Photosynthesis Activity: How to reduce the amount of carbon dioxide in the atmosphere: Controlling Global Warming

Photosynthesis Activity:

Listen to the NPR broadcast (take notes in your journal) Listen to NPR: http://www.npr.org/2011/04/08/135241853/growing-the-technology-for-artificial-leaves

1) Assign each article to 2 groups. (Table 1 & 2 read article #1; Table 3 &4 read article #2; Table 5 & 6 read article #3; Table 7 and 8 read article # 4)

2) Keep in mind while reading your article how closely the scientists are mimicking nature and how different the processes have to be from natural photosynthesis

3)Each group must come up with 4 questions that address the major concepts presented in their article that shares the points raised in item 3.

4)Write a personal reflection (typed) about the similarities and differences between natural and artificial photosynthesis, make sure you bring in ideas and concepts from your textbook readings and the articles presented in class. Reflect on the importance of carbon dioxide sequestering (look it up) and the effect on global climate change. NOTE: include the references you have used (APA style) at the end of your reflection paper.



Article 1: ‘The only silver bullet’: Successfully mimicking the leaf has the potential to profoundly change the global energy industry

http://business.financialpost.com/2013/12/17/the-only-silver-bullet-successfully-mimicking-the-leaf-has-the-potential-to-profoundly-change-the-global-energy-industry/?__lsa=f2e6-1224

Dan Ovsey | December 17, 2013 8:00 AM ET

 

Artificial photosynthesis is the concept of mimicking the natural processes of the leaf by taking the 46 gigatons of carbon dioxide in the earth’s atmosphere and combining it with sunlight and water to create hydrocarbons such as methane (a key ingredient in natural gas) or methanol (a core element for running fuel cells, making myriad chemicals and powering cars).

Geoffrey Ozin isn’t all that interested in climate-change politics. From the perspective of the University of Toronto chemistry professor, the science of artificial photosynthesis he and his team of approximately 30 researchers — and unaffiliated researchers around the world — are developing is something that should be pursued irrespective of whether or not one views climate change as real.

It is the only silver bullet we have,” he says in reference to the potential for generating a viable renewable energy source.

Artificial photosynthesis is the concept of mimicking the natural processes of the leaf by taking the 46 gigatons of carbon dioxide in the earth’s atmosphere and combining it with sunlight and water to create hydrocarbons such as methane (a key ingredient in natural gas) or methanol (a core element for running fuel cells, making myriad chemicals and powering cars).

Carbon dioxide is a fuel; it’s not a waste product

Carbon dioxide is a fuel,” says Prof. Ozin. “It’s not a waste product. It’s a whole new take. We just have to learn to run the world in reverse.”

While the concept of artificial photosynthesis has been around for the better part of three decades, the past 10 years have seen an explosion of research and published academic papers on the subject. In the U.S., Washington has funneled $120-million into the Joint Center for Artificial Photosynthesis (JCAP) at the California Institute of Technology and in his 2011 State of the Union address, President Obama referred to the program as an example of one of the “Apollo projects of our time”. In Europe, the Solar H Network project has been pursuing similar science with the support of the European Energy Research Alliance.

Will Royea, assistant director for strategy and communication at JCAP, says using artificial photosynthesis to generate a liquid-based hydrocarbon fuel remains the organization’s long-term goal. However, the more immediately achievable objective, he says, is to use the science to generate hydrogen fuel. Yet he’s quick to acknowledge the business case for hydrogen remains a significant challenge.

 “Hydrogen is a good fuel, but at this time … we’re not set up to handle hydrogen on a large scale. There have been some recent releases of vehicles that run off of hydrogen, so there appears to be quite a bit of movement in the development of infrastructure of running off of hydrogen, and because that’s also technologically easier to do than producing hydrocarbon fuels, that is our shorter-term goal within JCAP.”

Indeed, in most industrial economies where taxes and penalties on carbon emissions are rare, the viability of hydrogen as a cost-competitive model to non-renewable forms of fuel remains questionable. The Town of Whistler recently discovered that the hard way when B.C. Transit’s five-year $89.5-million plan to power 20 buses using hydrogen was scrapped due to numerous issues, including the fact the hydrogen buses cost four times that of conventional buses and required constant maintenance while the hydrogen fuel itself had to be trucked across Canada from Quebec.

