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J. Phys. Chem. B 2001, 105, 11796-11799
Elastic Behavior of Nanoparticle Chain Aggregates (NCA): Effects of Substrate on NCA Stretching and First Observations by a High-Speed Camera† Y. J. Suh, M. Ullmann, and S. K. Friedlander* Department of Chemical Engineering, UniVersity of California, Los Angeles, California 90095
K. Y. Park Department of Chemical Engineering, Kongju National UniVersity, Kongju, Korea ReceiVed: May 8, 2001; In Final Form: September 10, 2001
Chain aggregates of ∼7 nm titania particles exhibit elastic behavior when observed in the transmission electron microscope (TEM);1,2 under tension produced by an expanding hole in the film, these nanoparticle chain aggregates (NCAs) stretch, and when the tension is relaxed, they contract. The film that coats the TEM grid on which the NCAs are deposited has an amorphous carbon side and a Formvar side. When the NCAs are deposited on the carbon side, the strained NCAs detach easily from the carbon layer and stretching does not occur. On the other hand, NCAs deposited on the Formvar side exhibit stretching and contraction probably because of the adhesion of the Formvar (poly(vinyl formal), mol wt 70 000-150 000) molecules to the ends of the particle chains. A similar phenomenon may explain the reinforcement of rubbery polymers by particulate fillers. Earlier observations of NCA behavior were made by using a conventional video camera with a speed of 30 frames/s (FPS). This was fast enough to follow the stretching but not the contraction process. An attempt was made to follow contraction using a high-speed camera (9000 FPS). The titania NCA stretched by about 10% and then contracted to 78% of the initial length. Inspection of the aggregate shape showed that chain segments folded after contraction. The length of the NCA oscillated in the direction of motion during stretching (∼2 kHz) and contraction (∼1 kHz).
Introduction Nanoparticles generated by high-temperature gas phase (aerosol) processes are usually produced in the form of chain aggregates that result from collisions between smaller aggregates in Brownian motion.3 The primary (individual) particles in the chain aggregate are held together by bonds whose strength may range from van der Waals attractions to stronger ones such as ionic or covalent bonds. Previous studies of the behavior of nanoparticle consolidates have emphasized the importance of individual particle size. The interparticle forces and transient aspects, such as aggregate restructuring, have received less attention.4 In recent studies, our group has reported the discovery of a remarkable elastomeric behavior of chain aggregates of metal oxide nanoparticles through observations made by TEM.1,2 The TEM grids are covered with a film composed of a bilayer of carbon and Formvar, which serves as a substrate for the sample. The stretching and contraction of titania NCA are shown in Figure 1. The initial morphology of the NCA is the result of stochastic processes associated with the Brownian motion, the size of the primary particles and the temperature-time history of the aggregate (Figure 1a). Exposure of the TEM grid to the electron beam causes thinning of the film followed by the formation of one or more holes near the NCA (Figure 1b). As the hole enlarges, the stretched NCA resists tension (Figure 1c). When the particle-to-substrate attachment cannot withstand the tension produced by the receding film, the attachment breaks. †
Part of the special issue “Howard Reiss Festschrift”. * Corresponding author. E-mail:
[email protected]. Tel: 310-825-2206. Fax: 310-206-4107.
When the tension is relaxed, the NCA assumes a more compact form (Figure 1d), probably due to van der Waals attractions between the particles. This phenomenon may have important practical implications: nanoparticle carbon black and silica are used in large quantities as reinforcing fillers (additives) in the manufacturing of rubber.5 Both carbon black and silica are composed of NCAs made by high-temperature gas phase processes. The reinforcing fillers increase the elastic modulus, while the rubberlike properties of the base material are retained. In addition, they significantly increase the tensile strength of low-strength rubbers. The mechanism by which the fillers affect the mechanical properties of elastomers is not well understood.6 Our studies provide direct experimental support for a reinforcing mechanism based on the energetics of the stretching of NCAs.7 In our previous studies, NCA stretching was produced by the movement of the edges of the holes in the film on the TEM grid. The holes resulted from weakening of the film by the electron beam. The time scale of the stretching process could be followed easily by the observer and recorded by videotape using a conventional video camera with a speed of 30 frames/s (FPS). When the NCA had reached its maximum extension (up to 100%), it broke loose from the edge of the expanding hole and contracted. The time scale of the contraction process was much shorter than that of the stretching, less than 4 ms.8 This was too fast to be observed by eye or to be captured by the camera used to study stretching. A goal of this study was to obtain a better understanding of the contraction process. For this purpose, a high-speed video camera with a speed of 9000 FPS was used. We also report
10.1021/jp011744h CCC: $20.00 © 2001 American Chemical Society Published on Web 10/27/2001
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Figure 1. Elastic behavior of titania NCA. (a) Initial shape of NCA on an ultrathin carbon film. (b) NCA beginning to stretch as a hole developed in the carbon film after 7.5 min. (c) NCA stretched by 36% after 10 min. (d) Contracted NCA after the anchor point to the film (bottom right) broke at 11.3 min.2
