RESUMEN
Remote epitaxy is an emerging materials synthesis technique which employs a 2D interface layer, often graphene, to enable the epitaxial deposition of low defect single crystal films while restricting bonding between the growth layer and the underlying substrate. This allows for the subsequent release of the epitaxial film for integration with other systems and reuse of growth substrates. This approach is applicable to material systems with an ionic component to their bonding, making it notably appealing for III-V alloys, which are a technologically important family of materials. Chemical vapour deposition growth of graphene and wet transfer to a III-V substrate with a polymer handle is a potentially scalable and low cost approach to producing the required growth surface for remote epitaxy of these materials, however, the presence of water promotes the formation of a III-V oxide layer, which degrades the quality of subsequently grown epitaxial films. This work demonstrates the use of an argon ion beam for the controlled introduction of defects in a monolayer graphene interface layer to enable the growth of a single crystal GaAs film by molecular beam epitaxy, despite the presence of a native oxide at the substrate/graphene interface. A hybrid mechanism of defect seeded lateral overgrowth with remote epitaxy contributing the coalescence of the film is indicated. The exfoliation of the GaAs films reveals the presence of defect seeded nucleation sites, highlighting the need to balance the benefits of defect seeding on crystal quality against the requirement for subsequent exfoliation of the film, for future large area development of this approach.
RESUMEN
The rational control of forming and stabilizing reaction intermediates to guide specific reaction pathways remains to be a major challenge in electrocatalysis. In this work, we report a surface active-site engineering approach for modulating electrocatalytic CO2 reduction using the macrocycle cucurbit[6]uril (CB[6]). A pristine gold surface functionalized with CB[6] nanocavities was studied as a hybrid organic-inorganic model system that utilizes host-guest chemistry to influence the heterogeneous electrocatalytic reaction. The combination of surface-enhanced infrared absorption (SEIRA) spectroscopy and electrocatalytic experiments in conjunction with theoretical calculations supports capture and reduction of CO2 inside the hydrophobic cavity of CB[6] on the gold surface in aqueous KHCO3 at negative potentials. SEIRA spectroscopic experiments show that the decoration of gold with the supramolecular host CB[6] leads to an increased local CO2 concentration close to the metal interface. Electrocatalytic CO2 reduction on a CB[6]-coated gold electrode indicates differences in the specific interactions between CO2 reduction intermediates within and outside the CB[6] molecular cavity, illustrated by a decrease in current density from CO generation, but almost invariant H2 production compared to unfunctionalized gold. The presented methodology and mechanistic insight can guide future design of molecularly engineered catalytic environments through interfacial host-guest chemistry.
RESUMEN
Studying surface of catalyst nanoparticles in a flowing liquid is important for understanding the underlying mechanism of a reaction performed in liquid. We report the design of a reaction cell system of Si3N4 window covering the flowing liquid with an electron-transmissible membrane. By using metal nanoparticles as a catalyst dispersed in a solvent, examination of the surface of catalyst nanoparticles in a flowing liquid was demonstrated by observation of Ag 3d photoemission feature when a liquid containing Ag nanoparticles was flowing through this system.
RESUMEN
For nanoparticles active for chemical and energy transformations in static liquid environment, chemistries of surface or near-surface regions of these catalyst nanoparticles in liquid are crucial for fundamentally understanding their catalytic performances at a molecular level. Compared to catalysis at a solid-gas interface, there is very limited information on the surface of these catalyst nanoparticles under a working condition or during catalysis in liquid. Photoelectron spectroscopy is a surface-sensitive technique; however, it is challenging to study the surfaces of catalyst nanoparticles dispersed in static liquid because of the short inelastic mean free path of photoelectrons traveling in liquid. Here, we report a method for tracking the surface of nanoparticles dispersed in static liquid by employing graphene layers as an electron-transparent membrane to separate the static liquid containing a solvent, catalyst nanoparticles, and reactants from the high-vacuum environment of photoelectron spectrometers. The surfaces of Ag nanoparticles dispersed in static liquid sealed in such a graphene membrane liquid cell were successfully characterized using a photoelectron spectrometer equipped with a high vacuum energy analyzer. With this method, the surface of catalyst nanoparticles dispersed in liquid during catalysis at a relatively high temperature up to 150 °C can be tracked with photoelectron spectroscopy.
RESUMEN
Chemical waves in the H2 + O2 reaction on a Rh(111) surface alloyed with Ni [ΘNi < 1.5 monolayers (ML)] have been investigated in the 10-7 and 10-6 mbar range at T = 773 K using scanning photoelectron microscopy and x-ray photoelectron spectroscopy as in situ methods. The local intensity variations of the O 1s and the Ni 2p signal display an anticorrelated behavior. The coincidence of a high oxygen signal with a low Ni 2p intensity, which seemingly contradicts the chemical attraction between O and Ni, has been explained with a phase separation of the oxygen covered Rh(111)/Ni surface into a 3D-Ni oxide and into a Ni poor metallic phase. Macroscopic NiO islands (≈1 µm size) formed under reaction conditions have been identified as 2D-Ni oxide. Titration experiments of the oxygen covered Rh(111)/Ni surface with H2 demonstrated that the reactivity of oxygen is decreased by an order of magnitude through the addition of 0.6 ML Ni. An excitation mechanism is proposed in which the periodic formation and reduction of NiO modulate the catalytic activity.