That’s why the ideal end goal of scientists like Mr. Ozin and those at JCAP is to create a liquid or gas-based hydrocarbon fuel — one Mr. Royea envisions would be generated at remote fuel-generating stations (similar to solar farms) located on non-arable land parcels (e.g. the desert) and transported via pipeline to fuelling stations in urban centres where it would then be placed onto tankers and shipped to various points within the city.

If you can turn the thinking around that there’s a new potential for a new carbon dioxide economy, large numbers of jobs and huge amounts of money can be made

For his part, Prof. Ozin doesn’t see the practical implementation of hydrocarbon fuels via artificial photosynthesis as a means of replacing fossil fuels, but rather a way to have the two working in tandem to create carbon neutrality by having the former use the latter’s carbon emissions as a key ingredient to hydrocarbon fuel generation.

You’re not going to stop people from burning fossil fuels; you’re not going to stop them,” he says. “They need it for chemicals and they need it for energy, and everything we have comes from it. If you can turn the thinking around that there’s a new potential for a new carbon dioxide economy, large numbers of jobs and huge amounts of money can be made.”

As Prof. Ozin notes, “this ain’t going to happen fast”. The timeline for commercializing hydrocarbon fuel will depend on a variety of factors. Mr. Royea predicts JCAP would be able to deliver to private industry within seven years something it could commercialize but “how long it takes industry to build that up and roll that out, I wouldn’t venture a guess,” he adds.

Regardless of the timeline, what’s clear is that the science has the potential not only to neutralize the environmental impact of fossil fuels but also to profoundly change the economics of the entire energy industry.

Article #2:

Friday, July 12, 2013 Why the future of fuel lies in artificial photosynthesis
By Carol Clark http://esciencecommons.blogspot.com/2013/07/why-future-of-fuel-lies-in-artificial.html


Most people, especially technical experts, may agree that we have an energy crisis, but it’s much harder to come to a consensus on how to solve it.

Fossil fuels, wind power, biofuels, geothermal power, nuclear energy and solar power are all pieces in the puzzle for how to keep Earth’s burgeoning civilization running, says Emory inorganic chemist Craig Hill.

He adds, however, that an energy source that will be essential to manage the crisis in the coming decades is the least developed: Artificial photosynthesis.

Hill and other top experts in the nascent field of artificial photosynthesis co-wrote an opinion piece on the topic published in the journal
Energy and Environmental Science.

“Humanity is on the threshold of a technological revolution that will allow all human structures across the earth to undertake photosynthesis more efficiently than plants,” the authors write.

The 18 authors on the opinion piece, from leading research universities and national laboratories in the United States, Europe and Australia, represent the broad range of expertise, from chemistry to biology to engineering, working on the problem.

The aim of artificial photosynthesis is to use solar energy to split water, to generate hydrogen as a cheap and abundant source of carbon-free fuel.


“The development and global deployment of such artificial photosynthesis (AP) technology,” the authors write, “addresses three of humanity’s most urgent public policy challenges: to reduce anthropogenic carbon dioxide emissions, to increase fuel security and to provide a sustainable global economy and ecosystem. Yet, despite the considerable research being undertaken in this field … AP remains largely unknown in energy and climate change public policy debates.”

“Globally, our energy requirements our expected to double in the next 30 to 40 years, maybe less,” Hill says. “It’s a staggering problem that puts everything else in perspective. Everything derives from energy. If we don’t have enough energy, we’re not going to have enough food and water.”

Fracking has opened up new sources of fossil fuels in the United States, but ultimately fossil fuels are going to run out. Fossil fuel use is also coming at a rapidly escalating environmental cost, including rising global temperatures and acidification of the oceans.

The only energy source that can come close to sustainably powering our long-term needs is terrestrial sunlight, Hill says.

The solar power industry, which converts sunlight into electricity, continues to grow, but it has severe limitations, Hill says. A great deal of space is required for solar panels to collect the sun’s energy, and that energy must be stored in batteries.

