I don't think it's too far from the truth to say that humanity is facing a number of simultaneous environmental crises, any one of which, if not addressed, could lead to dramatic changes to the quality of life for millions of people. One of those crises is energy. We can continue to use coal, gas, and oil for quite some time, but the cost is huge. The price of extraction is going up, the amount to be extracted is going down, and the emitted CO2 is dramatically changing our climate. With all of these downsides, an alternative is called for. One such alternative is the generation of fuels via solar energy, where we either burn hydrogen directly, or reform carbon dioxide and hydrogen into hydrocarbons to form a closed carbon cycle. If sufficient scale could be reached, solar fuels could be used to meet transport energy requirements in the future.
But for this to work, we need to make a number of processes more efficient on a per-molecule basis. One of those is more efficient water-splitting. The problem actually lies with the oxygen side of the reaction. When we split water each molecule produces a single, highly reactive oxygen atom. This atom must combine with another oxygen atom to create molecular oxygen. Making that happen efficiently requires a good catalyst, and four charges with rather high energy. Recent work published in Nano Letters shows how this might be achieved.
Haven't we being splitting water for ages?
For a long time the big problem with water splitting had been that oxygen, even molecular oxygen, is highly reactive, so it tended to oxidize the catalyst, rendering it useless in a short time. A couple of years ago, however, a new class of catalysts based on cobalt were developed. These catalysts appear to be long-lasting and efficient. With this development, some of the focus has turned to the high-energy charges. Typically, the energy possessed by an electron is something close to the amount of energy used to free it up. In this case, where the excitation is due to light, that means that only photons from the ultraviolet part of the spectrum have sufficient energy to partake in the water splitting reaction.
Luckily for us, the Earth's atmosphere does a pretty good job of absorbing ultraviolet light, but that means that solar water-splitting seems doomed to low efficiency simply because the number of photons is so small. There is, however, an alternative: surface plasmon resonances. A surface plasmon is generated through electrons moving back and forth in concert with an exciting light field. The point is that the energy stored in a plasmon can be increased simply by turning up the light intensity. The light drives the electrons to move further and faster, just as pushing on a swing drives it to ever larger amplitude swings and faster speeds—more precisely, the speed of the swing is faster when it passes through its rest position. Even though the individual pushes have insufficient energy to generate such a large motion, the swing stores and combines that energy with those from earlier pushes.
Surface plasmons being able to store that energy is all well and good, but if you set yourself up to use that energy, then you usually damp out the swinging motion. Think of it like a shock absorber: the spring excites a bouncy motion every time you hit a bump. Given the chance, the spring will keep the car bouncing along, giving you motion sickness and reducing your control of the vehicle. The shock absorber provides resistance to the bouncing, extracting the energy stored in the spring and turning it into heat. That is very useful in the case of a car, but, for surface plasmon resonances, it tells you that if you extract the energy from the electrons efficiently, your plasmon is going to vanish. In other words: no high energy charges will be available for water splitting.
A group of researchers from the University of California Santa Barbara have shown how to extract just the right amount of energy from the surface plasmons, so that efficient water splitting can occur. The surface plasmons are excited by light illuminating an array of tiny golden rods—the rods are 90nm in diameter, and around 200nm long. The rods are isolated at the bottom and top by a thin layer of insulating material (titanium oxide). The cobalt-based catalyst was attached to the gold nanorods around their middle. All of this was placed on a transparent electrode, which was connected to a platinum electrode, where hydrogen is evolved.
So how does this work? The gold nanorod limits the motion of the plasmon and the plasmon reflects off the end surfaces and meets itself coming the other way. The resulting interference pattern generates a huge electric field near the ends of the nanorod. Physically, the electrons tend to repeatedly pile up at the ends of the nanorods and then spread along the length of the rod. The insulating cap prevents most of the electrons from escaping, unless they have sufficient energy to efficiently tunnel through the insulating barrier. And this is precisely what happens. The electrons tunnel through the oxide layer into the electrode, over to the hydrogen evolving electrode, where they provide the electrons necessary to produce molecular hydrogen.
The loss of these electrons creates high-energy holes in the gold nanoparticle—a hole is the absence of an electron that behaves just like a positively charged particle—which get sucked up by the catalyst and given to the surface oxygen. The oxygen then comes off the electrode as a gas.
The clever part of this strategy is the tunnel barrier between the plasmon and the electrode. By creating this, the researchers can ensure that only the highest energy electrons have a significant chance of making it to the electrode. This sucks some energy out of the plasmon, but does not damp it out completely, allowing the light to replenish the plasmon's energy. The plasmons themselves are most efficiently excited by light in the visible part of the spectrum, as well as wavelengths a little bit longer than that. This just happens to be where most of the light in the solar spectrum is—measured after passing through the Earth's atmosphere. A single photon cannot generate a single electron with the requisite energy, and the plasmon has some natural damping, so it takes at least two, and more likely three or four photons to generate a single electron, depending on the photon wavelength. Nevertheless, that's still better than having direct excitation by ultraviolet light.
Am I going to get a gold solar cell now?
The results are an interesting mix of nice and confusing. Most—around 80 percent—of the generated electrons contribute to the water splitting reaction. They show that with ultraviolet light only, the current density drops by a factor of five to ten. They also show that without the cobalt catalyst, the reaction rate is much poorer. This is in spite of the fact that the insulating material is also a catalyst (TiO2): it is ineffective compared to the cobalt catalyst. The general picture is that it is the catalyst and the plasmons that contribute to the increased reaction rate, rather than any particular component.
There is, however, a very confusing result. In these experiments, the general procedure is to use a standard lamp—this is a lamp that, to some extent, mimics the solar spectrum. More importantly, it allows different solar devices to be compared, and to switch it on and off periodically. The changes between on and off parts of the cycle, combined with changes between different cycles, allow researchers to understand how the cell is performing. The researchers note that during the on-time, the current jumps to some high level, and then, instead of plateauing, it slowly increases by about a third. There is no explanation of this slower process.
To give you an idea of why this is confusing: the electron processes that would allow the plasmon to reach equilibrium occur on the scale of picoseconds. Their interaction with the crystalline gold atomic cores should have stabilized in a few nanoseconds. The water and electrolytes take a bit more time to respond—call that a few seconds. After that, everything should be stable, so what is going on here? The researchers offer no comment at all.
You might say that thermal processes mean that you are simply heating the cell up. This may very well be true. But for the visible and near infrared part of the spectrum, the processes described above are the microscopic description of thermal processes. That only leaves direct infrared absorption. It seems to me that they have filtered out the longer infrared contribution from the lamp, so where did the heat come from? This requires more explanation.
What next? Get rid of the gold. Gold makes an excellent carrier for surface plasmons, but there isn't a lot of it around. This needs to be replaced with a more common metal, like aluminum or copper. If they can do that without a substantial decrease in current density, then they will have a winner. It would also help if the hydrogen was in a more useful form—hydrogen gas is hard to store efficiently—like a hydrocarbon. That will involve splitting CO2, which is much more difficult.
Nano Letters, 2012, DOI: 10.1021/nl302796f