Christian Danvad Damsgaard

Bioinspired molecular co-catalysts bonded to a silicon photocathode for solar hydrogen evolution

The production of fuels from sunlight represents one of the main challenges in the development of a sustainable energy system. Hydrogen is the simplest fuel to produce and although platinum and other noble metals are efficient catalysts for photoelectrochemical hydrogen evolution, earth-abundant alternatives are needed for large-scale use. We show that bioinspired molecular clusters based on molybdenum and sulphur evolve hydrogen at rates comparable to that of platinum. The incomplete cubane-like clusters (Mo(3)S(4)) efficiently catalyse the evolution of hydrogen when coupled to a p-type Si semiconductor that harvests red photons in the solar spectrum. The current densities at the reversible potential match the requirement of a photoelectrochemical hydrogen production system with a solar-to-hydrogen efficiency in excess of 10%. The experimental observations are supported by density functional theory calculations of the Mo(3)S(4) clusters adsorbed on the hydrogen-terminated Si(100) surface, providing insights into the nature of the active site.

Exploring the environmental transmission electron microscope

The increasing interest and development in the field of in situ techniques have now reached a level where the idea of performing measurements under near realistic conditions has become feasible for transmission electron microscopy (TEM) while maintaining high spatial resolution. In this paper, some of the opportunities that the environmental TEM (ETEM) offers when combined with other in situ techniques will be explored, directly in the microscope, by combining electron-based and photon-based techniques and phenomena. In addition, application of adjacent setups using sophisticated transfer methods for transferring the specimen between specialized in situ equipment without compromising the concept of in situ measurements will be exploited. The opportunities and techniques are illustrated by studies of materials systems of Au/MgO and Cu(2)O in different gaseous environments.

Spin injection between epitaxial Co2.4Mn1.6Ga and an InGaAs quantum well

Electrical spin injection in a narrow [100] In0.2Ga0.8As quantum well in a GaAs p‐i‐n optical device is reported. The quantum well is located 300nm from an AlGaAs Schottky barrier and this system is used to compare the efficiencies and temperature dependences of spin injection from Fe and the Heusler alloy Co2.4Mn1.6Ga grown by molecular-beam epitaxy. At 5K, the injected electron spin polarizations for Fe and Co2.4Mn1.6Ga injectors are 31% and 13%, respectively. Optical detection is carried out in the oblique Hanle geometry. A dynamic nuclear polarization effect below 10K enhances the magnetic field seen by the injected spins in both devices. The Co2.4Mn1.6Ga thin films are found to have a transport spin polarization of ∼50% by point contact Andreev reflection conductivity measurements.

Stability and resistance of nickel catalysts for hydrodeoxygenation: carbon deposition and effects of sulfur, potassium, and chlorine in the feed

The long term stability and resistance toward carbon deposition, sulfur, chlorine, and potassium of Ni/ZrO2 as a catalyst for the hydrodeoxygenation (HDO) of guaiacol in 1-octanol (as a model compound system for bio-oil) has been investigated at 250 °C and 100 bar in a trickle bed reactor setup. Without impurities in the feed good stability of the Ni/ZrO2 catalyst could be achieved over more than 100 h of operation, particularly for a sample prepared with small Ni particles, which minimized carbon deposition. Exposing the catalyst to 0.05 wt% sulfur in the feed resulted in rapid deactivation with complete loss of activity due to the formation of nickel sulfide. Exposing Ni/ZrO2 to chlorine-containing compounds (at a concentration of 0.05 wt% Cl) on-stream led to a steady decrease in activity over 40 h of exposure. Removal of the chlorine species from the feed led to the regaining of activity. Analysis of the spent catalyst revealed that the adsorption of chlorine on the catalyst was completely reversible, but chlorine had caused sintering of nickel particles. In two experiments, potassium, as either KCl or KNO3, was impregnated on the catalyst prior to testing. In both cases deactivation was persistent over more than 20 h of testing and severely decreased the deoxygenation activity while the hydrogenation of guaiacol was unaffected. Overall, sulfur was found to be the worst poison, followed by potassium and then chlorine. Thus, removal/limitation of these species from bio-oil is a requirement before long term operation can be achieved with this catalyst.

