AN EXAFS STUDY OF PHOTOGRAPHIC DEVELOPMENT IN THERMOGRAPHIC FILMS

Transcription

1 96 AN EXAFS STUDY OF PHOTOGRAPHIC DEVELOPMENT IN THERMOGRAPHIC FILMS T. N. Blanton 1, D.R Whitcomb 2, and S.T. Misture 3 1 Eastman Kodak Company, Kodak Research Laboratories, Rochester, NY , USA 2 Eastman Kodak Company, 1 Imation Way, Oakdale, MN 55129, USA 3 Alfred University, New York State College of Ceramics, Alfred, NY 14802, USA ABSTRACT Silver K edge extended X-ray absorption fine structure (EXAFS) spectroscopy of films containing silver behenate (AgBeh) in the unprocessed, fully processed, and step-processed states has been performed. The results of the EXAFS analysis indicate that the intensity for the real-space peak for the Ag-O distance (~2.3 Å) decreases while the real-space peak for the Ag- Ag distance (~2.9 Å) grows with increasing thermal processing of the film. The changes observed in the real-space EXAFS signal indicate the growth of metallic silver at the expense of AgBeh. The X-ray absorption near-edge spectroscopy (XANES) portion of the signal shows that the absorption edge position varies stepwise, with unprocessed films and pure AgBeh having an edge location at ev, films processed from steps 1 through 10 have an absorption edge at ev, and the fully processed film has an edge location at ev. INTRODUCTION Silver behenate, (AgBeh, Figure 1) is a crystalline long-chain silver carboxylate, CH 3 (CH 2 ) 20 COOAg, utilized in commercially available photothermographic (PTM) and thermographic (TM) imaging elements [1]. Figure 1. Molecular structure of a silver behenate dimer molecule. The primary commercial applications of this technology include black and white microfilm, medical diagnostic output media (Figure 2), thermal printing and graphic arts applications. The silver metal image formation is based on the heat-induced reduction of AgBeh dispersed in a binder incorporated with toner and development chemistry. The wide acceptance of PTM and TM products is primarily the result of the elimination of wet processing steps, used in traditional film products, a result of the incorporation of the heat-sensitive development chemistry in the imaging layer. In the absence of this development chemistry, AgBeh exhibits four phase transformations when heated from room temperature to 200 ºC [2]. Further increases in temperature lead to decomposition at 226 ºC.

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3 97 Figure 2. Medical diagnostic images displayed on thermographic films. One method of evaluating the development of thermographic films is to study silver metal, Ag(0), generation as a function of thermal processing. By controlling the temperature and time of thermal processing, images with differing optical densities can be generated. If one can understand the mechanism associated with the conversion of AgBeh to Ag(0), development chemistry can be optimized to enhance development efficiency, which can result in the reduction in the amount of AgBeh required in a coated thermographic film. Tolochko and coworkers [3] used extended X-ray absorption fine structure (EXAFS) spectroscopy to refine the structure of silver stearate (CH 3 (CH 2 ) 16 COOAg). They confirmed the dimer structure of the silver carboxylate and were able to study the effects of moderate and high temperature exposure. EXAFS refers to the oscillatory variation of X-ray absorption as a function of photon energy beyond an absorption edge for a specific element [4]. The EXAFS effect is small compared to the absorption resulting from the electron ejection, hence a high intensity X-ray source such as a synchrotron is required to produce spectra strong enough to be analyzed. EXAFS experiments differ from X-ray diffraction experiments in that EXAFS is sensitive only to short-range order, meaning EXAFS can be applied to crystalline as well as amorphous solids, liquids, and gases [5]. In an absorption experiment, a sample is irradiated with an X-ray source. A deep core electron (K,L shell) is excited above the Fermi energy level. A photoelectron is generated resulting in a spherical wave that scatters from surrounding atoms. The presence of atoms around the absorber atom causes oscillations in the absorption coefficient near the absorption edge. These oscillations can be analyzed to provide information about the local environment. As an example the EXAFS spectra for silver metal is shown in Figure 3. Figure 3. EXAFS spectrum for silver metal.

