe nanoporous AAO templates is reported. The prepared nanowires and nanotubes have been characterized by transmission electron micro.scopy (TE.VI). Raman spectroscopy and SEM. The results show the typical morphology and properties of muliiwall carbon nanotubes and other nanostructures.
Keywords: AAO templates, muliiwall carbon nanotubes. nanowires Classification numbers: 4.00, 4 08, 5.00. 5.10. 5.14. 5 16. 5.18
1. Introduction synthesize a vanety of nanostructured maienals such as nanowires. nanorods and especially nanotubes (1-6]. The Anodic aluminum oxide (AAO) template, also well known anodic aluminum oxide fabrication lechnique is one of the as AAO membrane, exhibits Ihe morphology of hexagonally ke\ methods for the fabncation of nano channel arrays Mnsi arranged parallel pores It is a .self-organized nano.structured recent published studies [2. 4, 5. 7-11 ] ha\c used the two-sicp matenal containing a high density of uniform cylindrical method using the same type of electrolyte solution. Thus, pores lhat are aligned perpendicularly to the surface of the ihe pore size depends on the conditions of both anodization material and penetrate its entire thickness. The pores can steps: type of acid, applied voltage, temperature and easily be controlled between 5 and 400 nm in diameter nme
and several tens of micrometers in depth. Therefore. Since 1990 many scientists have succeeded in prepanng nanoporous anodic aluminum oxide has been employed to tubular nanostructures or fiber from a variely of different precursor materials lo create nanostniclures such as metal from Ihis work may be u\od under ihe terms of (jbers and CNT by this method Polymeric nano-structures
^ihcCrc.iiiv.>Co.nnions Ailnbuiion .VOIiccnc^ ^^^j^ .|^ nanorods. nano%\ires and nanotubes can be manufactured by wetting and full> liiling the nano-porcs of liMnhuiion •>! this work must mainuiin ailnbuiion to the authorlsl and thc
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2. Experimental 2.1 Materials
All of the chemical compounds, perchloric acid, ethanol, sulfuric acid, oxalic acid, epoxy DER 331 and curing agent DETA. u ere purchased from Merck. The AAO templates were fabricated from plates (30 mm x 4 0 m m x 0.3 mm) of 99.95%
pure aluminum.
2 2 Fabrication process of carbon nanostructures 2.2.1. Manufacturing of AAO nanopore templates. The fabrication process of AAO templates is described in figure I.
The aluminum plates are annealed ai 5 5 0 ' C for 180min to remove internal stresses and increase the crystallization of aluminum [5. 6. 10], The electrochemical pohshing of aluminum sheets in a mixture of perchlonde acid and ethanol (HCIO4 (70%): C2H5OH = I , 4 by volume) under 15 V dc for 10 min was performed in order to remove surface impurides.
The surf:icc structure of the alununum sheets after the
Figure 3. FE-SEM images of the aluminum surface after removin|
of the aluminum oxide layer.
electrochemical polishing is shown in figure 2. The two-step anodization follows the pretreatment process to create the nano-pores.
In this experiment, the first-step anodization was carried out using a 3 wt% sulfuric acid solurion at a voltage of 12V at 2 C in 120 min. In this step, the surfaces will be etched 10 remove the faults and heterogeneous oriented pore's struclures are formed on the surface.
The aluminum oxide layer was removed via chemical etching in a mixture of 6 wt% of phosphoric acid and 1.8 wi^
of chromic acid at 60 C [ 2 , 4 , 5, 7-11]. Subsequently, on tbe aluminum surface there are honeycomb-like structures wilh relatively uniform diameters, shapes and surface distnbution (figure 3). These structures are pore nuclei directing the pore growth in the second anodization step.
The goal of the second anodization step is to enlarge pores in depth as well as in diameter unifonnly. Thus, the electrolyte used in this step should fulfill two conditioos:
no or low heal released and fast growlh of the pott depth. Oxalic acid as electrolyte is a good choice as the anodizadon released low heat, and the relative growth rate of the pore diameter is slower than lhat of the p<ff
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Figures. Influence of different conditions in second anodization step on morphologies of AAO tempi ale surfaces (a.c.e), on pores diameter and on distances between pores centers (b.d.O-
Table I. Fabrication conditions for AAO templates First anodization step ir
Concentration Tempera lure (%) ( X ) 3 2
sulfuric acid Voltage Time
(V) (h) 12 I
Second anodization step in oxalic acid Concentration Temperature Voltage Time
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Adv NaL Sci.. Nanosci Nanotechnol. 4 (20131 035013 XTHoangciot
Figure 7. SEM images of nano structures by immersion methods itilo cpoxy/MEK solulion 5% (a) and 10% (b).
depth. The influence of second-step conditions, such as lemperalure (at 0,3 M oxalic acid, 40 V. 120min), voltage (0.3 M oxalic acid. 5 ' C , 120 min) and lime (0.3 M oxalic acid, 40 V, 5 C ) on the structure of AAO template was studied.
