Engineering Aluminum Nanocomposites for Sensing, Photocatalysis and Photothermal Conversion
Halas, Naomi J
Doctor of Philosophy
As the field of plasmonics continues to expand, researchers are seeking for other possibilities beyond noble metals, and an alternative plasmonic material that has tremendous potential is aluminum (Al). This thesis focuses on the study of both fundamental and practical aspects of surface plasmon excitations in Al nanostructures. Al, with its low cost, abundance, and better optical tunability compared to noble metals, has demonstrated its potential in sensing, photocatalysis, optoelectronics, and many other areas. The world of Al plasmonics has been greatly expanded with the development of Al nanocrystals (NCs) synthesis. The Al NCs have a native oxide layer which provides more possibilities of chemical bonding. We have demonstrated the potential use of Al NC aggregates as a plasmonic substrate for surface-enhanced Raman spectroscopy (SERS). The native oxide layer serves as a valuable linker between molecules and substrate, prohibiting non-specific adsorption on the Al NC surface. Al NC aggregates, as synthesized, are SERS substrates that enable the first quantitative label-free detection of ssDNA with no modification to either the ssDNA or the substrate surface. Besides the external field enhancement, the internal field induced hot carrier generation is also investigated. Al NCs generate hot carriers at plasmon resonance, as well as interband transitions. However, the lifetime of hot carriers is on the order of picoseconds before they decay into heat. Instead, we developed Al@TiO2 core-shell nanoparticles as antenna-reactor with efficient hot carrier generation and excellent photocatalytic performance. Analysis of the Al-doped TiO2 interlayer in Al@TiO2 core-shell heterostructure greatly extends our knowledge on the interface at the nanoscale. Unlike the native oxide layer of Al NCs, this interlayer does not block the hot carrier transfer pathway. Instead, it enables direct contact between Al nanoantenna and TiO2 reactor, where the aligned Fermi energy level allows almost barrierless charge transfer. We demonstrate experimentally that Al@TiO2 nanoparticles can drive the photoreduction of 4-nitrophenol. By comparing wavelength-dependent results with the simulated hot carrier generation, we conclude that the photocatalytic reactivity is generated from plasmonic Al nanoantenna under visible, even near IR illumination. The combination of Al and TiO2 presented in this thesis is a new demonstration of antenna-reactor geometry for plasmon-induced photocatalysis with low cost and promising large-scale industrial applications. We further investigated the optical and photothermal properties of small Al NCs with a plasmon resonance in UV region. The color of UV absorbing solution is almost colorless due to little interaction with visible light. However, the color of Al NCs solution is observed to change from almost colorless to totally black with increasing concentration. The simulation results indicate that besides the dipolar plasmon resonance in UV, the Al NCs also serves as a pure absorber in the visible to near IR spectral region. This is because of the larger imaginary part of dielectric function of Al in the visible range, which makes Al NCs a great candidate in photothermal applications. In order to investigate the photothermal performance of Al NCs in aqueous environment, a silica layer is coated with controlled thickness to improve their water stability. The photothermal conversion measurements shows the temperature increase both at the laser spot and in bulk, demonstrating the absorber nature of Al@SiO2 nanoparticles. The photothermal conversion efficiency reaches 54.67% under 800 nm laser illumination, which make Al@SiO2 a low cost but efficient candidate for solar applications. In summary, we have investigated the plasmonic properties of Al in three aspects: hot spots induced near field enhancement, hot carrier generation followed by photocatalysis, and the photothermal conversion. These observations and results, both experimental and theoretical, have demonstrated that Al NCs, along with its nanocomposites, are promising candidates for many different areas of plasmonics.
Plasmonics; SERS; Photocatalysis; Photothermal