Nanotechnology
uses phenomena and structures that occur on the scale of small atoms and molecules
— a DNA helix, for example, is 2 nanometers in diameter — to make
an array of tiny tools. Early forms of nanotechnology already pervade the modern
world, in everything from paint pigments to biomedical devices.
Future strategies for solar energy should follow on principles in nature, where energy is stored in chemical bonds. Some promising research into this artificial photosynthesis focuses on nanotechnology for semiconductors — essentially altering solar energy cells on the molecular scale. Image is courtesy of Corbis.
On the distant horizon is molecular nanotechnology, literally the organization of matter at molecular scales. Imagine, for example, “smart” clothing, in which the molecules comprising the fabric can change in response to weather. The idea, as sketched by the late Richard Feynman at a meeting of the American Physical Society at Caltech in 1959 and elaborated on later by many others, is to enable the manipulation of individual atoms and molecules, using proportionally smaller tools to build and operate even smaller tools.
Biological systems are the perfect models for molecular nanotechnology, inspiring future nanotechnology. The capabilities of biological systems put present technology to shame.
Although nanotechnology is often relegated to the field of materials science, by taking a closer look at biological systems, it could one day have a profound effect on the earth sciences. In particular, millennia-old notions of what a “resource” is, and the collection and use of energy, are both likely to change beyond recognition.
Fuel laws
Current technology squanders energy because most of it is used as heat. Indeed, we could speak of the “heat” crisis rather than the “energy” crisis. Fuels, after all, are burned! Two-thirds of gas in an automobile’s tank, for example, goes right out the radiator. Due to the Carnot limit, a law that stems directly from fundamental constraints imposed by thermodynamics, even the most efficient heat engines waste at least half the applied energy.
Because electric batteries and motors are not heat engines, they are not subject to the Carnot limit, making them much more efficient. Conventional batteries, however, have other engineering issues such as low energy density and slow recharge times. Instead, fuel cells are a promising alternative.
While similar to a battery, fuel cells allow for continuous replenishment of the reactants consumed — producing electricity from an external supply of fuel and oxygen as opposed to relying on the limited energy storage capacity of a battery. And contrary to popular belief, fuel cells do not necessarily require hydrogen.
Practical fuel cells using, say, hydrocarbons or alcohols lie beyond present technological capabilities, as converting the chemical energy of fuels directly into electricity requires a highly controlled molecular-scale reaction. This process requires catalysts that are both extremely specific and robust, and hence well structured at the nanoscale. Better catalysts in general are an obvious application of near-term nanotechnology, and will have further profound and ramifying effects on energy efficiency.
Another near-term nanotechnology solution involves solar energy. It is often claimed that the high energy density of conventional fuels is not reproducible by any conceivable alternatives, at least at the scale required for modern civilization. But the high energy density of conventional fuels is merely a brute-force solution that is compensating for the inefficiency of burning them. Thus, it is simply not true that solar power is incapable of powering a technological culture.
A high-tech culture is the only sort that can be run on solar power. After all, life itself, with its extraordinary capabilities of self-organization, synthesis and element separation, runs on solar power. That’s why it is amusing to consider, for example, the oft-proposed use of biomass for fuel: Burning material originally assembled, atom by atom, from diffuse sources of both energy and materials.
Artificial photosynthesis
Why is
solar power usually thought to involve converting sunlight into electricity?
Biology doesn’t do it that way. Natural photosynthesis stores the energy
of sunlight in chemical bonds. That makes a lot more sense biologically, as
well as technologically.
The conventional disadvantages of solar power are that it is intermittent, and difficult to transport and store. The last two disadvantages are true of electricity, no matter how it is created. Using sunlight to make fuels, however, would solve the intermittency problem: Fuel can accumulate whenever the sun is shining and then be used later when needed.
Acidic drainage from mines is a pollution problem that could one day be a potential resource, by using nanotechnology to separate valuable minerals and dispose of contaminants. Photo is courtesy of Stephen L. Gillett.
Artificial photosynthesis is now receiving much attention by industry and research groups. The most promising approaches are based on semiconductors — materials for controlling conductivity that make possible most of modern-day electronics, including computer chips and lasers.
