Quantitative Characterization of the Morphology of 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 AVenue, Cambridge, Massachusetts 02139 ReceiVed: December 22, 2006; In Final Form: February 22, 2007 Films of multiwall carbon nanotubes (MWCNTs) grown by thermal chemical vapor deposition were studied using small-angle X-ray scattering (SAXS). We assessed the extent of alignment of carbon nanotubes (CNTs) by examining relative SAXS intensities as a function of azimuthal angle. We also identified features in the SAXS patterns that correspond well to CNT diameters measured through high-resolution transmission electron microscopy. For the case of thick films, corresponding to CNTs with lengths on the order of a millimeter, we were able to study the morphology of the films as a function of distance from the catalyst substrate. We examined two different films in which the morphologies of CNTs range from vertically aligned to entangled and tortuous. We determined that the alignment of CNTs as well as their average diameter can vary significantly throughout the film, demonstrating the utility of SAXS for quantitative structural analysis of CNT films, indicating the potential to reveal new information about the CNT growth process, and relating variations in morphology to evolution of the catalyst and reaction conditions. Introduction Films of carbon nanotubes (CNTs), grown by chemical vapor deposition (CVD) processes, have shown promise for use in nanoelectronics, 1 energy absorbing foams, 2 superhydrophobic surfaces, 3 and power applications. 4 The bulk morphology of these films, as well as the CNT dimensions, can be tailored through careful selection of catalyst and growth conditions. 5,6 Recently, control of the size, location, and areal density of multiwalled CNTs (MWCNTs) was demonstrated through the use of a novel catalyst system based on iron oxide nanoparticles templated from poly(styrene-block-acrylic acid) micellar thin films. 7-11 A significant amount of tortuosity and CNT entangle- ment can be present despite efforts to create vertically aligned films. 9,12 This entanglement can decrease the performance in certain applications and enhance suitability in others. Therefore it is important to understand the bulk morphology of CNT films to determine the appropriate choice of applications, as well as to relate this morphology to the reaction conditions. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) have been the methods of choice to characterize the structure and arrangement of MWCNTs that make up these films. 6,9 TEM is a powerful tool that gives direct visualization of CNT dimensions and internal structure with resolution down to the angstrom scale; however, it is inherently a local measurement. To obtain average CNT diameters with TEM requires many individual measurements from multiple micrographs, making the analysis of the macroscopic films tedious. In addition it is difficult to maintain the spatial correspondence between the location of the film and the TEM sample. SEM provides a mesoscopic view of MWCNT film morphology, but subjective image analysis is required to compare order and alignment and the resolution of SEM is typically insufficient to measure CNT diameters. Scattering methods, such as small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS), have been useful in interrogating features on the order of 1-100 nm. 13-16 Though SAXS and SANS are not direct visualization techniques, they provide the Fourier transform of real space information and have been used successfully to study the orientation of single-wall CNT (SWCNT) bundles, 17,18 the effect of sonication on the dispersion of SWCNT and MWCNT bundles, 19,20 average diameters of individual CNTs, 21,22 and the deformation of CNT nanocomposites. 23 Recently, Wang et al. demonstrated the utility of SANS as a technique to study the evolution of CNT orientation in vertically aligned MWCNTs. 16 For a typical SAXS setup with a beam spot size on the order of a few hundred microns, the resulting data samples millions of CNTs and can reflect a “locally averaged” measurement of CNT morphology. Thus SAXS provides a larger length scale counterpart to electron microscopy in studying MWCNT films, while extracting information based on nanoscale characteristics. In this paper we provide additional validation of scattering techniques, specifically edge-on SAXS measurements in our case, for studying the bulk morphology of MWCNT films. Using SAXS we verify measurement of spatial variations in MWCNT orientation, which was first demonstrated by using SANS in the work of Wang et al. 16 In addition we use SAXS to measure a locally averaged spatial variation in CNT diameters within our films and we verify the average results using TEM imaging. We also use SAXS to study the morphological characteristics of two different MWCNT films to confirm earlier reports that changes in morphology from entangled bundles of CNTs to well-aligned vertical growth can occur. 6 Relatively little prior effort has been directed toward studying morphological changes through the thickness of CNT films although Eres et al. * Address correspondence to this 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. 5859 J. Phys. Chem. C 2007, 111, 5859-5865 10.1021/jp068895a CCC: $37.00 © 2007 American Chemical Society Published on Web 03/30/2007