Browsing by Author "Tian, Shu"

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    Engineering Aluminum Nanocomposites for Sensing, Photocatalysis and Photothermal Conversion
    (2019-12-18) Tian, Shu; Halas, Naomi J
    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.
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    Metal-organic frameworks tailor the properties of aluminum nanocrystals
    (AAAS, 2019) Robatjazi, Hossein; Weinberg, Daniel; Swearer, Dayne F.; Jacobson, Christian; Zhang, Ming; Tian, Shu; Zhou, Linan; Nordlander, Peter; Halas, Naomi J.
    Metal-organic frameworks (MOFs) and metal nanoparticles are two classes of materials that have received considerable recent attention, each for controlling chemical reactivities, albeit in very different ways. Here, we report the growth of MOF shell layers surrounding aluminum nanocrystals (Al NCs), an Earth-abundant metal with energetic, plasmonic, and photocatalytic properties. The MOF shell growth proceeds by means of dissolution-and-growth chemistry that uses the intrinsic surface oxide of the NC to obtain the Al3+ ions accommodated into the MOF nodes. Changes in the Al NC plasmon resonance provide an intrinsic optical probe of its dissolution and growth kinetics. This same chemistry enables a highly controlled oxidation of the Al NCs, providing a precise method for reducing NC size in a shape-preserving manner. The MOF shell encapsulation of the Al NCs results in increased efficiencies for plasmon-enhanced photocatalysis, which is observed for the hydrogen-deuterium exchange and reverse water-gas shift reactions.
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    Polydopamine-Stabilized Aluminum Nanocrystals: Aqueous Stability and Benzo[a]pyrene Detection
    (American Chemical Society, 2019) Renard, David; Tian, Shu; Ahmadivand, Arash; DeSantis, Christopher J.; Clark, Benjamin D.; Nordlander, Peter; Halas, Naomi J.; Laboratory for Nanophotonics
    Aluminum nanocrystals have emerged as an earth-abundant material for plasmonics applications. Al nanocrystals readily oxidize in aqueous-based solutions, however, transforming into highly stratified γ-AlOOH nanoparticles with a 700% increase in surface area in a matter of minutes. Here we show that by functionalizing Al nanocrystals with the bioinspired polymer polydopamine, their stability in aqueous media is dramatically increased, maintaining their integrity in aqueous solution for over 2 weeks with no discernible structural changes. Polydopamine functionalization also provides a molecular capture layer that enables the capture of polycyclic aromatic hydrocarbon pollutants in H2O samples and their detection by surface-enhanced Raman spectroscopy, when polydopamine-stabilized Al nanocrystal aggregates are used as substrates. This approach was used to detect a prime carcinogenic H2O pollutant, benzo[a]pyrene with a sensitivity in the sub part-per-billion range.