One-pot preparation of iron/alumina catalyst for the efficient growth of vertically-aligned carbon nanotube forests

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Highlights

  • One-pot fully wet method to prepare catalyst layers for the growth of VA-CNT arrays.

  • VA-CNT arrays with structure and quality comparable to those grown from standard PVD catalyst layers.

  • Concomitant formation of the Al-based buffer layer and Fe catalyst nanoparticles.

Abstract

The catalytic growth of vertically-aligned carbon nanotubes (VA-CNTs) forest usually requires thin catalyst films deposited by multi-step and costly physical vapor deposition techniques. Here, we demonstrate that an efficient catalyst and its supporting layer for VACNT growth can be prepared by using a simple solution of Fe(NO3)3 and Al(NO3)3 deposited on silica in a single step. This process being much simpler and cheaper than existing preparation methods, it can easily be transferred to industry for the low-cost, thin and large-area coating of catalyst for VA-CNT growth. Our study shows that aluminum hydroxides preferentially react with the SiO2 surface while iron hydroxides tend to form oxide or hydroxide nanoparticles, thus allowing preparation of an aluminum-based buffer layer with iron-based nanoparticles at its surface. Optimization of the Fe/Al ratio and salt concentrations yielded catalysts with performances similar to standard Fe/Al2O3 catalysts prepared by physical vapor deposition.

Graphical abstract

A simple and inexpensive route of large-scale elaboration of VA-CNTs.

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Introduction

During the last 25 years, carbon nanotubes (CNTs) have raised a great interest due to their unique structural and physical properties. Vertically aligned CNTs (VA-CNTs) arrays in particular have shown great potential for many applications such as field emission [1], [2], energy storage [3], [4], gas sensors [5], [6], membranes [7], [8], structural composites [9], [10] or thermal interface [11], [12]. Methods to produce tall and high-quality arrays of VA-CNTs are now well established, especially using the water-assisted catalytic chemical vapor deposition (C-CVD) process, the so-called “super-growth” [13]. Industrial manufacturing of VA-CNTs is on the rise but their production costs remain high, which strongly hinders the commercialization and wide application of VA-CNT-based materials.

The growth process of VA-CNTs typically involves a carbon feedstock decomposed at high temperature on metal catalyst nanoparticles supported on an oxide layer, such as SiO2 [14], MgO [15], or Al2O3 [13], [16], which acts as a buffer layer to prevent catalyst ripening and diffusion in the bulk of the support (usually a silicon wafer). To date, the best and standard catalyst system for growing dense and tall VA-CNTs is a thin Fe film (0.4–2 nm) supported on an Al2O3 underlayer (10–100 nm) [17]. Al2O3 is particularly efficient as its role is not limited to a simple diffusion barrier but is also believed to reduce hydrocarbon contamination of the surface in the presence of H2O [18] and to stabilize the oxidation state of iron nanoparticles (Fe2+ and Fe3+) [19], restricting iron mobility on the surface, and therefore nanoparticle sintering [20]. In most studies to date on VA-CNT growth, the buffer and catalyst layers are typically prepared by physical vapor deposition (PVD) [21]. Although PVD systems are widely used in the semiconductor industry, a less expensive and demanding process of catalyst deposition would be highly beneficial for the large-scale and continuous production of VA-CNT arrays [22]. Our work was therefore motivated by the need for a simpler and cheaper method of catalyst preparation for the large-scale industrial production of VA-CNTs. Methods of wet deposition of metal (Fe, Co, Ni,…) have already been reported on alumina or silica, using metal salt solutions or metal colloid suspensions as starting materials. The as-made catalysts showed activities comparable to those of PVD-prepared catalysts [22], [23], [24], [25], [26]. Wet-deposition methods were also developed to prepare the Al2O3 underlayer [27], [28], [29]. For example, Wang et al. developed a fully wet procedure using boehmite nanoplates (γ-AlO(OH)) deposited on a silicon chip, which were converted in a 20-nm thick Al2O3 buffer layer by annealing at 750 °C in air. After deposition of a colloidal suspension of Fe3O4 nanoparticles, they obtained a catalyst yielding millimeter-thick VA-CNT arrays [30]. However, this approach remains complex and requires several steps: i) preparation and purification of the boehmite solution, ii) deposition of the particles, iii) annealing to form an Al2O3 layer, iv) preparation and deposition of the Fe2O3 colloidal suspension. Our goal was therefore to build on the versatility of wet methods while developing a simpler and cheaper process than those previously reported.