In Situ Observation of Cu-Ni Alloy Nanoparticle Formation by X-Ray Diffraction, X-Ray Absorption Spectroscopy, and Transmission Electron Microscopy: Influence of Cu/Ni Ratio

Silica‐supported, bimetallic Cu–Ni nanomaterials were prepared with different ratios of Cu to Ni by incipient wetness impregnation without a specific calcination step before reduction. Different in situ characterization techniques, in particular transmission electron microscopy (TEM), X‐ray diffraction (XRD), and X‐ray absorption spectroscopy (XAS), were applied to follow the reduction and alloying process of Cu–Ni nanoparticles on silica. In situ reduction of Cu–Ni samples with structural characterization by combined synchrotron XRD and XAS reveals a strong interaction between Cu and Ni species, which results in improved reducibility of the Ni species compared with monometallic Ni. At high Ni concentrations silica‐supported Cu–Ni alloys form a homogeneous solid solution of Cu and Ni, whereas at lower Ni contents Cu and Ni are partly segregated and form metallic Cu and Cu–Ni alloy phases. Under the same reduction conditions, the particle sizes of reduced Cu–Ni alloys decrease with increasing Ni content. Estimates of the metal surface area from sulfur chemisorption and from the XRD particle size generally agree well on the trend across the composition range, but show some disparity in terms of the absolute magnitude of the metal area. This work provides practical synthesis guidelines towards preparation of Cu–Ni alloy nanomaterials with different Cu/Ni ratios, and insight into the application of different in situ techniques for characterization of the alloy formation.

Theoretical study of in-plane response of magnetic field sensor to magnetic beads magnetized by the sensor self-field

We present a theoretical study of the spatially averaged in-plane magnetic field on square and rectangular magnetic field sensors from a single magnetic bead, a monolayer of magnetic beads, and a half-space filled with magnetic beads being magnetized by the magnetic self-field due to the applied bias current through the sensor. The analysis of the single bead response shows that beads always contribute positively to the average magnetic field as opposed to the case for an applied homogeneous magnetic field where the sign of the signal depends on the bead position. General expressions and analytical approximations are derived for the sensor response to beads as function of the bead distribution, the bias current, the geometry and size of the sensor, and the bead characteristics. Consequences for the sensor design are exemplified and it is described how the contribution from the self-field experimentally can be separated from that due to static magnetic fields.

Exchange-biased planar Hall effect sensor optimized for biosensor applications

This article presents experimental investigations of exchange-biased Permalloy planar Hall effect sensor crosses with a fixed active area of w×w=40×40μm2 and Permalloy thicknesses of t=20, 30, and 50nm. It is shown that a single domain model describes the system well and that the thicker film will have a higher signal as well as a lower noise. It is estimated that the signal-to-noise ratio for bead detection increases by a factor 2.1 when t is increased from 20to50nm and hence a higher t is beneficial for biosensor applications. This is exemplified with calculations on M-280 Dynabeads®.

High-resolution chemical imaging of gold nanoparticles using hard x-ray ptychography

We combine resonant scattering with (ptychographic) scanning coherent diffraction microscopy to determine the chemical state of gold nanoparticles with high spatial resolution. Ptychographic images of the sample are recorded for a series of energies around the gold L3 absorption edge. From these data, chemical information in the form of absorption and resonant scattering spectra is reconstructed at each location in the sample. For gold nanoparticles of about 100 nm diameter, a spatial resolution of about 20–30 nm is obtained. In the future, this microscopy approach will open the way to operando studies of heterogeneous catalysts on the nanometer scale.

Size-dependent effects in exchange-biased planar Hall effect sensor crosses

Exchange-biased planar Hall effect magnetic field sensor crosses with arm width w have been studied as function of w. For large values of w, the magnetic behavior is hysteresis-free and follows the single domain Stoner–Wohlfarth model. When w is decreased, hysteresis is observed in the sensor response. For intermediate values of w, the magnetization reversal takes place in two steps, and for small values of w, the magnetization reversal takes place in a single step. Based on electrical measurements, magnetic force microscopy, and micromagnetic simulations, the observations are explained by an increasing magnetic shape anisotropy of the arms of the cross. We propose a simple analytical model that captures the essential physics of the observations and parameterizes the effects of the cross-shape on the central part of the cross.

Influence of preparation method on supported Cu–Ni alloys and their catalytic properties in high pressure CO hydrogenation

Silica supported Cu–Ni (20 wt% Cu + Ni on silica, molar ratio of Cu/Ni = 2) alloys are prepared via impregnation, coprecipitation, and deposition–coprecipitation methods. The approach to co-precipitate the SiO2 from Na2SiO3 together with metal precursors is found to be an efficient way to prepare high surface area silica supported catalysts (BET surface area up to 322 m2 g−1, and metal area calculated from X-ray diffraction particle size up to 29 m2 g−1). The formation of bimetallic Cu–Ni alloy nanoparticles has been studied during reduction using in situ X-ray diffraction. Compared to impregnation, the coprecipitation and deposition–coprecipitation methods are more efficient for preparation of small and homogeneous Cu–Ni alloy nanoparticles. In order to examine the stability of Cu–Ni alloys in high pressure synthesis gas conversion, they have been tested for high pressure CO hydrogenation (50 bar CO and 50 bar H2). These alloy catalysts are highly selective (more than 99 mol%) and active for methanol synthesis; however, loss of Ni caused by nickel carbonyl formation is found to be a serious issue. The Ni carbonyl formation should be considered, if Ni-containing catalysts (even in alloyed form) are used under conditions with high partial pressure of CO.