results on the effects of the nature of the film covering the TEM grid on NCA stretching. Experimental Procedure Chain aggregates of titania nanoparticles were produced by the thermal decomposition of titanium tetraisopropoxide under a N2 atmosphere in a small tubular reactor.9 The nitrogen flow rate was controlled to give a space time of approximately 2 s in the reaction tube, in which the temperature was maintained at 800 °C. The titania NCA were deposited (probably by diffusion) from the gas flowing at a velocity of less than 1 m/s around a TEM grid (Ted Pella Inc., Carbon Type-A, 300 mesh, nickel, Model 01820N). The nickel grid was coated with a film 45-85 nm thick, composed of an upper layer of Formvar 3060 nm thick and a lower layer of amorphous carbon 15-25 nm thick (Figure 2). NCAs can be deposited on either the carbon or the Formvar side of the film. The behavior of the NCA deposited on the grid was observed using a TEM (JEOL, Model JEM-2000 FX) and a high-speed video camera (ROPER SCIENTIFIC, HS Motion Analyzer, Model 4540 mx). The TEM was equipped with a LaB6 filament to increase the electron beam intensity. The operating conditions of the TEM were 100 kV accelerating voltage, 130 000 times magnification, 0.8 × 10-4 Pa column pressure, and 57 µA beam current. The camera, which was placed in front of the viewing window of the TEM, was focused on the fluorescent screen at the center of the NCA. The NCA movement was recorded by the video at 9000 FPS. From these images, it was possible to analyze NCA displacement with a time resolution up to 0.11 ms. When the high-speed camera recorded images through the viewing window of the TEM with a recording rate over 2000 FPS, the lack of light resulted in unrecognizable images. An
Figure 2. Experimental setup for the observation of NCA contraction using a high-speed video camera (not to scale).
image intensifier (IMMCO, Image Intensifier, Model ILS-3) was combined with the high-speed camera to produce recognizable images. During observation of NCA movement, the image intensifier was adjusted and the recorded images in the camera system were continuously checked with a monitor. After a hole developed, we were ready to push the recording-control button of the camera at the moment of contraction. Recording of the images in the memory of the camera system was stopped just after contraction of the NCA and these images were transferred to the computer. The recording time was 1.82 s at 9000 FPS. The memory was enough to record the whole contraction process, because the contraction had finished within 4 ms according to Ogawa.8 The image on the fluorescent screen was projected on the camera lens at an angle, because the axis of the camera did not coincide with that of the fluorescent screen in the TEM. The
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Figure 3. NCA images during stretching and contraction. (a) A hole developed near the NCA (just before stretching). (b) Both ends of the NCA were still attached to the substrate when the NCA was stretched to 110%. (c) The upper end of the NCA was detached from the substrate and the NCA contracted to 78% at the end of contraction. (d)-(f) are drawings of the NCA for (a)-(c), respectively.
angle between the camera lens and fluorescent screen was about 48°; at an angle of 0°, two plane axes coincided. Thus, the recorded images require a transformation to correct this discrepancy. Negatives taken by the TEM to analyze morphological features in detail were used to compare with the images from the high-speed camera. Comparison of the negatives and still images provides an estimate of the angle between the camera and fluorescent screen and the extent of image reduction by the camera. The angle and the reduction rate were used to correct the aspect ratio and magnification, respectively. Thus, the length of the NCA could be more accurately estimated from the highspeed camera images. Results and Discussion Effect of Substrate on NCA Stretching. To study the effect of the TEM coating on stretching and contraction, NCAs were deposited (in separate studies) on the carbon and on the Formvar sides of the film. When NCAs about 0.5 µm long were deposited on either the carbon or Formvar layer, they formed loops with about two or three points of attachment. The NCA on the carbon layer moved back and forth in the electron beam before falling over and becoming attached to the layer. When a hole developed
in the film, the strained NCA detached easily from the carbon layer. As a result, stretching was rarely observed for NCA on the carbon layer. The behavior of NCA deposited on the Formvar side was similar to the NCA on amorphous carbon up to the point when the hole developed. Then the NCA remained attached to the receding Formvar film and stretched up to the point that one end of the NCA broke loose from the film. The adhesion of the Formvar [poly(vinyl formal)] molecules (70 000150 000 mol wt) to the titania chain may result from hydrogen bonding between Formvar CHO groups and the titania particles. Morphological Changes of NCA. A titania chain aggregate composed of about 300 primary particles, with a number averaged diameter of 8.3 nm, was observed under the electron beam. This titania aggregate was about 0.43 µm long and was mostly composed of anatase according to the selected area diffraction pattern observed with the TEM. After 23 min of electron beam irradiation, a hole developed in the film near the NCA. As the hole widened, the NCA started to stretch about 3 min after hole development and then suddenly contracted. NCA images during stretching and contraction are shown in Figure 3. After the original images from the high-speed camera were transformed to compensate for the difference due to the
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TABLE 1: Comparison of the Time Scales of Stretching and Contraction of Titania NCA
recording speed of camera (frames/s) primary particle diameter, d0 (nm) length of NCA (nm) % deformation time scale a
stretching contraction stretching contraction
Ogawa et al.2
present study
30 5.9 100 74 -68a ∼2 min
