March 1, 2016

A ‘Hole’ New World: Improving the Efficiency of Water Splitting

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Splitting water into hydrogen and oxygen atoms is a simple reaction that holds important implications towards energy and fuel needs. Photovoltaics — the process that converts solar energy into electricity — offers a feasible way to use light energy to split water.

Prof. Peng Chen, chemistry, and his team aimed to optimize this process by studying the surface of nanorods of semiconductor titanium dioxide with respect to levels of photocatalytic reactivity. Their research indicates that the variations in the structure of the surface of the nanorods lead to variable water-splitting activity.

Titanium dioxide nanorods can be used as photoanodes in a photochemical cell. Water is oxidized into oxygen on the surface of these rod-shaped crystals, according to Chen. The electrons extracted from water during oxidation is passed through an external circuit to generate a current.

Light can excite electrons in titanium dioxides, causing the electrons to jump to a higher energy state. This results in the formation of “holes” — empty states that acts like a positive charge since it is missing the usual electron — that tend to migrate to the surface of the nanorod and then react with water. Chen’s team worked to increase the amount of holes and increase the overall efficiency of this process, believing that applying a co-catalyst called cobalt-borate — a type of oxygen evolution catalyst — could prove pivotal.

Chen and his team observed that the surface of the nanoscale crystal of titanium dioxide has many different types of sites caused by the inherent characteristics of the nanoscale structure used.

“There are always structural defects and because of the shape, different facets may be exposed,” Chen said.

This begged the question of where to place oxygen evolution catalysts to yield the greatest improvement in efficiency, since certain sites are probably more efficient than others. The team investigated this by using a process called photoelectric deposition to precisely apply cobalt-borate at different sites on the crystal.

“A potential is applied to titanium dioxide, and a laser is focused at a particular location, exciting electrons and creating charged carriers that react with the precursors of OEC, generating and depositing OEC on the surface of titanium dioxide,” Chen said.

With the use of single molecule fluorescence microscopy, Chen and his team used specific probes to detect where the “hole”-induced reactions occurs on the titanium dioxide, identifying which sites are more or less active.

“The probe reacts with the hole or other hole-induced surface-oxidizing species and will generate a fluorescent molecule,” Chen said.

Sites with more hole-induced reactions will exhibit more fluorescence. Additionally, the amount of current generated from the hole-induced reactions — which is proportional to the number of electrons taken from water during oxidation — can be used as a measure of efficiency.

Chen and his team found that the areas that were already active did not yield a great improvement when the catalyst was placed there. However, there was a big improvement in efficiency when placed in the less active regions on the nanorods.

“If one wanted to have the most amount of improvement [in efficiency of oxidizing water], you would want to put the catalyst in the low active regions,” Chen said.

In terms of meeting energy and fuel needs, the current generated from the water-splitting activity can be used directly to power electronic devices. That current can also be delivered to a cathode and reduce protons to hydrogens, which can be used for fuel.

In the future, Chen hopes to further elucidate the mechanism behind the interaction between cobalt-borate and the activity of the holes on the surface of titanium dioxide. Testing and improving the efficiency of other semiconductors and oxygen evolution catalysts are also research questions that could be addressed later on.

Chen’s research on optimizing the semiconductor titanium dioxide by identifying specific regions for placement of oxygen evolution catalysts to yield the highest amount of water-splitting activity establishes important fundamental knowledge that can be used to engineer more effective photoanodes and improve current photovoltaic cells.