Crystal phase switching between the zincblende and wurtzite structures in III-V nanowires is crucial from the fundamental viewpoint as well as for electronic and photonic applications of crystal phase heterostructures. Here, the results of in situ monitoring of self-catalyzed vapor-liquid-solid growth of GaAs nanowires by molecular beam epitaxy inside a transmission electron microscope is presented. It is demonstrated that the occurrence of the zincblende or wurtzite phase in self-catalyzed nanowires is determined by the sole parameter, the droplet contact angle, which can be finely tuned

Galvanic replacement (GR) is a simple and widely used approach to synthesize hollow nanostructures for applications in catalysis, plasmonics, and biomedical research. The reaction is driven by the difference in electrochemical potential between two metals in a solution. However, transient stages of this reaction are not fully understood. Here, we show using liquid cell transmission electron microscopy that silver (Ag) nanocubes become hollow via the nucleation, growth, and coalescence of voids inside the nanocubes, as they undergo GR with gold (Au) ions at different temperatures. These direct in situ observations indicate that void formation due to the nanoscale Kirkendall effect occurs in conjunction with GR. Although this mechanism has been suggested before, it has not been verified experimentally until now. These experiments can inform future strategies for deriving such nanostructures by providing insights into the structural transformations as a function of Au ion concentration, oxidation state of Au, and temperature.

Nanoparticles (NPs) confined in thin layers of liquid within liquid cells used for in situ transmission electron microscopy (TEM) move very slowly, in contrast to free particles in bulk liquid. The reason is still poorly understood. Here, we tracked gold NPs moving in water at the liquid−solid interface with in situ TEM at rates of 100 frames per second. The recorded motion exhibited three key features: (1) it was made up of sustained sequences of "sticky" motion where NPs only moved a few nanometers each time; (2) sporadic long "flights" where the NPs traveled tens to hundreds of nanometers between frames; and (3) "flights" are accompanied by intermittent, fast pivoted rotations. Trajectory analysis shows that the displacements follow a truncated Levy distribution, pointing to desorptionmediated motion of NPs at the liquid−solid interface. We further associate pivoted rotations with a transient "weakly adsorbed" state between desorption and adsorption of NPs. The frequency of desorption was also controlled by electron flux and solution chemistry. We propose that the pattern of motion is the result of an inhomogeneous distribution of surface charges on silicon nitride (SiN x ). Such insight into the interactions between NPs and solid surfaces in liquids is useful for understanding dynamics at liquid−solid interfaces and has general implications for microfluidics, nanotribology, sensing, and self-assembly.

The catalytic performance of bimetallic nanoparticles (NPs) strongly depends on their structural and compositional changes under reaction conditions. At the fundamental level, these changes are driven by redox reactions that occur on the surface of the NPs. The degree of complexity in the redox reactions is further amplified in bimetallic NPs because both metals can have their own reactions with the reactant molecules, in addition to any synergistic effects between the metal nanocatalysts and their reducible oxides. Here, the gas phase oxidation and reduction reactions, and the oxidation of carbon monoxide (CO) over Pt–Ni rhombic dodecahedron NPs with segregated Pt frames and Pt–Ni alloy NPs are investigated using in situ gas cell transmission electron microscopy. The real‐time observations show that NiO shell formation and Pt segregation are two important features during the oxidation and reduction of Pt–Ni NPs, respectively. Moreover, the two types of NPs evolved in different ways. By combining high‐resolution imaging, mass spectroscopy, and modeling, it is shown that the evolution of NP morphology and composition during redox reactions plays an important role in controlling the catalytic activity of the NPs.

Both chemical and electrochemical etching are effective methods for tailoring the surface composition of Pt-based catalytic bimetallic nanoparticles (NPs). However, the detailed nanoscale etching mechanisms, which are needed for achieving fine control over the etch processes, are still not understood. Here, we study selective chemical and electrochemical Ni etching of Pt−Ni rhombic dodecahedron NPs using in situ liquid-phase transmission electron microscopy. Our real-time observations show that the intermediate NP structures evolve differently in the two cases. Chemical etching of Ni starts from localized pits on the NP surface, in contrast to the uniform dissolution of Ni during the electrochemical etching. Our study reveals how oxidative etching participates in the removal of a non-noble metal and the subsequent formation of noblemetal-rich NPs. The mechanistic insights reported here highlight the role of a native surface oxide layer on the etching behavior, which is important for the design of NPs with specific surface composition for applications in electrocatalysis.