2.2.2. Fabrication of polymer and carbon nanostructures.
The process to form the nanostructures is illustrated in figure 4. Nanopores of the AAO templates can be filled with a precursor polymer soludon or hot-melt polymer using various physical sorption technitjues such as dipping, reverse osmosis.
Figure 8, TEM images of nano stmctures by immersion methods into epoxy/MEK solution 5% (a), 10% (b). 15% (c) and 20% «))
dripping and vacuum pumping. After solvent removal, the epoxy resin was cured to form polymeric nanotubes in iw AAO template, which was recovered by dissolving the AAU template in a strong acid. The subsequent carbonization ofthe
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(a) (b) Figure 9. SEM images of nano .structures by reverse osmosis meihods with epox>/MEK solulion 5-7^ la) and 10'7o(b).
Figure 10. TEM images of
(a) ib) Figure 11. SEM (a) and TEM (b) images of nano structures by epoxy melt filling meihods.
polymeric nanotubes at 700-IOOO°C under inert atmosphere 3 . Results a n d discussion was perf'ormed to yield CNTs or carbon nanofibers
3.1. Manufacturing of AAO nanopore templates 2 2.3. Methods of characterization. Optical microscopy was
used to characterize the surface structure of aluminum sheets. F'gure 5 shows the surface structure o f l h e AAO template Scanning electron microscopy (SEM) was used lo analyze the obtained via a two-step procedure: the first anodization in a AAO template stmcture. Transmission electron microscopy sulfuric acid soludon and the second anodization in an oxalic (TEM) and Raman spectroscopy were used to charactenze ihe acid solurion with oprimum conditions summarized in table 1 synthesized nanostructures. and the morphology oflhe pore's wall is presented in figpre 6
Adv Nat. Sci. Nanosci Nanotechnol 4(2013)035013
Wavenumf (a)
Temperature (°C) (b)
Figure 12. Raman diagram of carbon nanotubes at carbonization temperamre 1000 ^C (a) and IUIIQ values at carbonization temperature 700, 800, 900 and 1000 ''C (b).
3.2. Effect of wetting methods on properties of polymeric structures
3.2.1 Immersion the AAO template in epoxy/methylethylketon {MEK) solution. Figures 7 and 8 show the SEM and TEM images, respectively, of the polymeric structures via dipping the AAO templates in epoxy/MEK solution.
The products contained both nanotubes and fibers with a lot of defects generated during the solvent evaporation inside the tubes, however, with nanotubes as the majority.
Upon increasing the epoxy concentration, the content of defects decreased, as a result of an enhanced resin AAO template adhesion and reduced formation of voids upon solvent evaporation.
3.2.2. Reverse osmosis method. SEM (figure 9) and TEM (figure 10) images show the polymeric structures, synthesized by reverse osmosis methods The products contained both nano-tubes and fibers. Upon increasing the epoxy concentralion, the lube wall thickness increased and the content of nanofibers also increased.
3.2 3 Filling fhe nanopores by epoxy-melt. Figure 11 shows SEM and TEM images of the polymeric structures synthesized by the pore-filling method. The products comprised most of the nanotubes with relatively uniform tube wall thicknesses and the tube surface containing few defects.
From the results of using different nanopore-filhng methods, the use of a molten epoxy resin allowed a considerable formation of nanotubes with belter shape and size compared lo the other approaches. The obtained nanotubes had diameters of 120-150 nm, wilh quite uniform wall thicknesses and smooth surface.
3.3. Carbonization and graphilization of nanostructures As above-mentioned, the use of a molten epoxy was the most suitable method for nanopore-filiing of the AAO template. In these expenments, the lemperalure for carbonization of the epoxy nanotubes was studied at 700, SOO, 900 and 1 0 0 0 ' C with a heating rate of 2 C min"' in 3h.
Figure 12{a) presents the Raman spectra of the nanotubes carbonized al 1000 "C wilh characteristic D and G peaks at approximately 1350 and I 6 0 0 c m " \ providmg yic ratios of the intensities /D and IQ of these characteristic D and G peaks as a measure of the amount of disorder in the CNTs, However, il should be noted that the carbon nanombes obtained by this method have graphite layers ui turbostratic struclures which are different from the structures of carbon nanotubes fabncaled by other methods. Figure 12(b) shows the decrease of / D / / G rario values by increasing of carbonization lemperature.
4. C o n c l u s i o n s
The AAO nanotemplates were successfully fabricated. The pore diameter and density can be controlled by applying voltage, lemperature, time and electrolyte. Using these templates, nanostructures including epoxy nanotubes, carbon nanotubes and carbon fibers were obtained.
A c k n o w l e d g m e n t
The authors acknowledge the financial support fi-om the Department of Science and Technology of Ho Chi Minh City.
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