Understanding interactions between manganese oxide and gold that lead to enhanced activity for electrocatalytic water oxidation.

To develop active nonprecious metal-based electrocatalysts for the oxygen evolution reaction (OER), a limiting reaction in several emerging renewable energy technologies, a deeper understanding of the activity of the first row transition metal oxides is needed. Previous studies of these catalysts have reported conflicting results on the influence of noble metal supports on the OER activity of the transition metal oxides. Our study aims to clarify the interactions between a transition metal oxide catalyst and its metal support in turning over this reaction. To achieve this goal, we examine a catalytic system comprising nanoparticulate Au, a common electrocatalytic support, and nanoparticulate MnO(x), a promising OER catalyst. We conclusively demonstrate that adding Au to MnO(x) significantly enhances OER activity relative to MnO(x) in the absence of Au, producing an order of magnitude higher turnover frequency (TOF) than the TOF of the best pure MnO(x) catalysts reported to date. We also provide evidence that it is a local rather than bulk interaction between Au and MnO(x) that leads to the observed enhancement in the OER activity. Engineering improvements in nonprecious metal-based catalysts by the addition of Au or other noble metals could still represent a scalable catalyst as even trace amounts of Au are shown to lead a significant enhancement in the OER activity of MnO(x).

Interfacial alloy hydride destabilization in Mg/Pd thin films.

Recently, a large increase in the equilibrium hydrogen pressure has been reported for MG thin films capped with a Pd layer. We show that this increase is due to intermixing of Mg and Pd, as opposed to a strain effect as previously claimed. Transmission electron microscopy and depth profiling x-ray photoemission spectroscopy are used to directly measure interfacial intermixing between Mg and Pd, and we find that intermixing and equilibrium hydrogen pressure both increase with annealing. We present a thermodynamic model of the effect of alloying on equilibrium pressure, and find that the observed equilibrium pressure increase is consistent with the observed thickness of the intermixed region, which is of the order of a few nm. We also show that stress measured during hydrogenation corresponds to a negligible increase in equilibrium pressure.

Plasmon enhanced solar-to-fuel energy conversion.

Future generations of photoelectrodes for solar fuel generation must employ inexpensive, earth-abundant absorber materials in order to provide a large-scale source of clean energy. These materials tend to have poor electrical transport properties and exhibit carrier diffusion lengths which are significantly shorter than the absorption depth of light. As a result, many photoexcited carriers are generated too far from a reactive surface and recombine instead of participating in solar-to-fuel conversion. We demonstrate that plasmonic resonances in metallic nanostructures and multilayer interference effects can be engineered to strongly concentrate sunlight close to the electrode/liquid interface, precisely where the relevant reactions take place. On comparison of spectral features in the enhanced photocurrent spectra to full-field electromagnetic simulations, the contribution of surface plasmon excitations is verified. These results open the door to the optimization of a wide variety of photochemical processes by leveraging the rapid advances in the field of plasmonics.

Engineering light absorption in semiconductor nanowire devices.

The use of quantum and photon confinement has enabled a true revolution in the development of high-performance semiconductor materials and devices. Harnessing these powerful physical effects relies on an ability to design and fashion structures at length scales comparable to the wavelength of electrons (approximately 1 nm) or photons (approximately 1 microm). Unfortunately, many practical optoelectronic devices exhibit intermediate sizes where resonant enhancement effects seem to be insignificant. Here, we show that leaky-mode resonances, which can gently confine light within subwavelength, high-refractive-index semiconductor nanostructures, are ideally suited to enhance and spectrally engineer light absorption in this important size regime. This is illustrated with a series of individual germanium nanowire photodetectors. This notion, together with the ever-increasing control over nanostructure synthesis opens up tremendous opportunities for the realization of a wide range of high-performance, nanowire-based optoelectronic devices, including solar cells, photodetectors, optical modulators and light sources.

Hydrogen storage in Pd nanodisks characterized with a novel nanoplasmonic sensing scheme.

A novel nanoplasmonic sensing scheme is introduced based on remote real-time detection of induced electronic and shape/structural changes in a metal nanoparticle during the metal-hydride formation process. The localized surface plasmon resonance (LSPR) of the nanoparticle is utilized as signal transducer for optical readout. As a model system, hydrogen storage through metal-hydride formation is studied in Pd nanodisks. The experimentally obtained plasmonic response to hydrogen uptake yields pressure-LSPR-response isotherms. These isotherms are found to obey Sievert's law in the low-pressure range and exhibit a characteristic "plateau" at 18 Torr upon hydrogen charging and 7.5 Torr upon hydrogen discharging. An additional experiment also clearly shows the typical temperature dependence of the plateau pressure. Conversion of the LSPR signal to absolute hydrogen concentration, based on a proposed linear dependence of the LSPR response to hydrogen uptake, results in p-C isotherms in excellent agreement with those in the literature. This puts forward that the LSPR response is an extremely sensitive, remote, and real-time probe for "bulk" changes in a metal nanoparticle and can readily be used to study processes such as metal-hydride formation for hydrogen storage applications, alloying on the nanoscale, thermal reshaping, and so forth.