In Situ Dynamic Atomic-Level Investigation of a Weak Charge Transfer Lamellar Intercalation Process
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transition metal dichalcogenide intercalation compounds, relatively little is known about their intercalation/deintercalation mechanisms, especially at the atomic level. Such knowledge is essential for determining the mechanism by which intercalation occurs and for understanding the chemical reactivity of these materials. A deeper understanding of the atomic-level intercalation process may also assist in the synthesis of new materials. In this regard, we recently used DHRTEM to complete the first atomic-level study of the onset and progression of deintercalation processes, including observations of staging and individual guest and host-layer behavior, for the model neutral system HgxTiS 2 [9]. We have also used environmental-cell DHRTEM to make the first atomic-level observations of intercalation for the reaction of NH 3 with 2H-TaS 2 [10]. This redox rearrangement intercalation process involves weak guest-host charge transfer, with the formation of (NHW)y(NH 3 )y TaS Y- (y' = 0.10) and one electron per NH] being transferred to the TaS2 conduction band [5,11]. Herein, we describe our DHRTEM investigations of the onset and layer-by-layer progression of this model weak-charge-transfer intercalation process. EXPERIMENTAL PROCEDURES Stoichiometric 2H-TaS 2 was synthesized and handled under inert conditions [9]. The DHRTEM samples were prepared by crushing pristine 2H-TaS 2 crystals in a special grinder at -196 'C in an inert-atmosphere dry box. The crystallites were then dry loaded on holey carbon films supported on copper grids. All DHRTEM studies were performed in situ in an environmental cell [12] fitted in the column of a PHILLIPS 400T microscope operated at 120 kV 183 Mat. Res. Soc. Symp. Proc. Vol. 404 0 1996 Materials Research Society
with 0.34 nm pt. to pt. resolution. TV rate (30 frames/s) reaction sequences were recorded in real time using a video recording system. 99.999% NH 3 (Alphagaz) was used as the reactant. Ammonia intercalation of 2H-TaS 2 was observed for thin crystallite fragments oriented with their c-axis perpendicular to the beam. The ammonia pressure in the environmental cell was varied from 0.1 - 5.0 torr. Some crystallites were initially observed prior to introducing NH 3 into the environmental cell, while others were exposed to NH 3 for several minutes before viewing. The still images shown herein were digitized from videotape and depict the crystallites parallel to their TaS2 layers. RESULTS AND DISCUSSION General Observations DHRTEM observation of the fully intercalated and undisturbed empty guest galleries show that they generally have layer spacings of 0.91 and 0.60 nm, respectively, consistent with XPD investigations [13]. The broad and narrow light lines in each image correspond to the fully intercalated and empty galleries, respectively. The optimal pressure range for observing the onset and progression of intercalation was 2.0 - 3.5 torr. Furthermore, we found that electron beam heating stimulated the intercalation process for crystallites exposed to > 2.0 torr of NH 3 prior to obse
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