Coherent-x-Ray Imaging of Nanoparticle Alloy Electrodes and Catalysts

Abstract

Bragg coherent diffraction imaging (BCDI) studies of silver nanoparticles revealed a new dissolution mechanism that has not previously been observed.1 We find that a long incubation time, ~30 sec, before start dissolution reaction when the particle starts from the pristine state. During the incubation period, however, electrochemical potential-induced pressure nucleates a dislocation loop within the nanoparticle that eventually exposes to the surface. Only then, rapid dissolution is activated at the surface sites. This result points to an important connection between structure and stability of silver in an electrochemical environment and demonstrates the potential of BCDI to elucidate this relationship. Understanding silver dissolution process is important in studying dealloying process of binary alloys. Dealloying is a selective dissolution of one element in an alloy resulting in a porous, strained structure often with new properties, such as highly active electrocatalysts.2 In the past, the pore formation during dealloying has been visualized using electron microscopy and the dealloying-induced strain has been studied at the ensemble level using x-ray diffraction.3 However, the information on lattice strain, which has been unavailable, could play an integral role in understanding and controlling properties and activities for a variety of applications. In our BCDI studies, 3D strain distributions of individual nanoparticles and thin film grains of silver-gold alloys were investigated during dealloying in nitric acid.4 Isolated particles exhibit dramatic changes in their shapes as expected (See the graphical abstract). Our results (not shown) also demonstrate that significant local strains occur as a result of dealloying. The same dealloying process was used for nanoscale grains of a thin film instead of the isolated nanoparticles. The results show that isolated nanoparticles are significantly more strained than the grain embedded in the film texture,4 primarily because strain can be relieved via grain boundaries in the case of the single grain. These observations demonstrate that the unique capabilities of BCDI techniques. We also have begun working on alloys under catalytic environment where the compositions and shapes of alloy nanoparticles are highly sensitive to gas and temperature environment. We will present some preliminary results as time permits in my presentation. Liu, Y., et al., Nano Letters (2017) 17 (3), 1595Erlebacher, J., et al., Nature (2001) 410 (6827), 450Yang, R. Z., et al., Journal of Physical Chemistry C (2011) 115 (18), 9074Cha, W., et al., Advanced Functional Materials (2017) 27 (25) Figure 1