Nanopillar Arrays of Glassy Carbon by Anodic Aluminum Oxide

NANO LETTERS

Nanopillar Arrays of Glassy Carbon by Anodic Aluminum Oxide Nanoporous Templates

2003 Vol. 3, No. 4 439-442

Saifur Rahman† and Hong Yang*,†,‡ Department of Chemical Engineering and Laboratory for Laser Energetics, UniVersity of Rochester, Rochester, New York 14627 Received December 16, 2002; Revised Manuscript Received January 27, 2003

ABSTRACT This letter presents a method for making nanopillar arrays of glassy carbon. Anodic aluminum oxide (AAO) nanopores on aluminum were used as templates and furfuryl alcohol (FFA)-based resin was used as filling precursor. The templates and nanopillars were characterized by field emission scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and powder X-ray diffraction (PXRD). It was found that the dimensions of the nanopores and the heating scheme of carbonization are important for making pillar arrays.

This paper describes a method of making nanopillar arrays of glassy carbon using anodic alumina nanoporous membrane on aluminum as template. One of the critical issues to make such arrays is to create individually separated nanopillars by overcoming the sagging of nanorods or nanowires after the removal of anodic alumina templates. We have made nanopillar arrays of glassy carbon by designing pore dimensions of the templates and the heating scheme of carbonization. Nanometer scale materials have received broad attention in recent years because of their novel properties and potential applications.1-5 Creations of arrays of such materials have become important aspects to realize their promise in nanoelectronics, photovoltaics, and other applications.3,4,6-13 Current photolithographic techniques cannot be used for making patterns with feature size e60 nm, structures with high-aspect ratios in particular, and have limitations in the choices of materials. Various unconventional nanofabrication techniques, on the other hand, can have unique advantages.14 One such method is to use anodic aluminum oxide (AAO) nanoporous membrane as template in the fabrication of nanostructured materials. Anodic alumina is a self-ordered nanoporous membrane that consists of an array of parallel straight nanopores of almost uniform diameter and length.15-20 Using this template, one can not only make nanometer features in the size range of ∼10 nm and above but also modify the dimensions by changing the porous geometry of AAO nanopores. AAO nanoporous templates, therefore, have been extensively used in making nanometer-sized tubes, rods, * Corresponding Author: [email protected]. † Department of Chemical Engineering. ‡ Laboratory for Laser Energetics. 10.1021/nl0259479 CCC: $25.00 Published on Web 02/26/2003

© 2003 American Chemical Society

and wires of various materials with a great flexibility.5,10,20-37 In most of the studies, however, the focus has been on the synthesis of nanowires and nanorods using AAO membranes as templates. The applications of making arrays of nm-sized features using these nanoporous structures, on the other hand, have been explored to a less degree and have begun to attract attention in the fabrication of electronic, magnetic, and photonic devices.10,23,25,27,32,38-41 In this paper we describe a method of making nanopillar arrays of glassy carbon using anodic alumina membranes on aluminum as template. Glassy carbon is a class of materials that have excellent thermal, mechanical, and electronic properties.42,43 They have been explored recently as new materials in the fabrications of components for microelectromechanical systems (MEMS).44,45 A critical issue for the synthesis of these arrays using AAO templates is to avoid the sagging of nanorods or nanoplillars. We solve this problem by properly designing pore dimensions and the heating scheme of carbonization. AAO template was made using a modified procedure reported elsewhere.16-18,28,46 In a standard procedure, an aluminum substrate (99.99%, 0.1 mm thick, Alfa Aesar) was cleaned with ethanol and acetone, followed by an electrochemical polishing step in a 70% ethanol/30% perchloric acid (v/v) mixture for ∼4 min at 7 V using a direct current (dc) powder supply (Kenwood, PA 250-0.42 A). The anodization of the foil was performed in an aqueous solution of oxalic acid at different voltages ranging from 40 to 75 V. Figure 1 is a schematic illustration of the fabrication process. In an optimized preparation, freshly cleaned Al foil was anodized in an aqueous solution of oxalic acid (0.3 M) at 55 V (dc) for 1 min. After the anodization, the foil was

Figure 2. Field emission SEM images of (a) top and (b) side views of aluminum oxide nanoporous templates that have the geometry for making nanopillar arrays of glassy carbon at the relatively high aspect ratios.