“We’re at the point now where we have solar powered buildings and electric cars, but we are never going to be able to run airplanes and ships and most other forms of transportation on electricity,” Hill says. “That’s why we ultimately need artificial photosynthesis, which is just another way of saying solar fuel.”

The goal of artificial photosynthesis is to do what plants do, only better.

“Plants use sunlight, water and carbon dioxide to make fuel in the form of carbohydrates,” Hill explains. “The process, however, is incredibly inefficient. It works for plants because they don’t have to worry about finances.”

Scientists currently know how to mimic plant photosynthesis, but not in ways that are powerful and efficient enough for practical application. Breakthroughs are needed in both fundamental science and materials engineering, says Hill, who is working on perfecting a key aspect of the problem, a water oxidation catalyst. Hill’s lab has developed the fastest homogeneous water oxidation catalyst to date.

“Artificial photosynthesis is a tremendous challenge,” Hill says, “but it’s also tremendously exciting.”

Hill foresees that we will eventually make the necessary breakthroughs to generate solar fuel. We simply have no other choice, he adds, as the human population approaches 10 billion by 2050.

Meanwhile, Hill and the co-authors of the Energy and Environmental Science opinion piece are calling for a globalized approach to artificial photosynthesis, to help raise the field’s public policy profile, remove logistical and governmental hurdles to its development, and strengthen an international commitment to clean, sustainable energy.

They envision scenarios like a network of light capture facilities situated in coastal cities where seawater would be catalytically converted to hydrogen and oxygen.

“Photosynthesis is the great invention of life,” they write. “Like biodiversity, the atmosphere, the moon, outer-space, the human genome and the world’s cultural and natural heritage, it could be treated as subject to common heritage requirements under international law, perhaps through a specific UN or UNESCO declaration. Common heritage of humanity status putatively limits private or public appropriation; requires representatives from all nations to manage such resources on behalf of all, actively share the benefits, restrain from their militarization and preserve them for the benefit of future generations.”

Article # 3

How Artificial Photosynthesis Works http://science.howstuffworks.com/environmental/green-tech/energy-production/artificial-photosynthesis4.htm

by Julia Layton

If the smartest energy source is one that's abundant, cheap and clean, then plants are a lot smarter than humans. Over billions of years, they developed perhaps the most efficient power supply in the world: photosynthesis, or the conversion of sunlight, carbon dioxide and water into usable fuel, emitting useful oxygen in the process.

In the case of plants (as well as algae and some bacteria), "usable fuel" is carbohydrates, proteins and fats. Humans, on the other hand, are looking for liquid fuel to power cars and electricity to run refrigerators. But that doesn't mean we can't look to photosynthesis to solve our dirty-, expensive-, dwindling-energy woes. For years, scientists have been trying to come up with a way to use the same energy system that plants do but with an altered end output.

Using nothing but sunlight as the energy input, plants perform massive energy conversions, turning 1,102 billion tons (1,000 billion metric tons) of CO2 into organic matter, i.e., energy for animals in the form of food, every year [source: Hunter]. And that's only using 3 percent of the sunlight that reaches Earth [source: Boyd].

The energy available in sunlight is an untapped resource we've only begun to really get a handle on. Current photovoltaic-cell technology, typically a semiconductor-based system, is expensive, not terribly efficient, and only does instant conversions from sunlight to electricity -- the energy output isn't stored for a rainy day (although that could be changing: See "Is there a way to get solar energy at night?"). But an artificial photosynthesis system or a photoelectrochemical cell that mimics what happens in plants could potentially create an endless, relatively inexpensive supply of all the clean "gas" and electricity we need to power our lives -- and in a storable form, too.

In this article, we'll look at artificial photosynthesis and see how far it's come. We'll find out what the system has to be able to do, check out some current methods of achieving artificial photosynthesis and see why it's not as easy to design as some other energy-conversion systems.

So, what does an artificial photosynthesis system have to be able to do?

Artificial Photosynthesis Approaches

To recreate the photosynthesis that plants have perfected, an energy conversion system has to be able to do two crucial things (probably inside of some type of nanotube that acts as the structural "leaf"): harvest sunlight and split water molecules.