4 98 The spectrum in Figure 3 consists of an absorption edge (vertical oval) that can be used for X-ray absorption near-edge spectroscopy (XANES) and the oscillation peaks (horizontal oval) used for EXAFS analysis. Analysis of an absorption spectrum begins with determination of the absorption edge energy followed by fitting a spline to approximate the background of the EXAFS region. The oscillation peaks are then extracted and converted to a wave number (Figure 4a). A Fourier transform is then performed to obtain real space data (Figure 4b). The real-space data are then analyzed for local structure information, specifically bond distances. Figure 4. (a) Wavenumber spectrum obtained from EXAFS oscillation data in Figure 3, (b) real space data obtained from the Fourier-Transform of the wavenumber spectrum in Figure 4a. Upon exposure to elevated temperature, the composition of a thermographic coating is expected to change. In this study, absorption spectra were collected on a thermally processed thermographic film. XANES and EXAFS data were collected as a function of different processing steps to assess if these techniques can be used to monitor the composition and structure characteristics of silver phases in TM films. EXPERIMENTAL Sample Preparation A 20.4 cm x 25.4 cm thermographic film (Eastman Kodak Company) comprising a 20 µm layer containing AgBeh in a Butvar binder, at a loading of 4.24 g/m 2 AgBeh (equates to an elemental Ag loading of 1.02 g/m 2 ), was coated on a 175 µm blue polyester film support. The coated imaging element was heat processed to produce a developed sensitometric sheet (12 steps) with an optical density range from D = 0.2 (D-min) [high AgBeh, low Ag(0)] to D = 3.1 (D-max) [low AgBeh, high Ag(0)]. A reference sample for Ag(0) was obtained using a thermographic film heated to 230 ºC for 5 minutes, insuring all AgBeh was converted to Ag(0). AgBeh reference spectra were collected from neat AgBeh powder and an unprocessed thermographic film. Data Collection Samples were measured at the National Synchrotron Light Source (NSLS) on a dedicated EXAFS beamline, X18 (Figure 5). Bulk AgBeh was measured as a pellet in fluorescence mode, while the films were measured as received, but stacked 10 films together to increase the signal. A 13-element, high-purity Ge detector was used to collect the fluorescence signal, providing 13 measurements of the fluorescence per monochromator scan. At least 4, and as many as 8,

5 99 monochromator scans were made for each specimen resulting in (13) x (4 to 8) independent measurements. All data were merged to provide the final data set that is presented. Figure 5. NSLS X18 beamline (a) EXAFS experimental hutch, set up for transmission mode; (b) germanium detector; (c) monochromator. Data Analysis EXAFS analysis was carried out using the programs Athena [6] and WinXAS [7]. Background fitting was carried out with the highly automated feature of Athena using a single-variable fit such that the long wavelength components of the EXAFS signal were removed from the data, thus removing any peaks in the real-space distribution below ~1 Å. The Fourier-Transforms were performed using the Kaiser-Bessel window with a sill length of 2 Å -1 over the k range of 2 12 or 2 10 Å -1, depending on the noise level. Phase shift corrections of ~0.5 Å and ~0.2 Å were applied to AgBeh and Ag(0) respectively, for determination of bond distance lengths. RESULTS The XANES portion of the absorption data is shown in Figure 6. These data demonstrate the edge position shift as a function of film processing. The edge position is a function of oxidation state and therefore might be expected to change monotonically with degree of film development if the AgBeh decomposes to Ag metal in a bulk homogeneous fashion. As shown in Figure 6, however, there are three distinct edge positions. The first, ev, is for bulk AgBeh and the unprocessed film. The second, ev, is a grouping of all films processed from step 1 through step 10, while the third, ev, is for the fully-processed film and the film processed to step 12. This result at first glance, is puzzling, and will be discussed further after describing the results from the EXAFS signal.

6 100 Figure 6. XANES silver K-edge region absorption spectra. A comparison of EXAFS real space spectra for Ag(0) and AgBeh are shown in Figure 7. For Ag(0) there are four dominant peaks at small R located at approximately 2.2, 2.7, 3.9, and 4.9 Å. The 2.7 Å peak after phase shift correction is 2.86 Å and is attributed to the Ag-Ag distance in Ag(0) with a coordination number of 12. This result is in good agreement with the crystallographic Ag-Ag distance of Å and a Ag-Ag distance of 2.89 Å reported by Bullut and coworkers using glancing angle EXAFS to study silver films deposited on glass [8]. a) b) Figure 7. EXAFS real space spectra for (a) Ag(0), (b) AgBeh. For AgBeh, there are three dominant peaks at small R located at approximately 1.8, 2.3, and 2.6 Å. With phase correction these peaks are 2.3, 2.8, and 3.1 Å respectively. These peaks can be assigned to bond distances present in an 8-membered dimer ring comprised of two silver atoms bridged by two carboxylates, consistent with results reported by Tolochko for silver stearate [3]. For example, the 2.3 Å distance is attributed to the shortest Ag-O distance. Attempts to model

