Above: Optical setup used in an experiment to study the aluminum chemistry in water.

Scientists gain a new perspective in understanding the chemistry behind aluminum

February 2014 - If it works, go with it—but why? Why does a material work the way it does, in this case, aluminum? A detailed understanding of this metal’s water-based forms proved elusive until recently, when researchers unlocked a new world of potential for this ductile material. Now the metal complex can be studied at a molecular level, creating possibilities for advances in numerous industries. 

After more than 100 years of commercial use without a breakdown of aluminum at a molecular level, this latest breakthrough is significant. “Suddenly we have this solution-based, bottom-up approach starting from the molecular-level construct and that’s how cool this is—scientists have developed these protocols to perform targeted synthesis of aluminum hydroxide clusters (Al13) in water, which can be readily used as a ‘green’ solution precursor for large-scale preparation of aluminum oxide thin films and nanoparticles for electronics, catalysis, photovoltaics and corrosion prevention,” says Chong Fang, assistant professor at Oregon State University’s Department of Chemistry. Oregon State, the University of Oregon and the Center for Sustainable Materials Chemistry, have combined efforts to work on the metal’s breakdown. 

Fang is one of nine researchers involved in the collaborative report, “Electrolytic synthesis of aqueous aluminum nanoclusters and in situ characterization by femtosecond Raman spectroscopy and computations.” Published in Proceedings of the National Academy of Sciences, the report lays out how a new electrolysis method is used to control the solution pH and counter-ion content precisely during aqueous cluster synthesis without steep pH gradients commonly associated with base titrations. 

“To achieve the atom- and step-economical electrolytic synthesis of aqueous aluminum clusters, we used an aluminum nitrate solution as the single reagent, which is cheap and widely available,” Fang says. “If you start from that and end up making something usable in everyday life—the implications could be huge for industry applications.” 

Fang points out the automotive industry may benefit from this research in particular as part of the uncovering includes an effective way to create scalable thin films with precision growth control—films that have the potential to strengthen already available aluminum. Once a thin film of aluminum oxide (Al2O3) is applied as a coating on an aluminum part using a step-economical solution process, the material hardness will be increased and corrosion can be prevented.

Bottom-up approach

Until now, there has been no effective way to study aluminum. “Before, researchers used a limited arsenal of tools to characterize certain types of aluminum species, but usually with limited access to detailed molecular-level structure and dynamics,” Fang says.

After silicon, aluminum is the second most abundant metallic element in the Earth’s crust, according to the U.S. Geological Survey. While plentiful in supply, it has only been produced for commercial use for a little more than 100 years. At one point in the mid-1800s, aluminum was harder to find and considered more valuable than gold. Today, however, aluminum is produced on a massive scale, with five companies operating 10 primary aluminum smelters in the U.S. The USGS reported the value of primary domestic metal production in 2012 was $4.32 billion. 

Aluminum is obtained from bauxite, which contains a mixture of hydrous aluminum oxides difficult to extract from ore because of its high reactivity and high melting point. Producing aluminum is labor and resource intensive. Four tons of bauxite are required to produce two tons of alumina, resulting in one ton of aluminum, according to The Aluminum Association. Over the years, aluminum producers have reduced the amount of energy consumption per unit of aluminum by 70 percent and perfluorocompounds, which are greenhouse gas emissions of perfluorocarbons, trifluoromethane (CHF3), nitrogen trifluoride (NF3) and sulfur hexafluoride (SF6), associated with primary aluminum, are also down 70 percent since 1995. 

While expensive, this lightweight, recyclable metal is used in a wide range of applications. Recycled aluminum is cheaper than alumnium produced from ore and promotes sustainability. It can be recycled over and over again without losing quality, according to The Aluminum Association, which also notes 75 percent of all aluminum ever smelted is still in use today.

Complementary uses

A silicon-based solar cell is typically used when working with aluminum films. Such films are useful in energy applications, particularly within the solar power sector. “It has been demonstrated that coating a thin aluminum-based film on top of a semiconductor can actually increase the light trapping and harvest sunlight more effectively. A good example is that Al2O3 thin film has gained popularity as a high-quality surface passivation material,” Fang says. 

Implications for this breakthrough in aluminum research go even further. When broken down, researchers were able to produce flat Al13 hydroxide clusters involving only octahedrally coordinated aluminum instead of what was previously achieved. 

Being able to efficiently produce these flat Al13 clusters is the key to using them as a solution precursor to make aluminum-based films. “I don’t look at this research as simply finding a new way to produce aluminum species,” Fang says. “Rather, we are interested in selectively synthesizing aluminum nanoclusters and unraveling their formation pathways.” 

By uncovering this new approach to aluminum nanoclusters, Fang says the future holds new ways to generate new species of aluminum. “As far as we know, people have not extensively explored this territory using solution-based aluminum speciation, where you easily encounter a variety of complex potentials, different compositions,” he says. “Once water goes away after dehydration at an elevated pH, then you’re left with versatile metal oxide films. Those are the products that hold transformative impact as society shifts toward more sustainable, greener, cheaper ways of getting and using energy.” MM