Article pubs.acs.org/JPCC
Characterization of Carbon Nanoparticles in Thin-Film Nanocomposites by Confocal Raman Microscopy E. Enríquez,* M. A. De la Rubia, A. Del Campo, F. Rubio-Marcos, and J. F. Fernández Electroceramic Department, Instituto de Cerámica y Vidrio, CSIC, Kelsen 5, 28049, Madrid, Spain S Supporting Information *
ABSTRACT: Confocal Raman spectroscopy coupled with an atomic force microscopy (AFM) study is carried out in conductive films of carbon black−silica obtained by the sol− gel process. The thin films are nanocomposites that consist of low graphitized carbon black nanoparticles (CB) forming agglomerates in a silica matrix obtained by the sol−gel process. High spatial resolution Raman images in both the surface and the depth profile are obtained by using confocal Raman microscopy in which the topographic information provided by the coupled AFM technique became relevant. The nanostructure is resolved by analyzing both the Raman intensity and the Raman shift of the carbon modes. From these results it can be concluded that increasing the sintering temperature produces microstructural changes that induce a Raman shift to higher wavenumbers (blue-shift) of the ordered phase (G peak). This blue Raman shift is higher in the regions where the CB is bound by the silica matrix as a consequence of interface interactions. The structural resolution followed by confocal Raman microscopy coupled with atomic force microscopy is an interesting experimental procedure for obtaining a detailed analysis of the nanocomposite thin-film structure.
1. INTRODUCTION
In addition to the complexity of fabricating highly conductive thin films, it must be added the difficulty of characterization of such films, as their small size and composite nature may result in siginificant limitation of adequate structural characterization: the film thicknesses are in the optical resolution limit; the composite is nanostructured; and both the carbon black and the silica matrix are amorphous. Moreover, the CB nanoparticle agglomeration state is still relevant, and the samples show a significant roughness. Therefore, these difficulties in characterizing the film structure are a current challenge to understanding nanostructured materials. Confocal Raman microscopy is a powerful nondestructive technique with high spectral and spatial resolution that is able to resolve structure−microstructure of a wide variety of materials and composites.20−22 A preliminary Raman analysis of different carbon fillers in a silica matrix was carried out in previous works.17,19 Specifically, the Raman analysis of CB−silica coatings showed a relevant shift to higher wavenumbers (blue-shift) for the graphitic Raman mode (G) (∼12 cm−1) and less so for the disordered Raman mode (D). This blue-shift was correlated with an increase of the covalent CC bonding force constant in the graphitic plane as a consequence of silica matrix densification that creates a compression effect on the CB nanoparticles and therefore in the CC bonds. In addition, by means of Fourier transform infrared attenuated total reflection (FTIR-ATR) measurements, it was demonstrated that the thermal treatment promotes the formation of Si−C and Si−O−C bonds that could contribute to the compression effect.
Carbon materials have been extensively investigated, especially graphene and carbon nanotubes, for a great variety of applications such as optical devices,1 electrodes,2 biosensors,3 as filler to reinforce materials,4,5 or conductive composites.6−8 Conductive coatings based on these carbon materials have also been further investigated. However, the conductive coatings are usually thick films composed of a polymer host with the conductive filler (graphene or carbon nanofibers) providing resistivities of 10−1−10−3 Ω m.9−12 Carbon black is a conductive filler with a wide particle size distribution (10 nm to 100 μm)13,14 which is mainly used as rubber filler or reinforcement in paints, polymers, or gums.15 Obtaining thin films with thickness
