One solution [to the problem of Cu oxidation] is to alloy Cu with stabilizing metals...Alloys with Au are desirable because bulk Au and Cu form a solid solution, so alloy composition is tunable, enabling optimization of optical and/or catalytic properties. A few of solution-based approaches for synthesizing AuCu NPs have been reported, but studies in how composition, structure, and size affect NP activity and optical properties have been limited. Because oxidation of Cu is facile, and can be accelerated in high surface area systems like NPs, an important property that needs to be understood is the stability of AuCu NPs with respect to oxidation, which has not been reported.

Here we study the oxidation of AuCu NPs as a function of composition. We find that the Cu oxidation rate depends on NP composition, where increasing Au% improves stability and alloying Au with Cu potentially lowers the overpotential for CO₂ reduction.

—Xu et al.

A paper detailing the results is in press in the RSC journal Chemical Communications; the research was funded by the National Science Foundation. Co-author Kimberly Hamad-Schifferli of MIT says the findings point to a potentially energy-efficient means of reducing carbon dioxide emissions from powerplants.

Hamad-Schifferli worked with Yang Shao-Horn, the Gail E. Kendall Associate Professor of Mechanical Engineering at MIT, postdoc Zichuan Xu and Erica Lai ‘14. To make the nanoparticles, Hamad-Schifferli and her colleagues mixed salts containing gold into a solution of copper salts. They heated the solution, creating nanoparticles that fused copper with gold. Xu then put the nanoparticles through a series of reactions, turning the solution into a powder that was used to coat a small electrode.

To test the nanoparticles’ reactivity, Xu placed the electrode in a beaker of solution and bubbled carbon dioxide into it. He applied a small voltage to the electrode, and measured the resulting current in the solution. The team reasoned that the resulting current would indicate how efficiently the nanoparticles were reacting with the gas: If CO₂ molecules were reacting with sites on the electrode mdash;and then releasing to allow other CO₂ molecules to react with the same sites mdash;the current would appear as a certain potential was reached, indicating regular turnover. If the molecules monopolized sites on the electrode, the reaction would slow down, delaying the appearance of the current at the same potential.

The team ultimately found that the potential applied to reach a steady current was much smaller for hybrid copper-gold nanoparticles than for pure copper and gold—an indication that the amount of energy required to run the reaction was much lower than that required when using nanoparticles made of pure copper.

Electrochemical CO₂ reduction is a complex process producing various surface and solution products, and faradic efficiency of certain products such as methane and methanol is typically used to evaluate catalyst selectivity and activity. While CV cannot definitively evaluate activity, the positive-shift of the onset potential indicates that alloying Au with Cu has potential to lower the energy used for the electrochemical CO₂ reduction, which must be considered in catalyst evaluation.

These results show that NP stability can be tuned by composition, which could impact applications using Au_xCu_y alloys, particularly catalysis and optical applications.

—Xu et al.

Going forward, Hamad-Schifferli says she hopes to look more closely at the structure of the gold-copper nanoparticles to find an optimal configuration for converting carbon dioxide. So far, the team has demonstrated the effectiveness of nanoparticles composed of one-third gold and two-thirds copper, as well as two-thirds gold and one-third copper.

Resources

• Z. Xu, E. Lai, Y. Shao-Horn, and K. Hamad-Schifferli (2012) Compositional dependence of the stability of AuCu alloy nanoparticles. ChemComm, in press.