6 Materialchemie
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Monodisperse iron oxide nanoparticles as reference material candidate for particle size measurements
(2020)
In order to utilize and rationally design materials at the nanoscale the reliable characterization of their physico-chemical properties is highly important, especially with respect to the assessment of their environmental or biological impact. Furthermore, the European Commission’s REACH Regulations require the registration of nanomaterials traded in quantities of at least 1 ton. Powders or dispersions where 50% (number distribution) of the constituent particles have sizes ≤ 100 nm in at least one dimension are defined as nanomaterials. This creates a need for industrial manufacturers and research or analytical service facilities to reliably characterize potential nanomaterials. Currently, BAM is developing reference nanoparticles, which shall expand the scarce list of worldwide available nano reference materials certified for particle size distribution and will also target other key parameters like shape, structure, porosity or functional properties. In this respect, materials like iron oxide or titanium dioxide are considered as candidates to complement the already available silica, Au, Ag, and polystyrene reference nanoparticles.
The thermal decomposition of iron oleate precursors in high boiling organic solvents can provide large quantities of iron oxide nanoparticles that can be varied in size and shape.[1, 2] The presence of oleic acid or other hydrophobic ligands as capping agents ensures stable dispersion in nonpolar solvents. Such monodisperse, spherical particles were synthesized at BAM and pre-characterized by electron microscopy (TEM, SEM including the transmission mode STEM-in-SEM) and dynamic light scattering comparing cumulants analysis and frequency power spectrum.
1. REACH regulations and nanosafety concerns create a strong need for nano reference materials with diverse properties.
2. Iron oxide nanoparticles are under development as new candidate reference material at BAM.
3. Narrow particle size distribution confirmed by light scattering and electron microscopy.
Based on practical examples of analysis with Electron Microscopy and Energy-Dispersive X-ray Spectroscopy, the minimum information requirements for reliable and reproducible nanomaterial characterization data such as particle size and shape distribution and elemental analysis are presented and discussed.
In this contribution different ways are explored with the aim to generate suitable training data for ‘non-ideal’ samples using various approaches, e.g., computer-generated images or unsupervised learning algorithms such as generative adversarial networks (GANs). We used these data to train simple CNNs to produce segmentation masks of SEM images and tested the trained networks on real SEM images of complex nanoparticle samples. The novel use of CNN for the automated analysis of the size of nanoparticles of complex shape and with a high degree of agglomeration has proved to be a promising tool for the evaluation of particle size distribution on a large number of constituent particles. Further development and validation of the preliminary model, respectively larger training and validation data sets are necessary.
In this paper one refined approach is applied to determine the exact orientation of bipyramidal TiO2 nanoparticles prepared with good dispersion as almost isolated particles on a carbon TEM grid. The advantages of the recently developed high-throughput Transmission Kikuchi Diffraction (TKD) are used to identify quickly and reliably the geometrical orientation of the crystalline TiO2 nanoparticle bipyramids (anatase) on a statistically relevant number of particles.
In this work, we present various promising sample preparation procedures aiming to avoid drying artifacts and guarantee homogeneously distributed NP samples for imaging techniques like TEM, SEM or AFM. This involves retarded drying in an environment of high relative humidity as well as spin coating or freeze drying of the sample on the substrate.
Depending on the type of the NPs, different techniques are successful in obtaining homogenous sample deposition. Simple drying at ambient conditions often results in heavily agglomerated NPs densely packed in coffee ring traces. Retardation of the drying speed in an environment of high relative humidity, however, yields homogeneous NP deposition in the case of bimodal SiO2 and bimodal gold NP’s. In contrast, a sample of bipyramidal TiO2, heavily agglomerated when commonly dried, is successfully dispersed by spin coating the suspension on the substrate. The analysis of the particle size distribution of the bimodal silica and gold samples yielded number ratios resembling the nominal number ratios. The reliable measurement of the NP concentration of polydisperse samples by electron microscopy, even if relative, is new and of special relevance because up to now only the size of the particles, i.e. the abscissa of the PSD, can be measured reliably.
Various promising sample preparation procedures aiming to avoid drying artifacts and guarantee homogeneously distributed NP samples for imaging techniques like TEM, SEM or AFM are presented. This involves retarded drying in an environment of high relative humidity as well as spin coating or freeze drying of the sample on the substrate.
Depending on the type of the NPs, different techniques are successful in obtaining homogenous sample deposition. Simple drying at ambient conditions often results in heavily agglomerated NPs densely packed in coffee ring traces. Retardation of the drying speed in an environment of high relative humidity, however, yields homogeneous NP deposition in the case of bimodal SiO2 and bimodal gold NP’s. In contrast, a sample of bipyramidal TiO2, heavily agglomerated when commonly dried, is successfully dispersed by spin coating the suspension on the substrate.
The analysis of the particle size distribution of the bimodal silica and gold samples yielded number ratios resembling the nominal number ratios. The reliable measurement of the NP concentration of polydisperse samples by electron microscopy, even if relative, is new and of special relevance because up to now only the size of the particles, i.e. the abscissa of the PSD, can be measured reliably.
Gold nanocubes with a monodispersed size distribution (edge = 55 nm) deposited on a silicon wafer. These tiny uniform cubes were produced as reference nanoparticles within the EMPIR project nPSize - Improved traceability chain of nanoparticle size measurements. SEM image acquired at 10 kV with an in-lens secondary electron detector. Image colored in ImageJ using LUT “Orange hot.” Published in Microscopy and Microanalysis 25(S2) (2019) 2328.
Gold nanocubes with a monodispersed size distribution (edge = 55 nm) deposited on a silicon wafer. These tiny uniform cubes were produced as reference nanoparticles in the frame of the European project nPSize - Improved traceability chain of nanoparticle size measurements. SEM Image acquired at 10 kV with an in-lens secondary electron detector. Image colored in ImageJ using LUT “Orange hot.” Published in Microscopy and Microanalysis 25(S2) (2019) 2328.
Understanding the correlation between the morphological and functional properties of particulate materials is crucial across all fields of physical and natural sciences. This manuscript reports on the investigation of the effect of polyethylene glycol (PEG) employed as a capping Agent in the synthesis of SrTiO3 crystals. The crucial influence of PEG on both the shape and size of the strontium titanate particles is revealed, highlighting the effect on the photocurrents measured under
UV–Vis irradiation.
In this paper one refined approach is applied to determine the exact orientation of bipyramidal TiO2 nanoparticles prepared with good dispersion as almost isolated particles on a carbon TEM grid. The advantages of the recently developed high-throughput Transmission Kikuchi Diffraction (TKD) are used to identify quickly and reliably the geometrical orientation of the crystalline TiO2 nanoparticle bipyramids (anatase) on a statistically relevant number of particles.