Figure 1. Schematic illustration of the fabrication of nanopillar arrays of glassy carbon using anodic alumina nanoporous membrane.

sonicated in a 20% chromic acid (1.8 wt %)/80% phosphoric acid (5 wt %) (v/v) aqueous mixture for 30 min in a sonication bath (Branson 2510) to widen the openings of pores,17 an approach that is important for making nanopores in alumina. This pretreated foil was anodized further in the mentioned oxalic acid solution at 55 V (dc) for 1.5 min followed by another pore widening treatment in the mixture of chromic acid and phosphoric acid for 30 min. Filling of anodic alumina nanopores with furfuryl alcohol (FFA, 99%, Aldrich, 5 µL/cm2),43,44,47-49 was conducted at 80 °C using a programmable digital hot plate (Dataplate, PMC 730 series). An aqueous solution of zinc chloride (ZnCl2, 50 wt %) was used as heat-activated catalyst. The amount of the catalyst used was ∼10% of the weight of FFA. (Caution: This catalytic reaction is exothermic and the process should be carefully carried out.) After FFA was applied on a porous template, the temperature of the hot plate was raised to 180 °C at a rate of 2 °C/min and kept at this temperature for 1 h. FFA transformed from liquid state to solid state at this temperature. A small amount of excess FFA was always used to make the base for the nanopillar arrays. The resin-filled template was then moved into a tube furnace (Lindberg/Blue M, model TF55035A-1) located 440

inside a fume hood, and the FFA-based resin was further pyrolyzed at 550 °C for 2 h under argon flow. After the carbonization, the template was dissolved in NaOH aqueous solution (4 M). The produced glassy carbon was cleaned with distilled water and dried using a nitrogen stream. Samples were characterized using field-emission scanning electron microscopy (SEM, LEO 982), Energy-dispersive X-ray spectroscopy (EDS), and PXRD (Philips MPD diffractometer, Cu KR (1.5405 Å) source). Entrances of a freshly made AAO nanoporous membrane tend to be blocked presumably by aluminum oxide. Attempts to fill these AAO pores using FFA were largely unsuccessful. We use an aqueous mixture of chromic and phosphoric acid to partially dissolve the formed aluminum oxide at the opening of the pores using a two-step approach. Instead of anodizing an aluminum foil once for 2-3 min followed by a single step17,18 of chromic and phosphoric acid solution treatment, we first anodized the aluminum for one minute, submerged the template in an aqueous mixture of chromic and phosphoric acid for 30 min, followed by a second anodization of ∼1.5 min and a similar second treatment in the mentioned acid solution. Using this two-step process, nanopores with wide openings can be readily obtained. The entrances of pores appeared to be enlarged after this treatment, a feature that favors the filling of nanopores with FFA-based resin. Figure 2 shows top and side views of a representative AAO nanoporous membrane that was designed for making nanopillar arrays from FFA-based resin with a final carbonization Nano Lett., Vol. 3, No. 4, 2003

Figure 3. Heating profile used for the conversion of furfuryl alcohol to glassy carbon.

