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Nanoscale Chemical Reactions Studied with In-Situ Transmission Electron Microscopy

Periodic Reporting for period 4 - NANO-INSITU (Nanoscale Chemical Reactions Studied with In-Situ Transmission Electron Microscopy)

Reporting period: 2021-03-01 to 2022-02-28

Great successes have been achieved in nanoscience and nanotechnology, where the development of functional properties and the assembly of nanoparticles into larger 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 of the electron microscope. 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 can elucidate what really happens during chemical reactions, thereby enabling the development of new nanomaterials for optoelectronics, lighting, and catalysis. My research group has extensive experience in in-situ TEM and recently has achieved significant successes in Liquid Cell studies. We are therefore in an ideal position to develop this new technology and open up new research areas, which have a major impact on science, industry, and society.
As a result of the ERC project ‘NANO-INSITU’, we have been able to follow in real time the self-diffusivity of small nanoparticles in liquids, which thus far was not achieved before and which opens the door to in-situ studies of directed self-assembly of nanoparticles. The wet chemical etching of silica particles, and transformations of cobalt oxide and titanium oxide nanoparticles to lower oxidation states was followed in real time as well. The two most prominent highlights of the current project were, however, the first-time experimental derivation of interaction potentials between nanoparticles in liquids, and the selective vertical and horizontal growth of two-dimensional WS2, an important 2D semiconductor, which could be followed real-time in the electron microscope as well. The results of the project was disseminated by presentations in webinars and at international conferences, through informative websites, and by press releases.
Within the ERC project 'NANO-INSITU', Work package 1 dealt with the development of in-situ TEM technology for Liquid TEM, and was mainly owned by PhD student Tom Welling. Although the research was much aided by technological developments conducted at the in-situ EM companies, great challenges still needed 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. 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 gold nanoparticles in liquids that follow the theoretical expectation for Brownian motion. (T.A.J. Welling et al., Particle and Particle Systems Characterization 37 (2020) 2000003).Furthermore, so-called rattle particles were studied which consist of a moveable core inside a larger silica shell. The mobility of the core nanoparticles inside the liquid-filled shell was manipulated by changing the ion concentration in the liquid, and by applying AC electric fields of different frequencies. This enables spectacular control over the motion of nanoparticles in liquid confinement.[T.A.J. Welling et al., ACS Nano 15 (2021) 11137-11149]
Work package 2 concerned 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 were conducted, with much focus on mitigating the effect of secondary nucleation of metal nanoparticles from metal precursors. Also the wet chemical etching of silica nanoparticles was investigated, which required extensive research into low-dose imaging conditions, and which was published in a methods-type paper.[A. Grau-Carbonell, ACS Applied Nano Materials 4 (2021) 1136-1148]
Work package 3 on the in-situ study on self-assembly of nanoparticles was owned by PhD student Dnyaneshwar Gavhane, who intensively investigated the assembly and transformations of chalcogenide nanosystems such as CoSe2, NiSe2, and MoSe2. Already spectacular findings were recorded during in-situ TEM experiments, showing a successive transformation from orthorhombic CoSe2 to cubic CoSe2 to hexagonal CoSe to tetragonal CoSe.[D.S. Gavhane et al., npj 2D Materials and Applications 5 (2021) 24] Furthermore, the in-situ horizontal or vertical growth of the 2D material WS2 was deciphered in detail and published in the high-ranking journal Advanced Functional Materials.[D.S. Gavhane, et al., Adv. Funct. Mater. 32 (2022) 202106450] This paper also featured on the cover of the journal.
Work package 4 on the in-situ study of reduction, oxidation, and hydration of metal oxides was mainly owned by PhD student Xiaodan Chen. She investigated the thermal stability of anatase and brookite TiO2 nanorods [X.D. Chen et al., ACS Applied Nano Materials 5 (2022) 1600-1606], and witnessed the transformation of Co3O4 nanoparticles to CoO [X.D. Chen et al., J. Mater. Chem. C 9 (2021) 5662-5675], and massive temperature-induced delamination of MoO3 nanocrystals into MoO2 nanoflakes. A paper with density functional theory (DFT) calculations on the thermal stability of transition metal oxides (TMOs) was published in a high-ranking journal [H. van Gog et al., npj 2D Materials and Applications 3 (2019) 18].
Our team has been pioneering the in-situ investigation of the mobility 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. Furthermore, in so-called rattle particles where a mobile core particle is contained in a liquid-filled silica shell, we have for the very first time derived experimentally the interparticle potentials between nanoparticles. The rattles were synthesized by researchers of Tohoku University, Japan, and an ongoing collaboration with that Japanese team has been established during the ERC project.
During the ERC project, much time was invested in mastering the in-situ methodology, where the in-situ chips and dedicated holder, the beam conditions of the electron microscope, and the quality of the sample all need to be perfect. Only after these skills and experimental conditions were mastered, new and surprising results were obtained as detailed above. The in-situ growth of the 2D materials WS2, which could controllably be grown either in horizontal sheets are in vertically oriented sheets and promoted by means of catalytic surfaces, is an astonishing result which was communicated in a press release and a webinar.
Journal Cover of Particle featuring the free diffusion of nanoparticles observed with liquid-cell EM
Schematic of the liquid cell setup, and observed Brownian motion of nanoparticles in fluids
Journal Cover of Advanced Functional Materials featuring the in-situ growth of 2D WS2