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Of Diatoms, Titanium Dioxide and A Bio-based Way to Increase Solar Power Efficiencies

Prolog:

It has been a while since last I discussed a scientific journal paper. The following post is based on a paper published in the American Chemical Society NANO Journal (Vol.2, No. 10, pp 2103-2112) with the title “Metabolic Insertion of Nanostructured TiO2 into the Patterned Biosilica of the Diatom Pinnulario sp. by a Two-Stage Bioreactor Culvtivation Process”.

What have they done?

The Rorrer lab is a nanotechnology/biotechnology engineering lab. They manipulate various microorganisms so that the microorganisms produce substances that are of use to human kind. Their latest series of publications has focused on diatoms which are single-celled algae that are able to make silica shells with very intricate patterns (see below). The Rorrer lab has a significant body of work inducing a particular diatom by the name of Pinnulario sp. to incorporate optoelectronic substances into its frustules (outer shells) that it normally would not. Optoelectronic substances can transform, transmit or sense different forms of light.

Figure 1. Microscopy Image of Pinnulario sp. Go to London Natural History Museum Website 
to read more about this organism. It's where this image is from.


In their latest work, Clayton Jeffryes et. al. have found conditions to cultivate these diatoms in such a way so that they incorporate titanium dioxide into the silica shell of the diatoms – something that these diatoms don’t do on their own. They did so by simply changing the media conditions. Initially, algal cells were starved for silicon dioxide. After starvation limited amounts of silicon dioxide mixed with titanium dioxide is fed back into the system. Under these “starved conditions” diatoms incorporate titanium dioxide into the frustules in addition to the normal silicon dioxide. When these diatoms are fixed onto glass slides and then gently washed with a detergent mixture (SDS/EDTA), the life cells are washed out of the silica shells, and the silicon/titanium dioxide skeleton is left behind.

Why is this significant?

The price per watt for solar panels is related to the cost for materials, manufacture, installation and maintenance and the amount of electricity that can be generated to payback the upfront cost. To increase distribution, the life-time cost for solar panels must come down. In different approaches, this could be achieved by reducing the use of the raw material through methods such as thin-film technology, and various light concentration technologies. Alternatively, the efficiencies of converting sunlight to electricity could be increased.

Titanium dioxide is known to be able to “trap” light particles called photons. The process of trapping these photons can dislodge additional electrons. In this way, small amounts of titanium dioxide can enhance photovoltaic properties of solar panels. The light trapping properties of titanium dioxide can be improved if titanium dioxide molecules are arranged in a certain periodic manner. However, this has been very difficult thus far. Using modified enzymes to precipitate titanium oxide out of solution has not thus far been successful in creating ordered titanium oxide structures. As an alternative, diatom frustules have been used as templates for “spraying on” titanium dioxide which requires much heat, and potentially harmful chemicals (titanium tetrachloride). This work by the Rorrer lab is significant because it describes a highly scalable, bio-based way of producing large of amounts of titanium dioxide that is arranged in a three-dimensionally ordered way.

Were this technology to be commercialized, at this time, it would still add too much cost to justify the increase. But one day, combining different technologies such as thin-film, light concentration with titanium dioxide doped bio-silica could make solar panels much more efficient than the mere ~ 20% current top-of-the line solar panel technology has to offer. The increased output could help to bring the cost of energy created per watt down sufficiently and make the dream of mostly relying on alternative energy sources come true.
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