temperature of 550 °C. This pyrolysis temperature was chosen based on the following facts and experimental observations. First, at temperatures g ∼600 °C, the FFAbased resin/glassy carbon started peeling off from the alumina template, most likely due to the difference of thermal expansion between FFA-based resin/glassy carbon and the AAO nanoporous membrane.44 Second, the AAO nanoporous templates with an average depth of less than several microns could have a large amount of unreacted pure aluminum sandwiched between two AAO layers that were filled with glassy carbon. Aluminum has a melting point of 660 °C, and the melting of attached aluminum substrate leads to distortion or even destruction of the template and causes substantial cracks in the glassy carbon monolith. To create an intact nanopillar monolith of glassy carbon, curing of the polymeric resin was done under controlled conditions. We noticed that heating and curing at various temperature segments can help to maintain the structural integrity of nanopillar arrays. Figure 3 is an optimized heat scheme for making ∼50 nm pillar arrays. Instead of increasing the temperature from room temperature to 550 °C directly, we used three intermediate curing temperatures at 250, 350, and 450 °C for 90 min each. The latter carbonization strategy resulted in slightly higher weight loss than that with no intermediate curing points. This slow and staged heating method allowed the pyrolysis to move toward to equilibrium at each stationary temperature and minimize the creation of voids in the glassy carbon due to the loss of volatile lowmolecular weight species.44 We were able to fabricate nanopillar arrays using nanoporous templates through the above controlled processes. Figure 4 shows SEM images and an EDS diagram of nanopillar arrays of glassy carbon made under the above optimized synthetic conditions. Figure 4a is a top view of the nanopillars of glassy carbon. The pillars are almost normal to the glassy carbon base and the average diameter of the pillars was ∼50 nm. Figure 4b is a SEM micrograph of the arrays taken with the specimen stage tilted at ∼30° with respect to imaging electronic beam. These nanopillars are essentially separate from one other. A region where pillars broke off from the glassy carbon base is shown in Figure 4c. The rough surfaces were mostly due to the nanometersized gold nanoparticles deposited to avoid the charging of the surface and enhance the image contrast in SEM study. The pillars are 50-60 nm in diameter and appear to be Nano Lett., Vol. 3, No. 4, 2003

Figure 4. Field emission SEM images of (a) top and (b) side views of a monolith of nanopillar arrays, (c) the individual pillars of glassy carbon, and (d) an EDS spectrum of the nanopillar carbon monolith. The surface roughness observed in (c) was gold nanoparticles that were thermally evaporated onto the exposed surfaces and used to enhance the contrast of the images.

hollow, Figure 4c. Solid nanopillars were also observed in some cases. The aspect ratio of the pillars shown is about 10, which is close to the maximum that we have made so far for the arrays. Length gradient of the nanopillar could exist in the same glassy carbon monolith. This gradient was originated from the aluminum oxide nanoporous templates. AAO nanopores generated in regions close to the air-water interface were among the deepest. We noted that the higher the aspect ratio the nanopillars had, the more difficult they were to remain separate after removal of AAO templates. Thin and long nanorods of glassy carbon tend to form bundles after the removal of aluminum oxide templates (see Supporting Information, Figure S1). Chemical composition of these materials was qualitatively characterized using EDS. Figure 4d shows the EDS trace of carbon pillars. Carbon (K line, 0.27 keV), oxygen (K line, 0.52 keV), and zinc (L line, 1.04 keV) were the only detectable elements. The oxygen could be from the oxygen-containing intermediates and surface absorbed species, and zinc was from the ZnCl2 catalyst. We could not quantify the exact chemical compositions of the pillars based on the EDS data due to the highly nonplanar nature of the nanopillar arrays. X-ray diffraction was used to examine the crystallinity of the materials, and no detected signal in the range between 10° and 90° 2θ from the produced carbon structures could be observed, which indicated that the carbon monolith was amorphous. In summary, we have demonstrated a method of making nanopillar arrays of glassy carbon using an AAO membrane. Although we have made the aspect ratio of the glassy carbon nanopillars up to about 10, we believe that it is possible to create arrays of nanopillars with higher aspect ratios using different precursors or other configurations of nanoporous membranes. The nanopillar arrays of glassy carbon could potentially be useful as supporting and functional nanostructures that are not easily fabricated using conventional photolithographic methods. We are currently exploring the applications of these nanopillar arrays in photonic applications. 441

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