Atmospheric-Pressure STEM
fixed in place with epoxy. Te stack was dried overnight. Te assembled flow cell and the flow cell with tubing is shown in Figures 2C and 3, respectively. Te flow cell, with tubing attached to it, was then placed in
Figure 2: Assembly of the flow cell. (A) Picture of the microchips being glued together with epoxy. The chips are kept in position by a loading device with two poles. (B) SEM image showing the backside of a microchip; the opening for the silicon nitride window is visible in the middle. (C) Picture of the assembled flow cell kept in place between the metal poles of the loading device. The plastic tubing is glued in place using epoxy.
room temperature activity of Au/TiO2 toward the oxidation of CO, and the exothermic nature of CO oxidation. Tis reaction provides heat, which promotes gold migration that could be studied in the electron microscope [10]. We used a catalyst with lower activity compared to other Au/TiO2 catalysts [10], thus avoiding gold particle migration and coarsening for the purpose of testing the flow cell. Te flow cell was assembled using a locally designed
mounting device. Te device consisted of two thin poles to support the chips from the bottom and to hold them in place from the top. Tere was also a movable arm to aid in balancing the chips on the bottom pole; this arm can be moved away from the chips for the final assembly steps. Te silicon chip with the catalyst sample was placed with the
SiN side facing up on the bottom pole of the mounting device. Using tweezers, short (<2 mm) pieces of plastic tubing (Peek tubing, Upchurch Scientific) with an inner diameter of 50 µ and an outer diameter of 0.36 mm were placed on the chip to serve as spacers. Te second chip was then placed with the SiN side facing down on top of this stack, and the top pole of the mounting tool was lowered. Te chips were precisely aligned by lightly squeezing the diced edges of the chips with tweezers. Te chips were then sealed with vacuum epoxy (Torr Seal, Varian), leaving one of the long sides open (the leſt side as seen in Figures 1 and 2). Aſter two hours of drying, the gas-flow tubing (typically ~1 m long each) was inserted into the open side of the flow cell and
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a locally designed specimen rod (Figure 3). Te rod consisted of two separable shaſts, where shaſt 2 fit into shaſt 1. Te smaller O-ring in Figure 3A provided the vacuum seal between the shaſts, and the tubing fit through their hollow interior. A vacuum seal was made with epoxy at the outer end of shaſt 1 (leſt side of Figure 3A), which sealed the tubing in place. Te flow cell was then placed into a cartridge, which was placed into the tip of the rod and held in place by a spring clamp and screw (Figure 3B). Te purpose of the cartridge was to provide the ability to reshape the tip region if needed, without having to change the whole shaſt. Te contents of the rod were separated from the vacuum of the microscope by the larger O-ring shown in Figure 3A.
Methods Electron microscopy was performed in high-resolution
mode with a Hitachi HD-2000 STEM at 200 kV using a probe current of approximately 0.1 nA. First, the vertical position of the stage was adjusted by focusing on the top window of the flow cell using the secondary electron detector. Te flow cell was then imaged in transmission mode with the ADF detector. Te brightness and contrast settings were adjusted for optimal visibility of the gold nanoparticles. Images of 1280 × 960 pixels were recorded with a 10-second acquisition time. Images were first recorded with the tubing open, thus with the interior of the flow cell at atmospheric pressure. Tis experiment verified that the windows did not rupture at atmospheric pressure when exposed to the electron beam. Te tubing was then connected using valves and fittings (Upchurch Scientific) to a premade mixture of 1-percent CO, 5-percent O2, He (Air Products) stored in an Al cylinder to prevent the formation of Fe(CO)5. Gas entered the cell at slightly higher than ambient barometric pressure (743 torr, for the location
Figure 3: Specimen holder for atmospheric pressure STEM. (A) Picture of the specimen rod (Hitachi style) consisting of two shafts. Shaft 1 (left) leads the tubing to the exterior of the microscope. Shaft 2 contains the tip positioned in the center of the microscope. The flow cell is placed in the tip, and its tubing is fed through the hollow interior of shaft 2 and epoxied in place at the end of the shaft. Shaft 2 fits into shaft 1, sealed by the small O-ring. The larger O-ring provides a vacuum seal in the microscope. (B) Close-up of the tip. The flow cell is fixed in place inside a cartridge with a clamp. Pictures from [8].
www.microscopy-today.com • 2011 May
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