Plants accomplish these tasks using chlorophyll, which captures sunlight, and a collection of proteins and enzymes that use that sunlight to break down H2O molecules into hydrogen, electrons and oxygen (protons). The electrons and hydrogen are then used to turn CO2 into carbohydrates, and the oxygen is expelled.

For an artificial system to work for human needs, the output has to change. Instead of releasing only oxygen at the end of the reaction, it would have to release liquid hydrogen (or perhaps methanol) as well. That hydrogen could be used directly as liquid fuel or channeled into a fuel cell. Getting the process to produce hydrogen is not a problem, since it's already there in the water molecules. And capturing sunlight is not a problem -- current solar-power systems do that.

The hard part is splitting the water molecules to get the electrons necessary to facilitate the chemical process that produces the hydrogen. Splitting water requires an energy input of about 2.5 volts [source: Hunter]. This means the process requires a catalyst -- something to get the whole thing moving. The catalyst reacts with the sun's photons to initiate a chemical reaction.

There have been important advances in this area in the last five or 10 years. A few of the more successful catalysts include:

·        Manganese: Manganese is the catalyst found in the photosynthetic core of plants. A single atom of manganese triggers the natural process that uses sunlight to split water. Using manganese in an artificial system is a biomimetric approach -- it directly mimics the biology found in plants.

·        Dye-sensitized titanium dioxide: Titanium dioxide (TiO2) is a stable metal that can act as an efficient catalyst. It's used in a dye-sensitized solar cell, also known as a Graetzel cell, which has been around since the 1990s. In a Graetzel cell, the TiO2 is suspended in a layer of dye particles that capture the sunlight and then expose it to the TiO2 to start the reaction.

·        Cobalt oxide: One of the more recently discovered catalysts, clusters of nano-sized cobalt-oxide molecules (CoO) have been found to be stable and highly efficient triggers in an artificial photosynthesis system. Cobalt oxide is also a very abundant molecule -- it's currently a popular industrial catalyst.

Once perfected, these systems could change the way we power our world.

Artificial Photosynthesis Applications

Fossil fuels are in short supply, and they're contributing to pollution and global warming. Coal, while abundant, is highly polluting both to human bodies and the environment. Wind turbines are hurting picturesque landscapes, corn requires huge tracts of farmland and current solar-cell technology is expensive and inefficient. Artificial photosynthesis could offer a new, possibly ideal way out of our energy predicament.

For one thing, it has benefits over photovoltaic cells, found in today's solar panels. The direct conversion of sunlight to electricity in photovoltaic cells makes solar power a weather- and time-dependent energy, which decreases its utility and increases its price. Artificial photosynthesis, on the other hand, could produce a storable fuel.

And unlike most methods of generating alternative energy, artificial photosynthesis has the potential to produce more than one type of fuel. The photosynthetic process could be tweaked so the reactions between light, CO2 and H2O ultimately produce liquid hydrogen. Liquid hydrogen can be used like gasoline in hydrogen-powered engines. It could also be funneled into a fuel-cell setup, which would effectively reverse the photosynthesis process, creating electricity by combining hydrogen and oxygen into water. Hydrogen fuel cells can generate electricity like the stuff we get from the grid, so we'd use it to run our air conditioning and water heaters.

One current problem with large-scale hydrogen energy is the question of how to efficiently -- and cleanly -- generate liquid hydrogen. Artificial photosynthesis might be a solution.

Methanol is another possible output. Instead of emitting pure hydrogen in the photosynthesis process, the photoelectrochemical cell could generate methanol fuel (CH3OH). Methanol, or methyl alcohol, is typically derived from the methane in natural gas, and it's often added to commercial gasoline to make it burn more cleanly. Some cars can even run on methanol alone.

The ability to produce a clean fuel without generating any harmful by-products, like greenhouse gasses, makes artificial photosynthesis an ideal energy source for the environment. It wouldn't require mining, growing or drilling. And since neither water nor carbon dioxide is currently in short supply, it could also be a limitless source, potentially less expensive than other energy forms in the long run. In fact, this type of photoelectrochemical reaction could even remove large amounts of harmful CO2from the air in the process of producing fuel. It's a win-win situation.

