Carbon nanotubes on nanoporous alumina: from surface mats to conformal pore filling
© Fang et al.; licensee Springer. 2014
Received: 27 May 2014
Accepted: 1 August 2014
Published: 12 August 2014
Control over nucleation and growth of multi-walled carbon nanotubes in the nanochannels of porous alumina membranes by several combinations of posttreatments, namely exposing the membrane top surface to atmospheric plasma jet and application of standard S1813 photoresist as an additional carbon precursor, is demonstrated. The nanotubes grown after plasma treatment nucleated inside the channels and did not form fibrous mats on the surface. Thus, the nanotube growth mode can be controlled by surface treatment and application of additional precursor, and complex nanotube-based structures can be produced for various applications. A plausible mechanism of nanotube nucleation and growth in the channels is proposed, based on the estimated depth of ion flux penetration into the channels.
63.22.Np Layered systems; 68. Surfaces and interfaces; Thin films and nanosystems (structure and non-electronic properties); 81.07.-b Nanoscale materials and structures: fabrication and characterization
Hybrid structures based on nanowires and nanotubes grown on solid matrices are promising materials for various applications ranging from nanoelectronics [1, 2] and biotechnology  to superhydrophobic surfaces , reinforced composite materials  and polymers . Application of the hybrid nanotube-based structures for water desalination can have alluring prospects [7, 8]. Among others, nanoporous aluminium oxide (alumina) membranes are often used as a base for such structures [9, 10]. Carbon nanotubes embedded in the nanoporous alumina membrane demonstrate promising properties , but controllability of the nanotube growth in the membrane is still a challenge. Carbon nanotubes and graphene flakes have been successfully grown using high-temperature reactions in the gas phase [12, 13]. However, this method has not been able to synthesize nanotube arrays and meshes with controlled structure and morphology. In particular, it is still a challenge to grow carbon nanotubes selectively in the channels only or on the membrane surface. Moreover, the mechanisms of nanotube nucleation and growth in channels and on the featured membrane surface are still far from being completely understood.
One possible way to enhance the controllability and outcome of the growth process and to fabricate sophisticatedly designed nanotube-based complex nanomaterials is to involve additional treatment methods, such as plasma-based processing . Atmospheric-pressure plasma jets [15, 16], microwave [17, 18], magnetron  and RF-based systems  are the common setups used for the plasma-enhanced nanofabrication. The atmospheric-pressure plasma jets and inductively coupled plasmas were particularly useful for the fabrication of one- and two-dimensional carbon-based nanostructures such as self-organized carbon connections  and graphene flakes . In the plasma- or hit gas-based growth processes, the precursors containing carbon (such as acetylene, methane, ethanol vapour or other similar gases) dissociate to molecular, atomic and ion species , then deposit onto the catalyst nanoparticles and nucleate on the catalyst surface. The further growth of carbon nanomaterials (graphene flakes, carbon nanowires or nanotubes) is sustained by the incorporation of carbon atoms via bulk and surface diffusion. The presence of ion and electron fluxes in the material flow to the substrate surface intensifies the surface-based growth processes and results in the formation of unique structures [24, 25].
In this paper, we demonstrate that by involving (i) plasma posttreatment of the nanoporous alumina membranes and (ii) additional carbon precursor (photoresist), one can control the morphology of the nanotube array grown on the membrane. Moreover, (iii) a plausible mechanism of the nanotube nucleation and growth in the channels is proposed based on the estimated depth of ion flux penetration into the channels. Our experiments show that denser arrays of nanotubes can be formed due to the plasma treatment, and importantly, the upper surface of the membrane can be kept free of nanotubes confined inside the membrane channels.
Conditions and results of experiments
Process ( T, °C)
CNT on top only
CNT on top only, curved, amorphous
Fe + S1813
CNT in channels and top
CNT in channels and top
CNT in channels and top
Fe + S1813 + Plasma
CNT in channels
CNT in channels
CNT in channels
The ready samples were then examined using field-emission scanning electron microscope (FE-SEM, type Zeiss Auriga, Carl Zeiss, Inc., Oberkochen, Germany) operated at electron beam energy of 1 to 5 keV with an InLens secondary electron detector. The structure of the nanotubes was studied by transmission electron microscopy (TEM) technique using a JOEL 2100 microscope (JEOL Ltd., Akishima-shi, Japan) operated at the electron beam energy of 200 keV. Micro-Raman spectroscopy was performed using a Renishaw inVia spectrometer (Renishaw PLC, Wotton-under-Edge, UK) with laser excitations of 514 and 633 nm and a spot size of approximately 1 μm2. Raman spectra from multiple spots were collected to perform the statistical analysis of the samples.
