Harnessing hot carriers from photoexcited metallic nanoparticles for catalysis is very challenging because these carriers have extremely short lifetimes. Here, we demonstrate that smaller particles have higher surface-to-volume ratios that allow hot carriers to diffuse to particle surfaces with a higher probability and thereby exhibit higher photocatalytic activities as quantified by quantum yields. The measured photocatalytic activities for photoinduced etching of gold nanospheres by FeCl3, and the previously unreported aqueous hydrogenation of styrene using sodium borohydride under interband excitation show perfect dependence on the reciprocal of particle size. The size-dependent photocatalytic activity for photoinduced etching of gold nanospheres by FeCl3 under plasmon excitation, however, slightly deviates from this scaling law and may be influenced by other factors such as the surface field enhancement effect. This scaling law is expected to apply to other nanomaterial-based photocatalysts that rely on hot carrier diffusion to a surface for catalysis. Future design of nanomaterials for the harnessing of hot carriers for catalysis should take this scaling law into account.

Metallic nanoparticle-based photocatalysts have gained a lot of interest in catalyzing oxidation–reduction reactions. In previous studies, the poor performance of these catalysts is partly due to their operation that relies on picosecond-lifetime hot carriers. In this work, electrons that accumulate at a photostationary state, generated by photocharging the catalysts, have a much longer lifetime for catalysis. This approach makes it possible to determine and tune the photoredox potentials of the catalysts. As demonstrated in a model reaction, the photostationary state of the photocatalyzed oxidative etching of colloidal gold nanoparticles using FeCl3 was established under continuous irradiation of different wavelengths. The photoredox potentials of the nanoparticles were then calculated using the Nernst equation. The potentials can be tuned to a range of 1.28 to 1.40 V (vs SHE) under irradiation of different wavelengths in the range of 450 to 517 nm. The effects of particle size or optical power on the photoredox potentials are small compared to the wavelength effect. Control over the photoredox potential of the particles using different excitation wavelengths can potentially be used to tune the activities and selectivities of metallic nanoparticle photocatalysts.

According to L-edge sum rules, the number of 3d vacancies at a transition metal site is directly proportional to the integrated intensity of the L-edge X-ray absorption spectrum (XAS) for the corresponding metal complex. In this study, the numbers of 3d holes are characterized quantitatively or semi-quantitatively for a series of manganese (Mn) and nickel (Ni) complexes, including the electron configurations 3d→ 3d. In addition, extremely dilute (<0.1% wt/wt) Ni enzymes were examined by two different approaches: (1) by using a high resolution superconducting tunnel junction X-ray detector to obtain XAS spectra with a very high signal-to-noise ratio, especially in the non-variant edge jump region; and (2) by adding an inert tracer to the sample that provides a prominent spectral feature to replace the weak edge jump for intensity normalization. In this publication, we present for the first time: (1) L-edge sum rule analysis for a series of Mn and Ni complexes that include electron configurations from an open shell 3d to a closed shell 3d; (2) a systematic analysis on the uncertainties, especially on that from the edge jump, which was missing in all previous reports; (3) a clearly-resolved edge jump between pre-L and post-L regions from an extremely dilute sample; (4) an evaluation of an alternative normalization standard for L-edge sum rule analysis. XAS from two copper (Cu) proteins measured using a conventional semiconductor X-ray detector are also repeated as bridges between Ni complexes and dilute Ni enzymes. The differences between measuring 1% Cu enzymes and measuring <0.1% Ni enzymes are compared and discussed. This study extends L-edge sum rule analysis to virtually any 3d metal complex and any dilute biological samples that contain 3d metals.

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