To combine the principles of in situ transmission electron microscopy (TEM) with 3D characterisation, we developed fast high angle annular dark field scanning TEM (HAADF STEM) electron tomography (ET). We reduced the acquisition time for ET from 1 hour to less than 5 minutes [1-3]. In this manner, we were able to investigate morphological transformations of Au and AuPd octapods at high temperatures [4]. Next, we measured 3D changes in composition for AuAg nanoparticles and could reveal the importance of grain boundaries, present in pentatwinned nanorods [5,6]. We furthermore exploited the use of secondary electrons to obtain a fast 3D characterisation [7,8].
Since gas cell holders do not enable conventional ET, we exploited a methodology based on atom counting [9]. The counting results are used to build a 3D starting model for molecular dynamics simulations. Such simulations may easily result in a closest local minimum in the potential energy landscape where the reconstructed structure deviates from the experiments. We proposed an iterative local minima search algorithm and applied this methodology to investigate the behaviour of CeO2 supported Au nanoparticles at high temperature [10,11]. We furthermore used in situ TEM and tomography to investigate Ni nanoparticles on CeO2 supports during CO2 hydrogenation [12]. Also strong metal-support interaction between Ni nanoparticles and TiO2 during CO2 hydrogenation was studied at the atomic scale under operando conditions [13].
To characterise surface ligands in realistic environments, we developed a novel protocol to prepare graphene TEM supports. Since drying of the ligands during sample preparation was found to be a limitation, we extended the graphene transfer methodology towards the creation of graphene pockets containing a colloidal solution. This approach allowed us to visualise the anisotropy and dynamics of ligand distribution at the Au nanorod surface. We additionally measured the chemical composition of the ligand shell. This work opened a direct visualisation of ligand distribution around nanoparticles, which contributes to explaining the influence of additives on the monodispersity of Au nanorods. This work was published in Nature Chemistry [14].
References:
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