As the world's energy consumption rapidly grows, we are exhausting finite stores of fossil fuels, but more importantly, we are emitting massive amounts of carbon dioxide and other greenhouse gases that contribute to global warming. One of the most significant scientific challenges facing the world is meeting global energy demand in ways that do not aggravate climate change or otherwise degrade the environment. It is becoming more and more urgent that we develop viable alternative sources of energy. I am interested in particular in solar energy: converting sunlight into electricity and fuels such as hydrogen. Why solar? While numerous methods exist for producing energy carbon-free, the only renewable source that can fully meet global demand is solar energy. More energy from sunlight strikes the earth in a single hour than the entire world uses in a year! Yet today only approximately 1% of the world's energy is produced directly from sunlight, whereas over three quarters comes from the burning of coal, oil, and natural gas. The reason for this disparity is the high cost of generating energy from sunlight using existing technologies. Our dependence on fossil fuels is not sustainable in the long term, and already today, it actively threatens our environment, our national security, and our health.

There are three ways to convert sunlight into energy, shown in the diagram above, each with advantages and disadvantages. Nature's method of choice is photosynthesis, shown on the left. Photosynthesis occurs in the leaves of plants, where sunlight and water are used to produces oxygen and sugars (what the plant uses to grow and sustain itself). Photosynthesis can only produce chemical fuels, and of course does not produce electricity. Photosynthesis is by far the largest consumer of solar energy, but the process isn't very efficient in converting sunlight to usable energy (estimated at ~0.5%). As humans, we use the energy produced by photosynthesis both from burning wood from trees we've cut down ourselves, but also from burning fossil fuels, such as oil and coal, which were formed from plant matter that grew millions of years ago.

By far, the most efficient way to convert sunlight to energy is with photovoltaic solar cells, shown on the right in the figure. These solar cells are made from various different kinds of semiconductors, such as silicon, gallium arsinide, and indium phosphide among others. Photovoltaic devices are almost one hundred times more efficient than photosynthesis at producing usable energy from sunlight, but their price tag is quite a bit higher... hundreds of times higher. These are the solar cells you might find on somebody's roof, or a solar-powered calculator, on satellites, and at some solar power plants. These have been engineered so well that little improvement in efficiency is even possible, but much work remains to be done to reduce their cost. Also, these units only produce electricity, and cannot produce fuels. They have no inherent ability to store the energy they produce, which is a problem at night! Unfortunately, electricity is a very difficult form of energy to store without loosing a lot of the energy in the storage process. Also, keep in mind that electricity is not always the most useful source of energy: the large majority (~85%) of energy used in the world today is from fuels, such as coal, oil, and natural gas, and not electricity.

The third way to convert sunlight to usable energy is in between the first two in all respects: it makes both fuels and electricity; it is more efficient than photosynthesis but less so than photovoltaics; and its price is also somewhere in between. These are known and semiconductor-liquid junction solar cells. They are generally much easier and cheaper to fabricate than photovoltaics. Because they can directly make fuels, such as H2, and much more efficiently that photosynthesis can, they are very promising as emission-free alternative energy sources. An ideal solution would be to store solar energy in the form of chemical bonds via catalytic conversion of water to hydrogen and oxygen gas (so-called water splitting), but no material known today is capable of driving this reaction in a cost-effective manner. Maximizing their efficiency while keeping their cost low is the name of the game here! Semiconductor-liquid junctions are what we study in the Katz Group.

Specifically, we seek to understand how the composition and physical structure of novel, low-cost semiconducting materials can be used to control the key chemical reactions and electron-transfer processes involved in the photocatalytic electrolysis of water. To do this, we synthesize new iron-based mixed-metal oxide semiconductors in nanostructured architectures, and systematically study their photoelectrochemical properties.

Iron oxide is a particularly attractive candidate for use as a photoelectrode due to its abundance, stability, and environmental compatibility. In particular, the hematite phase, alpha-Fe2O3, has several ideal properties for use in water-splitting solar cells:

it can absorb a large portion of the solar spectrum, it is stable, and it is thermodynamically capable of evolving oxygen from water.

Figure 3: Powder X-ray diffraction pattern of nanoparticles of the hematite phase of iron oxide formed directly by microwave heating.

Yet to date devices made from hematite have achieved only modest conversion efficiencies. While doped and surface modified electrodes have proven advantageous, the effect of loading and preparation is not yet fully understood. In addition, the maghemite phase, gamma-Fe2O3, while a commonly used magnetic nanomaterial, has been largely overlooked as a material for solar energy applications, despite having been shown to have higher photocatalytic activity than hematite.