Aqueous aluminum nanoclusters

Dr. Neil Canter, Contributing Editor | TLT Tech Beat February 2014

A new analytical technique monitors the synthesis of a flat aluminum nanocluster.

KEY CONCEPTS
• When aluminum is added to water, a variety of complex species that include monomeric, oligomeric and polymeric hydroxides are formed.
• A new technique known as Fermtosecond Stimulated Raman Spectroscopy shows how monomeric aluminum in water can be converted through an electrochemical process into a flat aluminum nanocluster known as Al₁₃ by slowly increasing the pH over a set range.
• Work is now underway to better understand the types of aluminum clusters and species that form in aqueous solution at pH values above 7.

ALUMINUM IS CONTINUING TO be an important metal used in the manufacture of automobiles. Its lighter weight (as compared to steel alloys), good strength and ability to elongate are important factors that enable automobiles to be produced with higher levels of fuel economy.

But aluminum does not have the mechanical strength of steel. In a previous TLT article, a new process known as high-pressure torsion was discussed that increases the strength of aluminum to a level comparable to carbon steel without sacrificing ductility (1). A well-known alloy, 7075 aluminum, was solution treated at 480 C for five hours followed by quenching in room temperature water. The resulting metal was found to display a strength of 1.0 GPa in a tensile strength test, which is comparable to a typical hardened and tempered carbon-steel alloy.

Aluminum is fabricated into components used in automobiles through a series of metalworking operations that occur mainly with water-based fluids. There are a number of challenges in finding optimum machining conditions for specific aluminum alloys.

But one of the intriguing issues is what happens to the aluminum alloy when it comes into contact with water, which is the primary component in a water-based metalworking fluid. Aluminum can readily form a series of metal salts with other additives used in MWFs such as fatty acids. These salts can become water insoluble and form residues that are similar to greases. Such contaminants are undesirable because they can degrade the performance of the MWF.

Chong Fang, assistant professor of chemistry at Oregon State University in Corvallis, Ore., says, “Addition of aluminum to water leads to the formation of a variety of complex species that include monomeric, oligomeric and polymeric hydroxides. These species are present in water as colloidal solutions and gels, but they can also form precipitates and crystals.”

Gaining a better understanding of the composition of these species is extremely difficult. Fang says, “Many of these species cannot be readily identified because they are difficult to detect using techniques such as 27Al nuclear magnetic resonance (NMR) and conventional Raman spectroscopy. The problem is water binds in many different positions with respect to aluminum, leading to the formation of different types of highly coordinated structures, and there may be transient species involved. The elucidation of aqueous aluminum speciation pathways demands a technique capable of monitoring molecular choreography.”

Some of these aluminum water species are known as hydroxide clusters that contain multiple aluminum atoms. Fang says, “Formation of aluminum clusters is dependent on factors such as reagent concentration and the method and rate of solution pH change.”

If specific aluminum clusters can be selectively synthesized, then these clusters can be studied to gain an understanding of their respective properties and how they may form when water contacts aluminum metal. One specific “flat” aluminum cluster has now been synthesized through a pH-controlled process monitored by a novel analytical technique.

FEMTOSECOND RAMAN SPECTROSCOPY
Fang and his fellow researchers synthesized an aqueous aluminum nanocluster known as Al₁₃ by slowly raising the pH of a solution and following the reaction using an emerging technique known as Femtosecond Stimulated Raman Spectroscopy (FSRS). He says, “We chose to produce Al₁₃ because this species represents a naturally occurring mineral that is octahedral in configuration. We have also pioneered a novel technique that enables thin metal-oxide films that are a few atomic layers thick to be prepared directly from solution instead of using more expensive methods. This integrated platform will enable Al₁₃ potentially to be used as a green solution in broad applications such as transistors, solar energy cells, catalytic converters and corrosion inhibitors.”

The researchers used an electrochemical process to slowly and precisely raise the pH of the reaction mixture to produce Al₁₃. Fang says, “In Stage I, we started at a pH of 2.2 where the dominant aluminum species prepared from a 1 molar aluminum nitrate solution is the monomeric aluminum hexa-aqua ion.”

The solution is placed in a two-compartment electrochemical cell, which contains an anode compartment and a cathode compartment. Nitrate ions migrate into the anode compartment where oxygen is produced. Aluminum ions migrate into the cathode compartment where hydrogen is produced. The charge balance is maintained. An electric current is used to control the process, which exhibits a net reduction in proton (hydrogen ions) concentration in the cathode compartment as the pH is slowly increased, wherein condensation of aluminum species occurs to produce larger aluminum nanoclusters.

FSRS was used to follow the reaction because of the limitation of conventional Raman spectroscopy. Fang says, “We needed to detect small changes in Raman vibrational modes down to between 300 and 500 cm-1. Unfortunately, this frequency is too close to the fundamental pulse. Instead, we used non-resonant (800 nanometer) FSRS spectroscopy with a newly developed Raman probe pulse based on our photonic advances to cover that spectral range.”

FSRS reveals that the reaction moves to stage II at a pH between 2.4 and 2.7 due to the formation of an intermediate identified as Al₇. Fang says, “As the pH increases to between 2.7 and 3.2, further deprotonation strips positive charges at the outer shell of Al₇, leading to the formation of the larger Al₁₃ cluster, which represents Stage III of the process. The key is to catch a glimpse of aluminum speciation as the chemistry proceeds in water.”

Figure 3 shows the two-compartment electrochemical cell and the reaction process as it moves from monomeric aluminum in Stage I to Al₁₃ Stage III via an octahedrally coordinated Al₇ intermediate in Stage II.


Figure 3. A study using a new technique known as Femtosecond Stimulated Raman Spectroscopy has enabled researchers to produce an aqueous aluminum nanocluster known as Al₁₃, and holds the promise for identifying other aluminum species present in water in the future. (Courtesy of Oregon State University)

The researchers deliberately ran this reaction sequence at a low pH because the involving aluminum clusters could be identified using FSRS aided by computations, and they represent the onset of larger aluminum cluster formation. Fang says, “Work is underway to characterize the different types of clusters and species that form in aqueous solution at pH values above 7. This effort might also bring us closer to the regime where dehydration and annealing yield metal oxide thin films with versatility.”

This work is also of interest to formulators of MWFs because they are designed to operate at a pH of 9. Potentially, the aluminum clusters identified at this alkaline pH may help formulators better understand how to prepare products that will minimize such concerns as staining.

Additional information can be found in a recent article (2) or by contacting Dr. Fang at chong.fang@oregonstate.edu.

REFERENCES
1. Canter, N. (2011), “Super-Strong, Ductile Aluminum,” TLT, 67 (1), pp. 10-11.
2. Wang, W., Liu, W., Chang, I., Wills, L., Zakharov, L., Boettcher, S., Cheong, P., Fang, C. and Keszler, D. (2013), “Electrolytic Synthesis of Aqueous Aluminum Nanoclusters and In Situ Characterization by Femtosecond Raman Spectroscopy and Computations,” Proc. Natl. Acad. Sci. U.S.A. 110 (46), pp. 18397-18401.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.