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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - NANO-INSITU (Nanoscale Chemical Reactions Studied with In-Situ Transmission Electron Microscopy)

Teaser

Summary

Great successes have been achieved in nanoscience where the development of functional properties and the assembly of nanostructures into nanomaterials have become increasingly important. In general, both the tuning of the chemical and physical properties and the self-assembly of nanocrystals into 2D or 3D superstructures take place in a liquid environment. When analyzing the structural properties of nanocrystals using Transmission Electron Microscopy (TEM), this liquid environment is contained between membranes to keep it in the high vacuum. The purpose of this research program is to devise methodologies which will turn real-time atomic resolution imaging and chemical analysis on nanoparticles in solution into reality.
This new in-situ technology will elucidate what really happens during chemical reactions, and will thereby enable the development of new nanomaterials for optoelectronics, lighting, and catalysis. Oriented attachment processes and self-assembly of nanoparticles, which are key to the large-scale production of 2D and 3D nanomaterials, can also be followed in the Liquid Cell. Furthermore, the hydration of nanoscale model systems of earth materials such as magnesia, alumina, and calcium oxide is of major importance in the geosciences.Â My research group has extensive experience in in-situ TEM and recently has achieved significant successes in Liquid Cell studies. We are in an ideal position to develop this new technology and open up these new research areas, which will have a major impact on science, industry, and society.

Work performed

The ERC-CoG project NANO-INSITU is fully deployed with the PI and 4 PhD students (Tom Welling, Albert Grau Carbonell, Dnyaneshwar Gavhane, and Xiaodan Chen) working on the several work packages, supported by temporary appointments of technical/postdoctoral staff members (Wun-Fan Li, Matthijs de Winter). All members of the team have received extensive training in transmission electron microscopy (TEM), both in-house and at international electron microscopy (EM) schools. The scientific progress is enabled by investments in new technology, amongst others a new liquid cell holder with heating and biasing modalities, as well as a co-investment in a new gas phase TEM holder, enabling to monitor transformations of nanoparticles and of nanostructures in liquid and gas environments, respectively. Because of the pioneering nature of the work, many liquid cell and heating chips, as well as TEM beam time hours, are consumed. The fruits of the research efforts have already been consolidated in a number of high-ranking papers, published in e.g. Advanced Materials, Chemistry of Materials, and npj 2D Materials and Applications. The progress in each of the work packages as defined in the research proposal is as follows.
Work package 1 deals with the development of in-situ TEM technology for Liquid TEM, and is mainly owned by PhD student Tom Welling. Although the research is also much aided by technological developments conducted at the in-situ TEM holder producing companies such as Protochips, great challenges still need to be overcome to prevent charging effects from the electron beam, both on the thin SiN windows containing the thin fluid, and in the fluid itself. In order to study self-assembly, first of all nanoparticles in solution have to be able to self-diffuse in a Brownian manner. By choosing low-dose electron beam conditions, and a solvent with a high dielectric permittivity (glycerol carbonate), we have been the first to achieve self-diffusion rates of titania and silica nanoparticles in liquids that follow the theoretical expectation for Brownian motion. The mobilities found in our research are orders of magnitude higher than the mobilities reported by others in the literature.
Work package 2 concerns the in-situ study of cation exchange reactions in heterogeneous nanocrystals. First, much time was invested in the synthesis of large PbSe nanocubes and CdS nanorods, which are very suitable to perform cation exchange on. Investigations of in-situ cation exchange in liquid are ongoing, with much focus on mitigating the effect of secondary nucleation of metal nanoparticles from metal precursors. In particular exchange of Pb and Cd ions with Cu and Ag ions will be pursued.
Work package 3 on the in-situ study on self-assembly of nanoparticles is owned by PhD student Dnyaneshwar Gavhane, who is intensively investigating assembly and transformations of chalcogenide nanosystems such as CoSe2, NiSe2, and MoSe2. Already spectacular findings were recorded during in-situ TEM experiments, showing a transformation from orthorhombic CoSe2 to hexagonal CoSe. A collaboration with the AMOLF Institute (Amsterdam, The Netherlands) resulted in a paper which was published in Advanced Materials (B. Sciacca et al.).
Work package 4 on the in-situ study of reduction, oxidation, and hydration of metal oxides is mainly owned by PhD student Xiaodan Chen. She has investigated in detail the thermal stability of brookite TiO2 nanorods, and witnessed spectacular transformations of Co3O4 nanoparticles to CoO, and delamination of MoO3 nanocrystals. A paper with density functional theory (DFT) calculations on the thermal stability of transition metal oxides (TMOs) was published in a journal belonging to the Nature Springer family, npj 2D Materials and Applications.

Final results

Our team is pioneering the in-situ investigation of the self-assembly of nanoparticles in liquids. By experimentation with particular nanoparticle-solvent combinations, we have been able to realize Brownian motion in accordance with theoretical expectations, with mobilities that are 5 orders of magnitude (!) higher than reported in the literature thus far. Worldwide, we are the most active group to experiment with high viscosity and high electrical permittivity solvents, to capture the real dynamics of nanoparticles in solvents. In the near future, using the acquired liquid cell TEM holder with heating and biasing modalities, we will move on to the next step: the in-situ study of self-assembly of nanoparticles steered by electric fields.
All PhD students are well on track with their research and have acquired results. Several papers are in preparation. Much time was invested in mastering the in-situ methodology, where the in-situ technology, the beam conditions of the electron microscope, and the quality of the sample all need to be perfect. Now that the PhD students have mastered these skills, results are expected in the coming two years at a higher pace. In particular the application of electric fields in the liquid cell are expected to provide novel results with respect to controlled self-assembly of nanoparticles. Also the experiments that will be performed with new gas cell TEM holder, where chemical and structural transformations during oxidation, reduction, and hydration can be followed in real time, are expected to generate novel scientific outcomes.