The localized surface plasmon resonance (LSPR) properties of nanocrystals (NCs) allow manipulation of optical responses by controlling their morphology, free carrier density, and local dielectric environment. In this context, semiconductor NCs, in which plasmonic properties arise due to various types of doping, provide unique opportunities in tailoring LSPR properties for a wide range of applications as viable alternatives to expensive noble metal NCs. Although extensive works have been done to control the LSPR properties of semiconductor NCs via doping, the role of surface ligand chemistry in the enhancement of LSPR properties remains poorly understood. Incomplete passivation of surface atoms creates dangling bonds and surface trap states that together could compromise the free carrier density and thus optoelectronic properties. Here, we report the impact of metal–ligand bonding interactions on the free electron density (N e) and the LSPR response of monoclinic, sub-stoichiometric, and two-dimensional tungsten oxide (WO3–x ) nanoplatelets (NPLs). The LSPR properties of WO3–x NPLs arise from the presence of free electrons in the conduction band as a result of oxygen vacancies in the monoclinic crystal. In situ surface passivation of unpurified colloidal WO3–x NPLs with X-type alkylphosphonate (R-PO3 2–) produces an LSPR peak in the near-infrared region of the electromagnetic spectrum. X-ray photoelectron, electron paramagnetic, and Raman spectroscopic data support the presence of a tridentate PO3–W3 bonding motif that allows increased passivation of shallow surface trap states, leading to an experimentally determined N e value of 8.4 × 1022 cm–3. Furthermore, experimentally determined bonding characteristics are correlated with density functional theory calculations. The effect of the high N e values of NPLs on their refractive index sensitivity is also evaluated. Together, the knowledge gained regarding surface-ligand-chemistry-controlled manipulation of the plasmonic properties in semiconducting metal oxide NPLs and the high N e values of WO3–x NPLs achieved may result in sizable advancement in various LSPR-driven applications such as sensing and energy storage and conversion schemes.
More than half of the world is fed and fueled through the Haber-Bosch process, and ammonia consumption and global population continue to grow in tandem. However, resulting nitrogen oxyanion (NOx) waste threatens aquatic ecosystems by feeding harmful algal blooms. Molecular transition metal electrocatalysts are poised to selectively reduce these NOx waste products: macrocyclic complexes are particularly robust to electrocatalytic N–O bond activations. Here, two cobalt-based macrocycles with redox-active ligand frameworks prove capable of reducing nitrate and nitrite, with an intriguing dependence on surface interactions. With an applied potential under monochromatic irradiation, a cobalt(III)-based diiminomethyl macrocycle, [Co(DIM)Br2]+, can more efficiently reduce NO2 – to ammonia by bypassing an auto-destructive ligand-based radical. Similarly, an intermediate with ligand-radical character is active toward NO3 – reduction in cobalt(II) tetraphenylporphyrin (CoTPP) in bulk systems, but can be stabilized in aqueous conditions for more affordable onset potentials once immobilized. Inspired by surface influence, homogeneous and heterogeneous systems are blended into molecular catalyst-support assemblies: physisorbing CoTPP on nanosupports imparts catalyst durability and nitrate electroreduction in coordinating buffers. These hybrid assemblies remarkably improve onset potentials in previously inaccessible conditions, ultimately overcoming many compromises between overpotential and turnover in purely homogeneous designs. Figure 1
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