Paper AS+NS-WeM2
XPS and SEM/STEM Characterization of Silver Nanoparticles Formed from the X-ray-Induced and Thermal Reduction of Silver Behenate

Wednesday, November 11, 2009, 8:20 am, Room C1

This study is the first reported use of X-ray photoelectron spectroscopy (XPS) to characterize the organometallic compound silver behenate and its X-ray-induced and thermal reduction. Silver behenate is a long-chain silver carboxylate, CH₃(CH₂)₂₀COOAg, that crystallizes as a dimer in a head-to-head configuration. Various silver compounds, including silver behenate, are used as primary components in commercially available photothermographic (PTG) and thermographic (TG) imaging elements. Individual particles of silver behenate powder typically exist as plate-like crystals with surface dimensions of 0.2-2 µm and thicknesses of 100 nm or less. Imaging in PTG and TG devices is based on the formation of silver metal nanoparticles in the 5-30 nm range (and larger aggregates) by the thermal reduction of silver behenate dispersed in a binder incorporated with toner and development chemistry. Silver behenate has also found use as a standard reference material for low-angle calibration of X-ray diffraction instruments because its crystal structure produces a unique multi-peak diffraction pattern.

In this study, the X-ray-induced reduction of silver behenate during exposure to monochromatic Al Kα X-rays in a micro-XPS instrument was investigated as well as its thermal reduction at 100 °C and 250 °C. The X-ray induced decomposition of other silver carboxylates (silver acetate, silver benzoate, and silver trifluoroacetate) was also investigated for comparison to the behavior of silver behenate. In addition, a combined high resolution scanning electron microscope/scanning transmission electron microscope (SEM/STEM) was used in this study to provide complementary morphological information to the XPS results.

Quantitative XPS analysis of silver behenate was consistent with the theoretical C:O:Ag atomic composition. However, brown discoloration of silver behenate powder begins within a few seconds of exposure to Al Kα X-rays and increases significantly with time. Noticeable changes to the XPS spectra and the observed surface composition begin to occur after about 30 minutes of X-ray exposure. Prolonged exposure to Al Kα X-rays resulted in significant changes in the C 1s, O 1s, and Ag 3d peak shapes and positions. Changes in the XPS spectra indicated that exposure to Al Kα X-rays results in the formation of silver metal particles and decomposition of the carboxylic acid potion of the molecule to hydrocarbon species. Thermal reduction of silver behenate powder produced similar changes in the XPS spectra. This study demonstrated that XPS and SEM/STEM are complementary techniques for investigating the chemical composition, morphology, and decomposition of nanomaterials.