But we're not there just yet. There are several obstacles in the way of using artificial photosynthesis on a mass scale.

Challenges in Creating Artificial Photosynthesis

While artificial photosynthesis works in the lab, it's not ready for mass consumption. Replicating what happens naturally in green plants is not a simple task.

Efficiency is crucial in energy production. Plants took billions of years to develop the photosynthesis process that works efficiently for them; replicating that in a synthetic system takes a lot of trial and error.

The manganese that acts as a catalyst in plants doesn't work as well in a man-made setup, mostly because manganese is somewhat unstable. It doesn't last particularly long, and it won't dissolve in water, making a manganese-based system somewhat inefficient and impractical. The other big obstacle is that the molecular geometry in plants is extraordinarily complex and exact -- most man-made setups can't replicate that level of intricacy.

Stability is an issue in many potential photosynthesis systems. Organic catalysts often degrade, or they trigger additional reactions that can damage the workings of the cell. Inorganic metal-oxide catalysts are a good possibility, but they have to work fast enough to make efficient use of the photons pouring into the system. That type of catalytic speed is hard to come by. And some metal oxides that have the speed are lacking in another area -- abundance.

In the current state-of-the-art dye-sensitized cells, the problem is not the catalyst; instead, it's the electrolyte solution that absorbs the protons from the split water molecules. It's an essential part of the cell, but it's made of volatile solvents that can erode other components in the system.

Advances in the last few years are starting to address these issues. Cobalt oxide is a stable, fast and abundant metal oxide. Researchers in dye-sensitized cells have come up with a non-solvent-based solution to replace the corrosive stuff.

Research in artificial photosynthesis is picking up steam, but it won't be leaving the lab any time soon. It'll be at least 10 years before this type of system is a reality [source: Boyd]. And that's a pretty hopeful estimate. Some people aren't sure it'll ever happen. Still, who can resist hoping for artificial plants that behave like the real thing?

Article #4: Capturing energy from the sun

Dörthe M. Eisele of the Research Laboratory of Electronics (left) and Moungi G. Bawendi of chemistry are investigating fundamental processes in light-harvesting nanotubes and quantum dots with a goal of one day developing a completely new approach to collecting energy from the sun.
http://web.mit.edu/newsoffice/2013/capturing-energy-from-the-sun.html

MIT investigators are inspired by a deep-sea bacterium that is able to harvest tiny amounts of incoming solar energy with exquisite efficiency.

Nancy W. Stauffer   January 23, 2013

This article first appeared in the Autumn 2012 issue of Energy Futures, the magazine of the MIT Energy Initiative. Subscribe today.

Much attention is now focusing on natural organisms that have evolved highly efficient light-harvesting capabilities over billions of years. The green sulfur bacterium is perhaps the champion light-harvester. It lives at the depths of the ocean, where light levels are extremely low. But it makes the most of the light it gets: It is able to harvest up to 95% of the solar energy it absorbs.

Research teams worldwide are trying to replicate the capabilities of the green sulfur bacterium. But such natural systems are exceedingly complex. “We’d like to break these systems down into their simpler components and see if we can find synthetic [structures] that mimic them,” says Moungi G. Bawendi, the Lester Wolfe Professor of Chemistry. “In the end, we want a set of building blocks that we can assemble to create a more complicated system — perhaps as complex as the natural system but where we have more control, where we can tune some critical parameters.”

Key to the light-harvesting success of the green sulfur bacterium is its light-harvesting antenna system. This system consists of long cylinders of closely packed bacteriochlorophyll molecules that absorb solar energy and transfer as much as 95% of it toward the “reaction centers” where critical chemical processes occur. Those cylinders are “perhaps nature’s most spectacular light-harvesting system,” says Dörthe M. Eisele, postdoctoral associate in the MIT Research Laboratory of Electronics. They could be an ideal building block for a practical device.

But applications are far down the road. The first step is to develop a fundamental understanding of how nature’s bacteriochlorophyll cylinders do their job. Chopping individual cylinders out of a bacterium for analysis would destroy their natural functioning. The solution is therefore to create and study an artificial model system that behaves like the in situ bacterio-chlorophyll cylinders.

