Award: OCE-1344241

Pyrite may be one of the most important minerals you've never heard of, at least not by this name. You might, however, know it by the name of "fool's gold": a mineral that's shiny and golden and bright, but allegedly of little worth. Pyrite, or fool's gold, has broken the heart of many gold prospectors. Though it did not bring those folks wealth and fame, pyrite has been -and continues to be- of immeasurable value to humankind. Pyrite is an incredibly versatile mineral that has a key role in a variety of manufacturing processes: making sulfuric acid, making and recycling aluminum, paints, batteries, paints and enamels, detergents, pharmaceuticals, and many other products. It is ironic that a mineral commonly called fool's gold is at the heart of so many multi-million dollar industries. Pyrite also happens to be an excellent semiconductor, which are materials that will conduct or insulate electrical current, depending on the voltage. Semiconductors are at the heart of our electronics industry, and every single transistor chip that humankind has ever made contains a semiconductor. Notably, pyrite is also a photovoltaic, meaning that pyrite can (in the appropriate configuration) act as a solar panel. In fact, pyrite absorbs 100 times more light than silicon dioxide (the current, favored solar panel material). Pyrite is also far cheaper to extract than silicon. Put simply, the economics of using pyrite as a solar collector at very compelling. What then, is impeding the use of Pyrite is solar panels? First, much more is known about how to deposit silicon dioxide onto wafers for solar cells. Pyrite, on the other hand, is trickier to prepare with current technologies. Pyrite must be deposited in the purest possible form, and with a microscale structure that allows the current from the crystals to be appropriately collected for energy harvesting. While much research has been done on Pyrite, far less has focused on developing a cost-effective means of growing them depositing pure pyrite crystals. Notably, at deep sea hydrothermal vents there are strikingly large pyrite crystals that have many of the properties that are necessary for pyrite solar cells (e.g. high purity, large crystal size, etc.). Curiously, these crystals seem to grow in association with hydrothermal vent microbes, although prior to our work no studies had adequately established the relationship between pyrite crystal growth and hydrothermal vent microbes. In this project, we set out to understand this relationship between pyrite crystal growth and microbes. Equally important, we did this work with an eye towards developing a micobially-mediated process wherein we could use microbes to deposit pyrite on photovoltaic substrates. To that end, we conducted a detailed study to evaluate pyrite deposition at vent conditions, and on vent-relevant substrates. We did this using so-called ?bioelectrochemical systems?, where we use anodes in the presence of hydrothermal vent fluid and vent microbes (Gartman et al., 2013). We ?poised? the electrode to the electrical potentials found naturally at vents. We found that electrical potential is really critical to the deposition of pyrite particles on the surface of the working electrode (no deposition occurred on the open circuit electrode), suggesting that naturally-occurring electrical currents are a key factor in the growth of the vent ?chimneys? that contain the large (and commercially interesting) pyrite crystals. Our later experiments showed that certain vent microbes called sulfate-reducing bacteria actually ?grow? pyrite nanocrystals on their cell membranes. These, in turn, are sites of crystal nucleation, which means the nanocrystals they produce promote the growth of even larger, purer crystals. Finally, we conducted a set of experiments that showed how one might grow iron sulfide crystals at hydrothermal vent conditions, which means that we could get a layer of crystal films to form on the surface of a conductive electrode that was incubated at 120? Celsius (~250? F). These observations further set the stage for the development of a system whereby one can use microbes to grow pyrite solar cells. The funds form this program allowed two scientists from markedly different disciplines (marine microbiology and materials science) to bring their respective strengths to bear on advancing our knowledge of how pyrite crystals grow. Our work showed that microbes are intimately involved in the growth of larger crystals, and that electrical potentials is equally important in the formation of large continuous crystal layers. Moreover, this support allowed us to promote the interdisciplinary training of graduate students and postdoctoral scholars alike, who have gone on to careers as federal lab scientists, professors, and even patent attorneys. This relatively modest investment has also led to the broad dissemination of our data, which we know has caught the attention of several companies who are using our findings to refine their processes for making particular iron sulfide compounds for industry, as well as potentially growing pyrite films for photovoltaics. Last Modified: 04/19/2019 Submitted by: Peter Girguis