7 101 the structure of the silver behenate from the EXAFS data were unsuccessful because of the complexity in constructing the scattering paths without a crystal structure. With the benchmark EXAFS data established for AgBeh and Ag(0), the observed real space peaks at 1.8 Å (2.3 Å corrected) for AgBeh and 2.7 Å (2.9 Å corrected) for Ag(0) can be monitored to assess the relative amounts of these two phases in processed TM films. In Figure 8, the 12 step processed TM film is shown with selected steps that were analyzed Figure 8. EXAFS real space spectra from selected optical density steps of a processed thermographic film. The real space spectra for the progression of the development process resulting in the conversion of AgBeh to Ag(0) (light dark) indicates that the peak for the Ag-Ag distance at 2.7 Å (uncorrected) increases in intensity while that for the Ag-O distance, 1.8 Å (uncorrected) decreases. The ratio of the Ag-Ag to Ag-O real space peak height data shown in Figure 8 are plotted as a function of exposure step number in Figure 9. In addition, Figure 9 includes an exponential growth curve fit to the data, consistent with previously reported in situ high temperature diffraction studies of AgBeh conversion to Ag(0) [9] where the nonlinear nature of the AgBeh conversion to Ag(0) has been attributed to AgBeh transformation to an intermediate phase before being reduced to Ag(0). Correlating the XANES edge shift with the EXAFS data is not straightforward because the former shows a stepwise change while the latter shows an exponential growth. Given that there is a dimer and that the Ag-Ag spacing in the 8-member ring is close to that in the metal preclude an accurate description of the effective oxidation state for Ag in the AgBeh crystals. Nonetheless, the absorption edge position does shift significantly and the stepwise transition found in the XANES signal might also indicate a two-step thermal decomposition of AgBeh to Ag(0).

8 102 Figure 9. Ratio of EXAFS real space peak heights for Ag-Ag to Ag-O as a function of exposure step number. SUMMARY Silver K-edge absorption spectroscopy analysis of thermographic film containing silver behenate has been described. The XANES signal shows three steps in the absorption-edge region representing silver behenate, silver metal, and likely an intermediate phase. The EXAFS signal was useful for tracking the development process in TM film. In addition, the EXAFS data for AgBeh are very similar to previously reported data for silver stearate, and hold promise for providing additional structural details. Modeling of the pure AgBeh EXAFS signal in an attempt to extract structural information was unable to provide enough data for complete structure elucidation, but did provide some bond length data that can be used for developing models for structure refinement that could be useful in solving the crystal structure. ACKNOWLEDGEMENTS The authors thank Syed Kahlid at NSLS for collection of the data presented in this study and Curt Wiens at Eastman Kodak Company for the imaged TM films. Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH REFERENCES [1] Blanton, T.N., Lelental, M., Barnes, C.L., Characterization of silver image formation in a silver behenate photothermographic imaging element using X-ray diffraction techniques, J. Imag. Sci. Tech., 2005, 49(4),

9 103 [2] Blanton, T.N., Zdzieszynski, S., Nicolas, M., Misture, S., An in situ high-temperature X-ray diffraction study of phase transformations in silver behenate, Adv. X-ray Anal., 2005, 48, [3] Tolochko, B.P., Chernov, S.V., Nikitenko, S.G., Whitcomb, D.R., EXAFS determination of the structure of silver stearate [Ag(O 2 C(CH 2 ) 16 CH 3 ] 2 and the effect of temperature on the silver coordination sphere, Nuc. Instr. Meth. Phys. Res., Sect. A, 1998, 405(2,3), [4] Teo, B.K., Extended X-ray absorption fine structure (EXAFS) spectroscopy: Techniques and Applications, in EXAFS Spectroscopy, edited by B.K.Teo and D.C. Joy, Plenum Press, New York, 1981, 3. [5] EXAFS Spectroscopy, [6] Athena , Bruce Ravel, Argonne National Laboratory, Argonne IL 60439, USA [7] WinXAS 3.0, Thorsten Ressler, Krabbenkamp 5D, D Reinbek b. Hamburg, Germany, [8] Bullut, A., Karabullut, K., Basaran, E., Robinson, J., A glancing incidence EXAFS study of evaporated silver films on glass, Turk. J. Phys., 2000, 24, [9] Blanton, T., Lelenatal, M., Zdzieszynski, Misture, S.T., In situ high-temperature study of silver behenate reduction to silver metal using synchrotron radiation, Adv. X-ray Anal., 2002, 45,