Energy Dispersive X-ray Tomography for 3D Elemental Mapping of Individual Nanoparticles

The overall goal of this experiment is to investigate the distribution of elements within silver-gold nanoparticles that are synthesized by the galvanic replacement reaction. This method can help answer key questions in the field of catalysis, such as why certain nanoparticle compositions display enhanced catalytic performance. The main advantage of this technique is the ability to probe the distribution of elements at the nanometer scale.

This allows incredibly high resolution in three dimensions. Although the method has been used here to study metallic nanoparticles, it can also be used to study other materials on the nanometer length scale, such as semiconducting quantum dots, where it can help us understand their optical properties and their performance inside the cells. To begin nanoparticle synthesis, dissolve five grams of PVP into 37.5 milliliters of ethylene glycol at room temperature.

Then add 200 milligrams of silver nitrate to this solution. Stir the solution until the silver nitrate is completely dissolved, and then heat the solution at a constant rate of one degree Celsius per minute on a hotplate to a temperature of 100 degrees Celsius. Let the reaction proceed at 100 degrees Celsius for the next one and a 1/2 hours.

Next, add 87.5 milliliters of distilled water and cool the solution back down to room temperature. Then centrifuge the solution at 8000 times G.Remove the supernatant, and redisperse the silver nanoparticles in 50 milliliters of distilled water. In a new round-bottom flask, dissolve 50 milligrams of PVP in 50 milliliters of ethylene glycol at room temperature.

Add 2.78 milliliters of the silver nanoparticle suspension, and heat the flask at 100 degrees Celsius for 10 minutes. Next, prepare 100 milliliters of 0.2 or hydrogen tetrachloroaurate trihydrate. Add 10, 20, 30, or 40 milliliter aliquots of the solution dropwise to separate preparations of the silver nanoparticle solution to obtain gold-silver nanoparticles at varying ratios.

Cool to room temperature, and wash with distilled water three times. Pipette approximately 0.02 milliliters of the nanoparticle solution directly onto a carbon TEM grid. If possible, use beryllium TEM grids to limit spurious x-rays, and a large mesh size to minimize shadowing from the grid.

After the nanoparticle solution dries, clean the TEM grids by pipetting 10 to 20 drops of 100%methanol, or ethanol, over the TEM grid while holding the grid with anti-capillary crossover tweezers. Gently remove the excess liquid after each drop using filter paper. Lastly, anneal the grid at 80 degrees Celsius under vacuum to remove or immobilize any remaining contamination.

Load the nanoparticle coated grid into a tomography holder, and then insert it into the TEM. When the microscope vacuum reaches a suitable stable value, open the column valves and ensure that the electron beam can be observed on the screen. In the scanning TEM mode, ensure that diffraction is selected, then go to the Search tab and tilt the stage to between plus or minus 15 degrees by pressing the Alpha Wobbler button.

Minimize any movement in the sample by adjusting the Z height to the eucentric height. Now move the sample so that the beam is over a hole in the sample. Ensure that the appropriate condenser aperture is correctly aligned by centering the shadow of the aperture in an underfocused image of the probe.

Wobble the intensity to ensure that the focused probe does not move to check for beam tail misalignment. Then trace the outline of the grid square by selecting show tracks in the Search tab and following the outline of the grid square while imaging. Find a representative particle within the middle portion of the grid square.

Tilt the sample to the maximal tilt range of the holder whilst imaging to ensure that the nanoparticle is not obscured by the grid bars at large holder tilts. Acquire high-angle angular dark field, and energy dispersive x-ray spectrum images at a constant acquisition time of five minutes over the full range of tilt angles using angular increments of between five to 10 degrees. First, acquire an overview high-angle angular dark field image by clicking Acquire in the Imaging pane.

Then select a mapping window around the nanoparticle by dragging the box over this image and pressing Acquire. Next, extract the characteristic x-ray counts by first opening the spectral data cubes as RAW files. Then sum the intensity of the data cube slices that correspond to the energy channels of the peaks of interest using the Slice 2D and Sum functions.

Acquire energy dispersive x-ray spectrum images in the same manner as the previous step, but using the time intervals determined from the data on the constant's acquisition time. Import the high-angle angular dark field TIFF images as an image sequence. Use them to compile a tilt series of high-angle angular dark field images into the MRC file format by using the MRC writer in the image visualization software.

Next, cross-correlate the high-angle angular dark field images to get a rough alignment of the tilt series. Save the alignment data as either a SFT file, or as the file XCORR.TXT. Subsequently, press Proceed in the Calculate Alignment Shifts window to perform cross-correlation for the entire tilt series.

Repeat the alignment until local shifts are below one pixel, being sure to save all shift files at each step as text. Next, load the acquired spectral data cubes and use the Slice 3D and Sum scripting functions in order to extract maps that are the summed slices of energy channels corresponding to the elements of interest. In the tomography reconstruction software, go to the Apply Alignment tab and select Use shifts from alignment data view to apply the high-angle angular dark field alignments to the extracted energy dispersive x-ray elemental maps.

If necessary, perform a tilt axis adjustment on the high-angle angular dark field image series and check the Use corrections from tilt axis adjustment task in the Apply Alignment tab, in order to apply the adjustment to both the dark field images and x-ray maps. Next, use a simultaneous iterative reconstruction algorithm implemented within the tomography software package to reconstruct the tomographic data set for each of the extracted energy dispersive x-ray elemental signals. In the image visualization software, select Image, go to Stacks, and select Orthogonal Views to extract the ortho slices of the separate elemental reconstructions.

Construct and visualize further ortho slices, volume, and surface renderings from the reconstructions, using the visualization software. To ensure that the scale is set correctly, load all of the elemental reconstructions and check them, as the scale is often not transferred well from REC files. Next, select the reconstruction object in the object pool and right-click, and select the Ortho Slice module to extract an ortho slice.

Right-click on the reconstruction and select Volume Rendering to extract a volume rendering. Finally, extract an isosurface by right-clicking on the reconstruction and selecting Isosurface. Three-dimensional elemental mapping performed using energy dispersive x-ray tomography revealed clear gold surface segregation within the low gold content silver-gold nanoparticles synthesized by the galvanic reaction.

However, as the gold content increases, this surface segregation switches, so that for the highest gold content, there was clear silver surface segregation. This technique has demonstrated the ability to characterize the surface composition for an individual nanoparticle. Complimentary techniques, such as x-ray photo-electron spectroscopy, can provide information on the ensemble nanoparticle population.

After watching this video, you should have a good understanding of how to use energy dispersive x-ray tomography to obtain three-dimensional chemical images of nanoparticles.