Fabrication of Uniform Au–Carbon Nanofiber by Two-Step Low Temperature Decomposition
This paper presents a facile and efficient way to prepare carbon nanofibers ornamented with Au nanoparticles (Au/CNFs). Gold nanoparticles were first deposited in the channels of an anodized aluminum oxide (AAO) membrane by thermal decomposition of HAuCl4and then carbon nanofibers were produced in the same channels loaded with the Au nanoparticles by decomposition of sucrose at 230 °C. An electron microscopy study revealed that the carbon nanofibers, ~10 nm thick and 6 μm long, were decorated with Au nanoparticles with a diameter of 10 nm. This synthetic route can produce uniform Au nanoparticles on CNF surfaces without using any additional chemicals to modify the AAO channels or the CNF surfaces.
KeywordsAu Carbon Nanoparticles Nanofibers AAO
Carbon nanostructures have attracted extensive attentions due to their unique physicochemical properties and promising applications in nanodevices [1, 2]. One of the major challenges for nanodevice fabrication is how to immobilize the functional nanostructures on electrically conducting electrodes. Amma et al. demonstrated that extremely hydrophilic carbon nanotubes (CNTs) could be immobilized on a hydrophilic Au surface by means of electrostatic interaction. However, this technique requires harsh chemical reactions (e.g. azo-coupling) to modify the CNT surface with a hydrophilic group (e.g., –SO3Na). The harsh reaction conditions and the bonded chemical groups may cause some changes of the intrinsic nature of the carbon nanotubes.
Therefore, an alternative immobilization method under relatively mild reaction conditions is desirable to assemble the carbon nanodevices without sacrificing the unique properties of the carbon nanotube. One of the most promising immobilization methods would be to attach Au nanoparticles on the carbon nanostructure as anchoring posts. The Au nanoparticles offer the great potential in nanodevice assembly because the Au surface can be easily modified to interact with a substrate through a mild chemical treatment [4, 5]. There are some known methods to attach Au nanoparticles to carbon nanostructures [6–8]. For example, Au nanoparticles can be attached to CNTs by chemical reductions of HAuCl4 either in a solution of CNT stabilized by surfactant  or surface-modified CNTs . However, the intrinsic surface properties of CNTs could not be sustained due to the surface modification chemicals [6, 7]. The Au/CNTs can also be prepared by a direct pyrolysis of HAuCl4/acetone mixture in a template . This may be the simplest method to prepare the Au-decorated 1D carbon nanostructures [1, 2, 9–14]. But this method could be applied only to get large the diameter (> ~180 nm) of carbon tubes .
We investigated a facile route to prepare the Au nanoparticle loaded carbon nanostructures by a two-step thermal decomposition method. This method consists of the thermal decomposition of HAuCl4 followed by the carbonization of sucrose in anodized aluminum oxide (AAO) channel, 80 nm in diameter. In the previous paper, we have shown that monodisperse Au-nanoparticles with a diameter of ~10 nm can be produced by thermal decomposition of HAuCl4 solution infiltrated in the AAO channel . This synthesis route produces carbon nanofibers ornamented with uniform sizes Au nanoparticles. Since the Au nanoparticles are synthesized first and the sucrose carbonization is carried out at the same temperature used for the Au nanoparticle synthesis, the property of Au nanoparticles will not be disrupted during the carbon nanofiber growth. Once the carbon fibers are produced, the Au/carbon nanostructure can be released from the AAO channel under mild chemical etching conditions. Therefore, the intrinsic properties of the complexed nanostructures will not be altered greatly.
The AAO membrane was prepared by the Masuda process  and the diameter of AAO channels was controlled to ca. 80 nm. The uniform Au nanoparticles (~10 nm) were embedded inside the AAO channels by a thermal decomposition of HAuCl4 at 230 °C. The detailed procedure was described elsewhere . Then the AAO membranes loaded with Au nanoparticles (Au–AAO) were soaked in a sucrose (Aldrich ACS grade) aqueous solution for 24 h followed by heating to 230 °C for 30 min under the nitrogen (99.9%) flow to carbonize the sucrose. The heating ramping rate was 4 °C/min. The AAO frame was removed in an etch solution (6% H2CrO4:1.8% H3PO4 = 1:1 in volume) and rinsed several times with deionized water. The carbon nanostructures decorated with Au nanoparticles (Au/C) were retrieved from the solution. For comparison, bare carbon nanostructures without Au nanoparticles were synthesized via the same carbonization procedure inside the pristine AAO channels. The samples were characterized with field-emission scanning electron microscopy (FE-SEM, JEOL JSM6700-F), transmission electron microscopy, energy dispersive X-ray spectroscopy and selected area electron diffraction (TEM/HRTEM/EDX/SAED, JEOL JEM-2010), powder X-ray diffractometry (XRD, Philips X’Pert MPD system, Cu Kα radiation), and X-ray photoelectron spectroscopy (XPS, VG-Multilab 2000).
Results and Discussion
Although the thermal decomposition process of sucrose includes complicated multiple steps,  it can be simplified as C12(H2O)11 → 12C(s) + 11H2O(g). If this reaction occurs in the AAO channels, the water vapor will be evolved and form bubbles. The bubble formed inside a channel, when escapes, pushes outward the sucrose phase at the outer region of the channel. At the same time, due to the strong adhesion between the aluminol groups of AAO and the hydroxyl groups of sucrose,  a thin layer of sucrose might remain on the wall of AAO channel. Finally, the decomposition of the coated sucrose on as-prepared AAO channel will result in the nanotubes as shown in Fig. 3.
In conclusion, we demonstrated a facile route for the synthesis of carbon nanotubes and dimension-confined (10 × 10 nm) nanofibers decorated with size-defined (10 nm) Au nanoparticles by the carbonization of sucrose in the AAO channels which were coated with Au nanoparticles without any chemical additives or further chemical reactions. The attached Au nanoparticles could be used as good anchoring posts to assemble carbon nanostructures on proper substrates.
This work was supported by Pukyong National University Research Abroad Fund in 2007(PS-2007-010). D.K. is grateful to Prof. S.H. Kim for his stay and research at Penn State.