A model system — and a challenge

For the past five years, Eisele and her collaborators at MIT’s Center for Excitonics, the University of Texas at Austin, Humboldt University of Berlin in Germany and the University of Groningen in the Netherlands have been working with an artificial system that is similar in size, shape, and function to the natural antenna system in the green sulfur bacterium.

The structure consists of molecules of cyanine dye that naturally aggregate and self-assemble, rolling up into long double-walled nanotubes when they are immersed in water. Each nanotube is about 13 nm wide and thousands of times that long, and each contains two concentric cylinders of closely packed cyanine dye molecules about 4 nm apart. That “supramolecule” with its two cylinders of light-absorbing material closely resembles the natural antenna system in the green sulfur bacterium.

In 2007, Eisele developed a carefully controlled technique to produce well-defined cyanine dye-based nanotubes. In early work, she and her colleagues demonstrated that her nanotubes have highly uniform properties — from tube to tube and also along the length of each one. Because the LH nanotubes are “all the same,” she can study their properties from the ensemble in solution with no need to isolate the responses of the individual nanotubes.

Nevertheless, determining how the LH nanotubes collect and transport energy from light is a challenge. Even advanced microscopes cannot show the details of their structure, so Eisele turned to another option: their optical spectra. Shine light on a supramolecule made up of closely packed molecules and it will absorb certain wavelengths and not others. The resulting spectrum can be the key to unraveling not only the optical behavior of the supramolecule but also its physical structure.

Early spectral analyses revealed a critical property of such cyanine-based supramolecules. On their own, the individual cyanine molecules have a characteristic absorption spectrum. Yet when they pack together, the supramolecule that results has a dramatically different spectrum — even though the molecules retain their individual structure, do not share electrons, and are held together only by weak attractive forces.

Why? When an individual molecule absorbs energy from the sun, it becomes “excited.” But when a supramolecule absorbs solar energy, the closely packed molecules in it “share” their excited states. Because of those shared excited states, the optical properties of the supramolecule are significantly different from those of the individual molecules. “So the unique ability of our nanotubes to harvest light so efficiently arises from the ensemble of closely packed, aggregated molecules,” says Eisele. Moreover, the details of how the molecules are packed together strongly affect those interactions, and thus the optical properties of the supramolecule. If the researchers could understand the relationship between the structural details and the optical properties, they might be able to fine-tune the optical behavior by altering how the molecules pack together.

Understanding the spectral evidence

Analyzing the spectrum of the supra-molecule would provide valuable insights — but there is a problem. The “electron excitation” is shared between neighboring molecules. But do such molecule-to-molecule interactions occur mainly within the inner and outer cylinders separately or throughout the entire LH nanotube? If the former, then the spectrum of the LH nanotube would show the absorption behavior of the two independent (or at most weakly interacting) cylinders superimposed on one another. But if the latter, the overall spectrum would reflect the combined optical response of the two strongly interacting cylinders.

A solution would be to observe the spectra of the cylinders separately. But using a beam of light to excite selected parts of the LH nanotubes in a vial isn’t feasible, and removing the outer cylinder to isolate the spectrum of the inner cylinder won’t work. “It’s a self-assembling system, and we can’t destroy its structure without altering its behavior,” says Eisele. “What we can do is change the light-absorption capability of the outer cylinder.”

To do that, she oxidizes the nanotubes using silver nitrate. Each silver ion removes one electron from a single molecule of cyanine dye, altering its absorption behavior. Her experimental procedure therefore consisted of simply adding silver nitrate to a vial of suspended nanotubes and then carefully taking the absorption spectrum of the mixture every 30 minutes for six hours as oxidation proceeded. The results appear in the diagram below.

 (click to see diagram)
Experimental results from oxidizing the cyanine-based nanotubes with silver nitrate (AgNO3). The plots show the absorption spectrum of the nanotubes in solution before the AgNO3 was added (red), spectra taken every 30 minutes after it was added (gray), and the spectrum at the end of the six-hour experiment (green). Peak 2 drops down quickly and finally disappears. Peak 1 and other portions of the spectrum change more slowly and drop only in amplitude, not shape. Analysis of the spectral data shows that those two responses reflect the separate behavior of the inner and outer cylinders in the light-harvesting nanotubes. Micrographs show that the outer cylinder is morphologically intact after oxidation but now 'decorated' with silver nanoparticles (as shown on the inset above). Isolation of the inner cylinder’s spectrum subsequently enabled the researchers to model the detailed supramolecular structure of the artificial nanotubes.

