We study the Fe-catalyzed chemical vapor deposition of carbon nanotubes by complementary in-situ grazing-incidence X-ray diffraction, in-situ X-ray reflectivity and environmental transmission electron microscopy. We find that typical oxide supported Fe catalyst films form widely varying mixtures of bcc and fcc phased Fe nanoparticles upon reduction, which we ascribe to variations in minor commonly present carbon contamination levels. Depending on the as-formed phase composition, different growth modes occur upon hydrocarbon exposure: For γ-rich Fe nanoparticle distributions, metallic Fe is the active catalyst phase, implying that carbide formation is not a prerequisite for nanotube growth. For α-rich catalyst mixtures, Fe3C formation more readily occurs and constitutes part of the nanotube growth process. We propose that this behavior can be rationalized in terms of kinetically accessible pathways, which we discuss in the context of the bulk iron-carbon phase diagram with the inclusion of phase equilibrium lines for metastable Fe3C. Our results indicate that kinetic effects dominate the complex phase evolution during realistic CNT growth recipes.

Video-rate environmental transmission electron microscopy of Au-catalyzed chemical vapor deposition of Ge nanowires shows that growth kinetics are linked to an oscillatory behavior at the liquid−solid interface near the triple phase boundary (TPB), where the nanowire surface consists of an oblique facet. A nanowire growth cycle starts by preferential Ge precipitation at the TPB, which increases the wetting angle and Ge supersaturation of the liquid AuGe catalyst. With a continuously rising Ge supersaturation, the kinetic barrier for Ge bilayer nucleation falls until step nucleation occurs. Subsequent step flow creates a (111)-type plane at the nanowire growth front and hence rapidly decreases the Ge supersaturation in the catalyst. This leads to the destabilization and partial dissolution of the Ge initially stored in the rough TPB area, i.e., completes the growth cycle. The results and arguments are general and based on few system specific assumptions, therefore may be applicable to other nanowire systems, and are relevant to general nucleation and liquid−solid interface dynamics in dimensionally confined systems.

Understanding the interaction between water and oxides is critical for many technological applications, including energy storage, surface wetting/self-cleaning, photocatalysis and sensors. Here, we report observations of strong structural oscillations of BaSrCoFeO (BSCF) in the presence of both HO vapour and electron irradiation using environmental transmission electron microscopy. These oscillations are related to the formation and collapse of gaseous bubbles. Electron energy-loss spectroscopy provides direct evidence of O formation in these bubbles due to the incorporation of HO into BSCF. SrCoO was found to exhibit small oscillations, while none were observed for LaSrCoO and LaCoO. The structural oscillations of BSCF can be attributed to the fact that its oxygen 2p-band centre is close to the Fermi level, which leads to a low energy penalty for oxygen vacancy formation, high ion mobility, and high water uptake. This work provides surprising insights into the interaction between water and oxides under electron-beam irradiation.

Alloy nanoparticle catalysts are known to afford unique activities that can differ markedly from their parent metals, but there remains a generally limited understanding of the nature of their atomic (and likely dynamic) structures as exist in heterogeneously-supported forms under reaction conditions. Notably unclear is the nature of their active sites and the details of the varying oxidation states and atomic arrangements of the catalytic components during chemical reactions. In this work, we describe multimodal methods that provide a quantitative characterization of the complex heterogeneity present in the chemical and electronic speciations of Pt-Ni bimetallic catalysts supported on mesoporous silica during the reverse water gas shift reaction. The analytical protocols involved a correlated use of in-situ X-ray Absorption Spectroscopy (XAS), Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS), complimented by ex-situ aberration corrected Scanning Transmission Electron Microscopy (STEM). The data reveal complex reactions occur between the metals and support in this system under operando conditions. These reactions, and the specific impacts of strong metal-silica bonding interactions, prevent formation of alloy phases containing Ni-Ni bonds. This feature of structure provides high activity and selectivity for the reduction of CO2 to carbon monoxide without significant competitive levels of methanation. We show how these chemistries evolve to the active state of the catalyst: bimetallic nanoparticles possessing an intermetallic structure (the active phase) that are conjoined with Ni-rich, metal-silicate species.

Lattice-resolved, video-rate environmental transmission electron microscopy shows the formation of a liquid Au-Ge layer on sub-30-nm Au catalyst crystals and the transition of this two-phase Au-Ge/Au coexistence to a completely liquid Au-Ge droplet during isothermal digermane exposure at temperatures far below the bulk Au-Ge eutectic temperature. Upon Ge crystal nucleation and subsequent Ge nanowire growth, the catalyst either recrystallizes or remains liquid, apparently stabilized by the Ge supersaturation. We argue that there is a large energy barrier to nucleate diamond-cubic Ge, but not to nucleate the Au-Ge liquid. As a result, the system follows the more kinetically accessible path, forming a liquid even at 240 degrees C, although there is no liquid along the most thermodynamically favorable path below 360 degrees C.