Here, we report a simple and fully wet approach to prepare catalysts able to grow dense and tall VA-CNT arrays on oxidized silicon wafers. The main novelty is that the aluminum-based buffer layer and the catalyst nanoparticles at its surface are formed together in a single step. This method is based on the dip-coating of a single solution of a mixture of Fe(NO3)3, Al(NO3)3 and NH4OH which are widely available and low-cost precursors. Although similar approaches were already tested in previous works, they did not yield tall VA-CNT arrays (<50 µm) [31], [32], [33]. We show here that a careful optimization of the concentrations of the different species in the solution leads to a growth activity and a VA–CNT quality comparable to those obtained with typical PVD-made catalysts in the same growth conditions.

Section snippets

Materials

Fe(NO3)3·9H2O (ACS Reagent, >98 %), Al(NO3)3·9H2O (ACS Reagent, >98 %), NH4OH (5 M) were purchased from Sigma-Aldrich and used without further purification. He (99.995 %), H2 (99.9995 %) and C2H4 (99.95 %) were purchased from Linde Gas. Gas flow were controlled using Brooks GF80 mass flow controllers. Silicon wafers were thermally oxidized on both sides to reach a layer of 600 nm of SiO2. For reference, PVD Al2O3 sublayers were prepared by depositing 20 nm of Al2O3 by radio-frequency

Results

We first studied the influence of the catalyst preparation parameters on the features of the grown CNTs. Fig. 1 shows the typical procedure for the experiments conducted throughout this study. First, we investigated the activity of a solution of Fe(NO3)3 in dilute NH4OH (Fe solution). The role of NH4OH is to allow the formation of metal hydroxides able to condense into nanoparticles. This solution was dip-coated on a thermally oxidized Si wafer (Fe@SiO2) and on a silicon wafer coated with an

Discussion

To understand the effect of the addition of the Al salt, the catalyst formation mechanism was investigated using Atomic Force Microscopy (AFM), X-ray Diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS).

AFM observations of the samples after deposition of solution FeAl[1] on Si/SiO2 and drying were first performed. As shown in Fig. 5a, large nanoparticle aggregates of 10–50 nm were observed on the surface. Since the diameter of a MWCNT is strongly related to the size of its catalyst

Conclusion

In this study, a fully wet process was developed to prepare, in a single deposition step, a catalyst to grow VA-CNT arrays on standard SiO2/Si wafers. This is important from a materials engineering point of view because the process is much simpler and cheaper than existing preparation methods. We therefore expect it to be easily transferred to industry for low-cost and large-area coating of catalyst for VA-CNT growth. This catalyst, prepared from a mixture of Fe(NO3)3 and an Al(NO3)3, yields

Acknowledgements

We thank Michel Ramonda and the Near-Field Microscopy service (CTM) of the University of Montpellier for AFM analyses. HRTEM studies were conducted at the Laboratorio de Microscopias Avanzadas, Instituto de Nanociencia de Aragon, Universidad de Zaragoza, Spain.

The Government of Aragon, and the European Social Fund are gratefully acknowledged. R.A. gratefully acknowledges the project “Construyendo Europa desde Aragon” 2014-2020 (grant number E/26). R.A. gratefully acknowledges the support from

Data availability

No raw/processed data are required to reproduce these findings.

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    Present address: Grenoble Alpes University, CEA-LITEN, 17 rue des Martyrs, 38054 Grenoble Cedex 09, France.

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