The red curve is the initial absorption spectrum before the start of the experiment; the gray curves are the 30-minute spectra; and the green curve is the final spectrum. Comparison of the curves shows that peak 2 goes down more quickly than peak 1 does — and it ultimately disappears. Eisele and her co-workers were able to show that this fast decrease in intensity of peak 2 reflects changes in the outer cylinder and that the slower decrease of peak 1 reflects changes in the inner cylinder. The conclusion: Peak 2 can be unambiguously attributed to the outer cylinder, peak 1 to the inner one. Moreover, the portions of the absorption spectrum that drop only in amplitude with no significant change in shape can be traced to the inner cylinder.

Detailed analysis of the experimental data confirmed that the original spectrum is made up of the spectra of two essentially independent chemical species superimposed on one another. The two LH cylinders can thus be treated as two electronically separate systems, with at most weak coupling between them. In addition, images taken with a cryogenic electron transmission microscope clearly showed the double-walled structure of the nanotubes — both before and after oxidation. Indeed, the only difference in the post-oxidation images was that the exterior surfaces of the nanotubes were “decorated with silver nanoparticles,” says Eisele. Those results confirm that the outer cylinder was still physically present. Only its optical behavior had changed.

The isolation of the inner cylinder’s spectrum made possible unprecedented theoretical advances. After three years’ work, collaborators led by Professor Jasper Knoester at the University of Groningen, the Netherlands, modeled a structure for the inner cylinder that reproduced the experimentally observed spectrum. The structure has molecules organized in a herring-bone fashion — a geometry previously proposed by others but with certain details that are different. For example, each tile is made up of two molecules, and as the tiles wrap around to form the cylinder, they tilt out from the surface at distinctive angles.

Knoester and his group next modeled the structure of the outer cylinder assuming the same packing geometry but adjusted to span the greater circumference. They then calculated the spectrum of a suprastructure formed from their two cylinders that shows remarkable agreement with Eisele’s measured absorption spectrum. By combining these experimental and theoretical results, the researchers were thus able to settle a long-standing argument about the geometry of such cyanine-based nanotubes.

Only the beginning

Armed with their new understanding, the researchers at MIT are now continuing their studies. For example, they are examining the nature of the weak interaction between the inner and outer cylinders, and they are looking into what happens when many cylindrical nanotubes cluster together, as they do in nature. Says Eisele, “Now we need to know whether we can think of them as a superposition of individual cylinders — or do they become a totally new system with different optical properties?”

But even a cluster of LH cylinders is just one building block for a future device, Bawendi says. He and Eisele are now working to connect the LH nanotubes to quantum dots (QDs) — nanometer-scale inorganic crystals that fluoresce when stimulated by light. That combination raises exciting possibilities. By controlling the size of the QDs, Bawendi — an expert in this field — will be able to “tune” them to absorb sunlight and then emit a specific wavelength that will generate maximum electron excitation in the LH nanotubes. In the lab, that focused light will enable the researchers to track how the excitation propagates along the LH nanotubes. In a practical device, such tailored QDs could deliver focused energy that LH nanotubes could efficiently transport and deliver to a system — perhaps including more QDs — where chemical reactions might, for instance, produce fuels.

Bawendi stresses that such concepts are very far down the line. “The idea is to create something from building blocks, so first we have to understand the building blocks themselves and how they interact,” he says. But if his “grand vision” succeeds, a device integrating such building blocks could one day provide a completely new way to collect energy from the sun — perhaps modeled in part on that solar-harvesting genius, the green sulfur bacterium.

This research was supported by the MIT Center for Excitonics, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, and by the Deutsche Forschungsgemeinschaft, the Integrative Research Institute for the Sciences in Berlin, the National Science Foundation, the Alexander von Humboldt Foundation, the US Army Research Office and the US