As in a conventional photovoltaic (solar) cell, semiconducting materials, such as silicon, absorb solar radiation. That radiation knocks electrons loose to create a flow of current, and each excited electron leaves behind a vacancy, or “hole,” that acts like a single positive charge. Conventionally, the electrons and holes are forced to drive an electric circuit before they recombine. Instead of driving an external circuit, however, technical alterations can make the hole and electron drive chemical reactions that store energy.
Most research has focused on “water splitting,” the production of hydrogen gas from water, but alternative fuel generation is possible. Familiar semiconductors, such as silicon, are too vulnerable to oxidation reactions to be used in such photochemical applications. Work instead has focused on oxide semiconductors, such as titanium dioxide, which remain stable indefinitely in an oxygen-water environment. Reaction takes place at the wetted interface between the water and semiconductor.
To be practical, however, such semiconductor surfaces will require near-molecular-scale structuring. Because the electrons and holes can combine to form heat, nanostructuring of the surface is necessary to ensure reasonable charge separation. At present, “decorating” the surface with nanoparticles of a precious metal, such as platinum, is the favored method of ensuring charge separation, but obviously this increases both expense and complexity.
An ironic result of such technologies is that desert areas, with their year-round sunlight, could become major fuel production centers. In particular, the nations of the Middle East could continue exporting fuel indefinitely, albeit in competition with other deserts throughout the world.
Pollutant v. Resource
A fundamental technical problem involves separating one kind of atom or molecule from a background of others: pollutants from wastewater, metals from ores, salt from seawater. Separation is basic to purification, pollution control and resource extraction. Defining the process is a question of context: If we want what we separated, it is a resource; otherwise it is a pollutant.
Traditionally, however, separation has been viewed as the source of a host of different problems. In particular, researchers have seen resource extraction not only as distinct from pollution control, but also as intrinsically energy-intensive. In turn, its profligate energy usage is typically justified by vague appeals to the laws of thermodynamics.
Yet, quantitatively, element separation is not intrinsically expensive. Do not merely believe thermodynamic calculations: Bio-logical systems underscore how woefully inefficient conventional separation processes are, as they perform feats that put conventional resource extraction to shame.
Organisms do not carry out thermally driven phase separation. Instead, they literally move individual atoms or molecules, using specialized mechanisms — for example the binding of nutrient elements by specialized proteins. These molecular-scale processes are vastly less costly energetically and allow separation from considerably lower concentrations.
Plant
roots extract both nutrients and water at low concentrations from the ambient
soil. Vertebrate kidneys extract only certain solutes out of the blood from
a background of many other solutes. For photosynthesis, plants extract carbon
dioxide from the air, where its concentration is only about 350 parts per million,
and furthermore do so using only the diffuse and intermittent energy of sunlight.
Diatoms are particularly impressive, building shells from silica extracted at
parts-per-million levels from the ambient water.
Organic compounds called “crown ethers,” shown here schematically, could be key players in nanotechnology designed to extract metals. The ring, or “crown,” changes in structure by substituting differently sized atoms, such as potassium or lithium, for oxygen in the crown. Image is courtesy of Stephen L. Gillett.
Again, the reason why conventional resource extraction is so energy expensive is because it largely relies on vast quantities of heat, in this case to drive the partitioning of elements into coexisting phases. Not only do such processes require a lot of energy, but they also are intrinsically polluting, both due to the combustion necessary to generate the heat and because the separation is never complete. Moreover, byproducts containing geochemically abundant elements, such as iron in copper ores, are usually uneconomic and discarded as waste.
Thermal-based separation is also impractical for pollution control and purification. Of course, that’s why such problems are traditionally viewed as distinct from resource extraction. Indeed, a number of embryonic molecular separation technologies already exist whose development has largely been driven by addressing purification issues.
Extracting solutions
In their simplest form, molecular separation techniques require that the material being separated be free to flow as a gas or a liquid. Selectivity of the separation is also fundamental: Usually only one particular dissolved species is of interest, but it is dispersed in a background of many others. Sometimes the species is valuable (for example, palladium and lithium), whereas in other cases it is toxic (for example, lead and cadmium).
One particular set of approaches toward selective molecular separation has been the focus of a tremendous amount of research in recent decades. Such efforts involve molecules with branched and ring structures that can bind tightly and specifically only with certain solutes. For instance, a group of organic molecules called crown ethers are highly effective extraction agents for many metal ions.
