Two of the major research aims in the Aikens group involve developing a theoretical understanding of the relationship between structure and properties of nanomaterials. These research areas include the structure and spectroscopic properties of gold and silver nanoparticles and nanoparticle arrays and the reactivity of nanostructured metal oxide particles.
Structure and Properties of Noble Metal Nanoparticles
Nanoparticles have a sharp peak in their absorption spectrum called a surface plasmon resonance (SPR) band. This band depends sensitively on the size, shape, and composition of these nanoparticles, and thus absorption characteristics can be tuned by adjusting these parameters. Gold nanorods have very tunable absorption properties and are finding application in cancer therapy. Other gold nanoparticles have been used for decades as biolabels. In addition, the SPR band is useful in techniques such as surface-enhanced Raman scattering (SERS) and LSPR sensing which can detect compounds at low concentration.
The Aikens group has examined the optical absorption spectrum of Au25(SR)18-,1 which is one of the few gold nanoparticles whose crystal structure has been determined. One of the interesting characteristics about gold nanoparticles in the 1-2 nm size regime like Au25(SR)18- is that their optical absorption spectra consist of a series of molecular-like peaks, rather than the single SPR band of larger nanoparticles. Silver nanoparticles also show similar discrete spectra and might be useful in light-harvesting applications.2,3 In addition to the anionic charge state, the Au25 nanoparticle exists in a neutral form which is paramagnetic and gives an axial signal in its electron paramagnetic resonance (EPR) spectrum due to occupation of a delocalized P-like orbital by the unpaired electron.4 The Aikens group is also examining the complicated chiroptical properties of gold nanoparticles.
In addition to monolayer-protected nanoparticles such as Au25, the Aikens group is interested in the optical absorption of highly symmetric bare nanoparticles. Pentagonal silver nanorods have two sharp peaks in their absorption spectra (which correspond to longitudinal and transverse excitations),5 while silver tetrahedra have only a single peak.6 In nanoparticle arrays such as silver nanorod dimers the plasmon resonances couple, which means that construction of materials with multiple interacting nanoparticles is another way to control and tune material properties. In general, the smaller nanoparticles examined by Prof. Aikens follow well-established trends for larger nanoparticles, which suggests that the symmetry of the system is more important than the size.
This work is currently funded by Kansas State University and the Air Force Office of Scientific Research.
Catalysis and Reactivity on Nanostructured Metal Oxide Surfaces and Clusters
Bioinspired Water Splitting Catalysts
Development of renewable and clean sources of energy is one of the most pressing needs of the 21st century. Hydrogen production via water splitting has the potential to provide a clean alternative to fossil fuels. The best catalyst currently known for the water splitting reaction is the oxygen-evolving complex of Photosystem II, which contains a manganese-calcium oxide core. The Aikens group is examining aspects of this system that make it a good water splitting catalyst in order to aid in the development of synthetic catalysts with similar efficiencies.
This work is funded by the National Science Foundation (CAREER).
Photocatalytic oxidation of volatile organic compounds
Volatile organic compounds (VOCs) are organic compounds with high vapor pressures, which means they can be present in significant concentrations in air at room temperature. Many VOCs are toxic, so removal of VOCs from air is important for improving environmental quality. Metal oxide materials are potentially good photocatalysts for the destruction of VOCs by oxidation to carbon dioxide and water. However, the relationship between catalyst structure, properties, and reactivity is not fully understood. The Aikens group is collaborating with the Klabunde group at Kansas State University to better understand the photocatalytic mechanisms involved in this process.
In order to develop a sustainable world, our current dependence on fossil fuels must be diminished. Because many of the polymers we use are derived from fossil fuels, replacement of petroleum-based monomers with biorenewable monomers is one possible way to decrease our reliance on nonrenewable resources. Polymers derived from biomass could potentially replace some petroleum-derived polymers, but fundamental research is required before this is viable. Starting materials derived from biomass typically possess a greater number of carboxylic acid and hydroxyl functional groups than petroleum-derived products. In consequence, these functional groups interact differently with catalyst surfaces than do hydrocarbons. Understanding the interactions of functional groups with catalyst surfaces is the first step to controlling the transformation of biomolecules into value-added chemicals. The Aikens group is currently interested in understanding how lactic acid binds to metal oxide surfaces. (This work is funded by the American Chemical Society Petroleum Research Fund.) The Aikens group collaborates with other researchers in the Center for Biobased Polymers by Design (CBPD) at Kansas State University.
1. Correlating the Crystal Structure of a Thiol-Protected Au25 Cluster and Optical Properties. M. Zhu, C. M. Aikens, F. J. Hollander, G. C. Schatz, R. Jin, J. Am. Chem. Soc., 2008, 130, 5883-5885.
2. Silver Nanoparticles with Broad Multi-Band Linear Optical Absorption. O. Bakr, V. Amendola, C. M. Aikens, W. Wenseleers, R. Lee, L. Dal Negro, G. C. Schatz, F. Stellacci, Angew. Chem. Int. Ed., 2009, 48, 5921-5926.
3. Origin of Discrete Optical Absorption Spectra of M25(SH)18- Nanoparticles (M = Au, Ag). C. M. Aikens, J. Phys. Chem. C, 2008, 112, 19797-19800.
4. Reversible Switching of Magnetism in Thiolate-Protected Au25 Superatoms. M. Zhu, C. M. Aikens, M. P. Hendrich, R. Gupta, H. Qian,G. C. Schatz, R. Jin, J. Am. Chem. Soc., 2009, 131, 2490-2492.
5. Electronic Structure and TDDFT Optical Absorption Spectra of Silver Nanorods. H. E. Johnson, C. M. Aikens, J. Phys. Chem. A, 2009, 113, 4445-4450.
6. From Discrete Electronic States to Plasmons: TDDFT Optical Absorption Properties of Agn (n = 10, 20, 35, 56, 84, 120) Tetrahedral Clusters. C. M. Aikens, S. Li, G. C. Schatz, J. Phys. Chem. C, 2008, 112, 11272-11279.