The ability to control size, shape, and composition of particles is essential when seeking to improve efficiency and lower costs in renewable energy applications. It is also useful for testing hypotheses regarding the spatial distribution of chemically active or photoactive sites. For example, the different dominant crystal surfaces on titanium dioxide particles each have a unique arrangement of titanium and oxygen atoms which can influence electron transport and chemical reactivity. After synthesizing different shapes of similar particles, we can then use the results of spectroscopy and electrochemistry experiments to guide us in designing the optimal particle for the desired application.
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| Here are electron microscope images of examples of morphologies for titanium dioxide particles. On the left are very flat and thin nanosheets that expose a high number of reactive sites for photocatalysis. The middle example is a micron-sized particle made to be large enough to enable imaging luminescence from specific crystal surfaces in an optical microscope. Both the left and middle particles are made by hydrothermal techniques using fluorine to increase the size of the large, flat [001] surfaces. The nanotubes on the right are made by anodizing titanium foil in an electrolyte containing fluoride ions. The lengths and diameters of the tubes are easily controlled by changing the applied potential and the nature of the electrolyte. |
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| These iron oxide particles are examples of different morphologies that I made with the goal of testing their optical and electronic properties using photoluminesce techniques. There has been considerable interest in using iron oxide nanoparticles for using solar energy to generate hydrogen fuels due to its resistance to corrosion, low cost, and low toxicity. Unfortunately, iron oxide was not amenable to study by my techniques due to very weak photoluminescence, even at low temperatures. |
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| The middle image here shows an array of hemispherical pits made of gold. They are often referred to as sphere segment void (SSV) arrays. The diagram on the left gives an overview of the process to make such arrays. In brief, plastic spheres are coated with sulfur-containing molecules and fixed to a gold substrate in a densely-packed arrangement. Then a gold-containing electrolyte is used to electrodeposit gold, filling up the space between the plastic spheres. The desired volume of gold is deposited by controlling the total number of electrons that move through the circuit. After depositing gold to the desired depth, an organic solvent is used to dissolve the plastic spheres, leaving behind voids in the gold surface. Our lab was interested in using SSV arrays for a technique known as surface-enhanced Raman spectroscopy (SERS). The abundant sharp edges on the array offer many sites for enhancing the signal of a sample molecule in this spectroscopy technique. The right image shows a particularly pretty example of a sample deposited on an SSV array. |
