Electron microscopy support film preparation

Figure 4. Inhomogeneity of silica-aluminas prepared by various methods. A series of 17 commercial samples of silica-aluminas from seven different producers was submitted to microanalysis. All of them showed considerable fluctuations of composition at the scale of several tens of nanometers to several micrometers. These samples were prepared by coprecipitation or by the sol-gel method. It is not known whether some of these samples were prepared from alkoxides. Smaller but significant fluctuations at the micrometer scale were also observed for two laboratory samples prepared from alkoxides. The samples were dispersed in water with an ultrasonic vibrator. A drop of the resulting suspension was deposited on a thin carbon film supported on a standard copper grid. After drying, the samples were observed and analyzed by transmission electron microscopy (TEM) on a JEOL-JEM 100C TEMSCAN equiped with a KEVEX energy dispersive spectrometer for electron probe microanalysis (EPM A). The accelerating potential used was 100 kV.

Hardacre el al. (7 75, 174) investigated the properties, structure, and composition of cerium oxide films prepared by cerium deposition on Pt(lll), finding that the activity for CO oxidation is enhanced on Pt(lll) that is partially covered by ceria. It was suggested that new sites at the Pt-oxide interface become available for reaction. A remarkable observation is the high activity for CO oxidation when the Pt(lll) sample is fully encapsulated by ceria (Pt was undetectable by XPS and AES). It was proposed that an ultrathin, disordered ceria film becomes the active catalyst. It was also demonstrated by XPS and AES that Pt dramatically increases the reducibility of cerium oxide that is in intimate contact with Pt. This result suggests that intimate contact between the noble metal and oxide phases is indeed crucial to facile oxygen release from ceria. High-resolution electron microscopy demonstrated the presence of direct contact between ceria and noble metal for supported Pt-Rh catalysts (775). Hardacre et al. (173,174) related the catalytic activity of the ceria phase to partially reduced cerium oxide. 

The support generally used for observation of catalysts in transmission electron microscopy comprises a thin film of amorphous carbon with holes, itself supported by a metallic grid. These carriers allow the observation of several zones of a preparation, of a few hundred square micrometres, without interference. We have pointed out the need to work in transmission electron microscopy on samples with a thickness not much above a hundred nanometres. In certain cases, which include that of reforming catalysts, this criteria is naturally satisfied due to the nature of the support. Transition aluminas are effectively made up of very fine platelets that can be separated by simple grinding. The preparation is then dispersed in a liquid and a drop is collected and placed on the carrier. 

Scanning transmission electron microscopy (STEM), coupled with EDX, has been used to determine metal particle sizes. The specimens for STEM are prepared by dispersing the sample ultrasonicaUy in methanol and placing one drop of the suspension onto a Formvar film supported on a copper grid. 

Thin films of Cu, Co and Ni on Si were prepared from different aqueous electrolytes containing sulfates of the respective metals as well as some supporting electrolyte/additive. Voltammetry and current transients were used to analyze the electrochemical aspects of the deposition. The electrodeposited layers were investigated by scanning electron microscopy (SEM), Rutherford backscattering (RBS), magnetooptical Kerr effect (MOKE), X-ray diffractometry (XRD) as well as by electrical measurements. 

Depending on the nature of the sample, different techniques of observations and sample preparations are used in electron microscopy. For particles produced in solution " or from inert gas aggregation,clusters are generally collected on a microscope grid covered with an amorphous carbon film or with micro-crystals. Particles can, however, diffuse on the substrate and structural instabilities sometimes occur during the measurements. For particles supported on a soluble substrate (NaCl, KCl, MgO, etc.), a carbon layer is evaporated onto the particles and the carbon layer containing the particles is then stripped from the substrate by interfacial dissolution. The particles are observed directly in top view. The... 

Electron microscopy represents the only direct method that permits to see with our own eyes the interior of a sample with a resolution of a few nanometers. To characterize the texture of hypercrosslinked polystyrenes both transmission and scanning electron microscopy have been appfied. In the former case, ultrathin sections with a thickness of about 600 A were cut firom a sample fixed in epoxy resin, and then directly examined in transmission mode. Alternatively, two-step replicas have been prepared firom the cleavage face. To prepare the repfica the surface was first coated with a collodion film appfied from amyl acetate solution, and then with a carbon—platinum film. Finally, the collodion support was dissolved and the free carbon—platinum repfica examined under a transmission microscope. 

The structures of the composites were observed by using trsnsmission electron microscopy ITEM). The samples were prepared by dissolving the composites iri m-cresoi and placing drops on a carbon thin film which was supported by an electron microscope grid. Radii ol the dispersed parlictes were elso measured directly with TEM. 

Our films were prepared from dilute solution (1 % of polymer) in chloroform. The evaporation of the solvent was carried out at a slow rate at 40 C and the films were directly prepared on the supports used for the various tests. The films were clear, transparent and smooth. Their surface was examined by scanning electron microscopy and was found reproducible from one film to the other. 

Sectioning is one of the most widely used methods in the preparation of polymers for electron microscopy. Microtomy permits the observation of the actual structure in a bulk material which is not possible by methods such as thin film casting or surface replication. Polymers (in common with biological tissues) require care in handling, embedding in resins for support and addition of stains to enhance contrast in the TEM. 

To examine semicrystalline block copolymer morphology by electron microscopy, the specimens need to be sectioned and stained to provide sufficient imaging contrast. Though challenging, the development of new sample-preparation protocols has recently enabled the examination of individual crystals within block copolymer microdomains [Loo et al. 2000b]. Optical microscopy is a long-established technique for investigating crystalline superstructure, especially the presence and structure of spherulites. Recently, atomic force microscopy (AFM) has been applied to study the structures of thin supported films of crystallizable block copolymers. Optical and AFM measurements are nondestructive and can be conducted in or near real time so the process of crystallization can be tracked. 

Transmission electron microscopy studies of the supported catalysts were performed in a JEOL JEM 2010 operating at 200 kV equipped with EDS capabilities. The microscope is equipped with an ultrahigh resolution polar piece (point resolution 1.9 A). Specimens for the TEM studies were prepared by depositing a drop of aqueous suspension onto 300 mesh Cu grid coated with a lacey carbon film. The Pd loadings of the different catalysts were determined by ICP-AES. 

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