(83r) Synthesis of Carbon Nanofibers on Silicon Substrates Using Sol-Gel Prepared Porous Alumina Layer

The main goal of one of the projects within our microreactor-research-program is to develop a microstructured multiphase microreactor, viz. a gas-liquid contactor with specially modified catalytic coatings on its microstructured internals, for studying three-phase (G-L-S) reactions. The microreactor will be fabricated using micromachining (cleanroom) technologies in silicon and vitreous materials (i.e., fused silica and/or Borofloat glass).

Currently, there is a trend to use structured catalyst supports, i.e., that the catalyst is deposited on a rigid, orderly arranged support. Catalytic support layers based on Carbon Nano Fibers (CNFs) offer a novel option to facilitate this task [1, 3]. With their inherent high surface area-to-volume ratio, CNF's provide more catalytic surface area [1, 2], thereby obtaining sufficient activity per unit volume of catalyst, while maintaining a low fluidic resistance (which is important in microfluidic devices). Additionally, diameter and length of the fibers and hence the bulk density of the CNF layer, can be manipulated to tailor porosity and overcome tortuosity problems. This also eliminates the internal diffusion limitations by preventing concentration gradients inside the CNF layer.

The attachment of CNFs to the flat surface (silicon or glass substrates) upon which they are grown is extremely important. Enhanced mechanical stability of CNFs inside microchannels in actual fluid flow conditions can directly address the critical issues like channel blocking due to loose CNFs as well as loss of catalytic activity over the period of operation.

Various approaches can be utilized to achieve improved mechanical stability of CNFs upon flat surfaces like silicon substrates. One of them is to utilize a porous ceramic layer coating to deposit the growth catalyst (e.g. nickel) necessary for growing CNFs. Figure 1 schematically illustrates the cross sectional view of microchannel filled with CNFs anchored to the channel walls using porous alumina layer.

A sol-gel dip-coating method [4, 5] was utilized to prepare alumina layer on silicon substrates using a boehmite sol. During dip-coating procedure various parameters viz. sol viscosity, dwell-time and withdrawal-speed as well as calcination temprature were monitored to obtain stable and uniform mesoporous alumina layer on silicon substrates. Prior to dip coating the silicon substrates were pretreated thermally (at 1100 ^oC for 15 h.) in stagnant air to vary the roughness via producing oxide layer (of about 600 nm) and investigate its effect on the mechanical stability of the deposited alumina layer compared to untreated smooth surface of silicon substrate. Further nickel, as a CNF growth catalyst, was deposited by impregnation on the stable alumina layer using nickel nitrate solution. Following calcination and reduction (with H₂) pretreatment, these alumina coated and nickel impregnated silicon substrates were subjected to synthesize CNF via catalytic vapor deposition (CVD) process using hydrocarbon gas (e.g. ethylene). The CNFs were grown with diameters in the range of few nanometers to 50 nm maintaining the underlying alumina layer intact. HR-SEM images shown in figure 2 illustrate different views involving porous alumina layer and grown CNFs.

It is expected that the mesoporous alumina layer deposited on silicon substrates can enhance the attachment and thus the mechanical stability of the CNFs to the walls of microchannels. This will further facilitate to realize the ultimate goal of designing smart and efficient multi-phase microreactors using CNFs as structured catalyst support.

References:

[1] M.-J. Ledoux , C. Pham-Huu, Catal. Today 102-103 (2005) 2.

[2] M. L. Toebes, F. F. Prinsloo, J. H. Bitter, A. J. van Dillen, K. P. de Jong, J. Catal. 214 (2003) 78.

[3] J. K. Chinthaginjala, K. Seshan, L. Lefferts, Ind. Eng. Chem. Res. 46 (2007) 3968.

[4] C. J. Brinker, A. J. Hurd, J. Phys. III France 4 (1994) 1231.

[5] J. ?W. Lee, C. -W. Won, B. ?S. Chun, H. Y. Sohn, J. Mater. Res. 8-12 (1993) 3151.