Crown ethers are strongly selective. The ring, or “crown,” changes in structure by substituting differently sized atoms, such as nitrogen and sulfur, for oxygen in the crown. For example, the crown ether 18-crown-6 forms a strong complex with the potassium ion, which fits nicely into the ring, whereas the smaller ring of 12-crown-4 strongly binds with the lithium ion, but is too small to accommodate potassium.
One application of such a separation system is to tether the extraction agent to a substrate to form a highly selective surface for extracting particular solutes from solution. For example, researchers have used a substituted crown ether tethered to a silica surface to recover palladium from scrap catalytic converters dissolved in acid. The palladium is bound, while the other much more abundant metals remain in solution.
The major problem with such approaches to separation is that eventually the solute must release its captured ions to regenerate the extraction agent. Typically this takes extreme chemical measures. In the palladium recovery system, for example, highly concentrated acid must be used to flush out the palladium.
Such steps generate a much larger volume of wastewater that now becomes a serious disposal problem. Separation requiring washing with fluids can be practical for recovery of highly valuable commodities like palladium, but its applications are obviously limited.
So-called switchable binding provides a way to solve the problem: Under one set of circumstances, binding occurs, but changing some variable causes the solute to unbind again. Again, biology has anticipated technology. Hemoglobin, for example, binds strongly to oxygen in the lungs, but under different chemical conditions elsewhere in the body, it gives up the oxygen to the tissues.
An example of switchable binding is “electrosorption,” which is based on straightforward principles of attraction and repulsion. Charging an electrode attracts ions with the opposite charge; reversing the charge of the electrode desorbs the ions again. Although first proposed in the 1960s for desalination, electrosorption remained impractical until the recent advent of nanostructured electrodes with very high surface areas. Because the “filled up” electrodes look like a charged capacitor, too, a great deal of the electrical energy can be recovered when the ions are desorbed.
More selective approaches require more molecular-scale structuring. For example, researchers have patented a process for extracting lithium ion from brine that uses electrodes made of a form of crystalline manganese dioxide. In the process, the electrode becomes negative, which leaves the crystal with an overall negative charge, so positive lithium cations are drawn into tunnels in the structure to compensate. Lithium cations can fit into these tunnels, whereas larger cations cannot. Reversing the charge on the manganese dioxide electrode then expels the lithium.
A similar system for extracting cesium ion is based on cesium nickel hexacyanoferrate. Here, the crystal structure contains large cavities that can accommodate the big cesium ion. Again, on applying a negative voltage, cesium cations are drawn in to compensate. The system is of great interest for extracting highly radioactive cesium-137 from nuclear waste.
An alternative potential trigger for switching binding is light. One way is to use molecules that change their structure upon absorbing a photon. The “backbone” of spiropyrans, a specialized class of molecules, for example, rearranges so drastically that a solution containing the molecule actually changes color when illuminated. Strategically arranging the extracting groups on the backbone can make the molecule go from binding solutes in its ground state to releasing them upon illumination.
A different approach uses the absorption of light by a semiconductor surface, but such systems are nascent. In this case, the photogenerated electric charge would drive molecular mechanisms at the surface. For example, a surface might adsorb ions from a solution in the dark, but desorb those ions when illuminated. Merely shining sunlight on a surface to desorb its solutes would obviously be much cleaner and “greener” than flushing it with strong acid solutions!
Blurring the lines
At present, pollution control and purification are the key economic drivers for these separation technologies. As they mature, however, they will blur the distinction between a “pollutant” and a “resource.” Moreover, recovered pollutants will begin to have an impact on resource extraction. After all, copper extracted from a wastewater stream is copper that does not have to be mined.
Ultimately, a great many aqueous solutions, of both natural and artificial origin, will become nontraditional resources. Wastewater streams, acid-mine drainage, seawater, concentrated natural brines such as those in oilfields or saline lakes — sometimes viewed now as problems — all could become potential sources of materials with the help of nanotechnology.
Gillett holds a Ph.D. in geology from SUNY Stony Brook and has worked on applying paleomagnetic techniques in resource applications. He is now with Nanotechnologies, Inc., involved in several startup ventures on the application of “proto-” nanotechnology to resource issues.
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