Characterizing the Morphologies of Mechanically Manipulated Multiwall Carbon Nanotube Films by Small-Angle X-ray Scattering † Benjamin N. Wang, ‡ Ryan D. Bennett, ‡ Eric Verploegen, § Anastasios J. Hart, | and Robert E. Cohen* ,‡ Departments of Chemical Engineering, Materials Science and Engineering, and Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts AVe, Cambridge, Massachusetts 02139 ReceiVed: March 5, 2007; In Final Form: June 5, 2007 We used small-angle X-ray scattering (SAXS) to quantitatively characterize the morphological characteristics of pristine and mechanically manipulated multiwall carbon nanotube (MWCNT) films. We determined that CNT diameters measured near the edges of a film were smaller compared to those measured in the interior. Uniaxially compressed MWCNT films exhibited a buckling deformation that was observable both in scanning electron microscopy (SEM) and SAXS. CNT films were also converted into cellular foams of CNTs through capillarity-induced densification. By examining spatially- and time-resolved SAXS data for the cellular foams, we identified low angle features in the scattering curves that correspond to the average spacing between CNTs, demonstrating that SAXS is a useful method for monitoring the packing density of CNTs in a film. For all of the morphologies that were examined (aligned, disordered, compressed, and densified), SAXS data showed good correspondence with SEM images. Introduction Carbon nanotube (CNT) films have attracted significant interest from the engineering community because of their remarkable thermal, electrical, and mechanical properties 1-4 and the ease with which their morphologies can be macroscopically tailored. 5-11 Several groups have shown that by controlling reactor growth conditions CNT films can be grown into complex structures. 12-15 Hart et al. have shown that alignment in films of multiwall CNTs (MWCNTs) can be controlled by applying mechanical pressure during growth. 3,12 Bennett et al., employing a novel iron oxide nanoparticle based catalyst, showed that varying the density of catalyst particles, and corresponding CNT density, can give rise to a wide range of morphologies. 5,7,16 Templating techniques such as microcontact printing have been employed in fabricating hierarchical CNT structures. 6,9,17 Chakra- pani et al. converted MWCNT films into cellular foams through surface tension driven collapse of the film. 2 Kaur et al. have extended the work on cellular foams through the use of colloidal silica particles making “buckled” CNT films, 18 and Salguerin ˜o- Maceira et al. have demonstrated visually appealing fractal- like structures based on similar particle templating. All of these studies relied primarily on electron microscopy for characteriza- tion of the morphology of CNT films. In a previous paper, we showed that small-angle X-ray scattering (SAXS) is useful in quantitatively studying the morphology of MWCNT films. 19 SAXS provides morphological insight into both nanoscopic properties such as average CNT diameters and ensemble properties such as CNT orientation, reflecting a “locally averaged” measurement. This arises from the mesoscopic size of the X-ray beam which is typically on the order of a few hundred microns. This dimension is simultaneously small in relation to the typical millimeter scale of films and large relative to the nanometer scale of individual CNTs. Thus the SAXS beam can probe a small region of the film to yield data that reflect information about millions of CNTs. The mesoscopic size scale of scattering techniques has been used successfully to demonstrate that the average diameter and orientation of MWCNTs can change as a function of distance from the film substrate. 19,20 In this paper, we use SAXS to study the morphologies of mechanically manipulated MWCNT films. We begin by char- acterizing a real variation in CNT diameters of pristine MWCNT films. We then examine the morphology of films that have been subjected to uniaxial mechanical compression. Finally, we investigate the morphology of cellular foams made from capillarity-induced densification of MWCNT films and dem- onstrate that CNT packing density can be estimated from the SAXS data. Experimental Methods Small-Angle X-ray Scattering (SAXS). Small-angle X-ray studies were performed at the G1 beamline station at the Cornell High-Energy Synchrotron Source (CHESS). The wavelength of the X-rays was 0.1239 nm, and the sample to detector distance was 1123 mm, calibrated with silver behenate (first-order scattering vector of q of 1.076 nm -1 (with q ) 4π sin(θ/λ) where 2θ is the scattering angle and λ is the wavelength). Slit collimation is used to achieve a resulting beam spot that is approximately 0.1 mm in height and 0.5 mm in width (the y and x axes, respectively). A slow-scan CCD-based X-ray detector, home-built by Drs. M.W. Tate and S.M. Gruner of the Cornell University Physics Department, was used for data collection. Additional studies were performed at the X27C beamline at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory (BNL). The wavelength of the † Part of the special issue “Richard E. Smalley Memorial Issue”. * Corresponding author. E-mail: recohen@mit.edu. Phone: 617-253- 3777. Fax: 617-258-8224. ‡ Department of Chemical Engineering. § Department of Materials Science and Engineering. | Department of Mechanical Engineering. 17933 J. Phys. Chem. C 2007, 111, 17933-17940 10.1021/jp071798c CCC: $37.00 © 2007 American Chemical Society Published on Web 08/02/2007