Results and discussion
Similar experiments on the growth of nanotubes in C2H2 atmosphere without S1813 have shown quite similar results (curved nanotubes on the alumina membrane top, no nanotubes in the membrane channels), but the TEM analysis has revealed a nearly amorphous structure. This observation is likely due to the rather low process temperature which was not sufficient for crystallization, even in the presence of Fe catalyst.
A better degree of control was obtained in Fe + S1813 + Plasma series, i.e. in growing the nanotubes on alumina plasma-treated membranes. Figure 5c,d shows SEM images of the nanotubes grown by 750°C process (C2H4 + S1813 + plasma). Importantly, the thick fibrous mat of entangled nanotubes was not found in this case, but all nanotubes look like they have been cut near the membrane surface. Moreover, the nanotube ends are not deformed, and the nanotubes are open. A similar experiment in CH4 (S1813 + Fe + plasma, at 900°C) has demonstrated a similar structure with many nanotubes protruding from the pores but not forming the mat (Figure 5e). Examination of the dissected membrane has revealed the presence of thin, straight carbon nanotubes within the channels (Figure 5f). More SEM images of the nanotubes grown on plasma-treated membranes can be found in Additional file 1: Figure S3.
It should be noted that SEM and TEM examinations reveal the open-end carbon nanotubes grown inside the channels and on the membrane top (see Figures 1, 4 and 5 in Additional file 1: Figures S2 and S3). Examination of many SEM images made at different tilt angles shows that most of the nanotubes have open ends. This important finding could be explained by the specific mechanism of the nanotube nucleation and growth on the nanoporous membranes. We believe that the surface features of the membrane surface play a major role in nanotube nucleation and sustaining the growth (a similar mechanism was described for the silicon surface with mechanically written features ). In this particular case, channel walls nucleate open nanotubes and sustain their growth with open ends. It should be also noted that the diameter of the channel-nucleated and grown nanotubes corresponds to the channel diameters (20 to 50 nm, Figure 5), whereas the diameters of the nanotubes nucleated on the membrane top can reach 70 to 80 nm (Figure 4). The number of atomic carbon layers composing the nanotube walls is also larger for the case of nanotubes nucleated on the membrane top.
Thus, the plasma posttreatment of the alumina membranes before the nanotube growth radically changes the outcomes. Indeed, nucleation of the nanotubes inside long channels becomes possible. Here, we should stress that we did not use any special catalyst applied into the channels (directly at the bottom), as it was demonstrated by other authors . In contrast, we used a rather simple technique of depositing cheap and commonly used S1813 photoresist and a thin Fe layer onto the upper surface of the membrane. Most probably, the plasma posttreatment changes the energy state of the alumina membrane and promotes deep penetration of the photoresist (which serves as a carbon precursor) into the channels. As a result, nucleation and efficient growth of carbon nanotubes in the pores become possible.
where ϵ0 is a dielectric constant, λD is the electron Debye length and kp is the constant, typically in the range between 1 and 5. The estimates using Equation 2 give the sheath thickness of the order of 10 μm to 0.1 mm, that is, much larger than the average diameter of the alumina membrane channels. This means that the ions extracted from the plasma edge will not be significantly deflected by the electric field distorted by nanosized features on the membrane surface. Hence, the ions move along straight trajectories and could penetrate deeply into the channels. As a result, one can expect that the surface of the channels will be treated by the ion flux penetrating relatively deeply under the upper surface of the membrane.
To conclude, we have demonstrated that effective control of nucleation and growth of carbon nanotubes in nanopores of alumina membranes is possible by using plasma posttreatment of the membrane and application of S1813 photoresist as an additional carbon precursor. A few options to control the growth of nanotubes inside the membrane channels or on the upper membrane surface were considered and successfully demonstrated. In particular, we have demonstrated the fabrication of multi-walled carbon nanotubes on plasma-treated membranes. The nanotubes conformally filled the membrane channels and did not form mats on the membrane top. Thus, the growth mode can be controlled, and complex structures on the basis of nanotubes can be produced for various applications. A plausible nucleation and growth mechanism was also proposed on the basis of analysis of the plasma parameters. Further experiments with different types of plasmas are warranted to reveal the potential of this method for applications of organic-inorganic nanohybrid materials for energy storage, sensing and other emerging areas.
This work was partially supported by CSIRO's OCE Science Leadership Research Program, CSIRO Sensors and Sensor Network TCP, and the Australian Research Council.
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