the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 28 Jan 2025
Spark ablation metal nanoparticles and coating on TiO2 in the aerosol phase
Abstract. Generation and characterisation of metal nanoparticles (NPs) gained attention in recent years due to their significant potential in applications as diverse as catalysis, electronics or energy storage. Despite the high interest in NPs, their characterisation remains challenging and detailed quantitative information on size, number concentration and morphologies are key to understand their properties. In this study we generated NPs from four metals, Au, Pt, Cu and Ni, via spark ablation in the aerosol phase, which allows to produce NPs as small as 1 nm in high quantities and purity. Particles were characterised with transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDX) as well as online aerosol particle size distribution measurement techniques. Particle size modes for the four metals ranged between 3 nm and 5 nm right after generation. Differences in number and size of particles generated can be rationalised with thermodynamic properties of the metals such as melting point or surface free energy. The four metal NPs were also coagulated with larger TiO2 NPs of about 120 nm size and the metal surface coverage of the TiO2 particles was characterised with electron microscopy and EDX spectroscopy. This detailed characterisation of NPs mixtures will be essential for a fundamental understanding of spark ablation generated particles and their applications for material sciences.
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RC1: 'Comment on ar-2025-2', Anonymous Referee #1, 10 Feb 2025
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The manuscript (MS) describes the production and characterization of aerosol nanoparticles (NP) as single material form and as hetero-aggregates (HA) using a spark discharge generator (SDG) combined with a nebulizer. The HA consist of TiO2-NP, on which four different metal NPs are deposited. The pure metallic particles and the HA are characterized online and offline using a variety of methods such as SMPS and TEM-EDX.
The MS contains many valuable and original results, which, however, still need to be bundled and formulated much better. In particular, there is no common thread linking the research question addressed, its answer and further consequences of the results. What are, up to the conclusion, new findings the authors want to report about the coating of larger NPs with metal NPs from spark ablation? The authors are encouraged to thoroughly rewrite the manuscript with a clear story line and targeted motivation. One interesting part to focus on could be the metal-depending layer quality (layer porosity, thickness, contact angles etc.) of spark generated NPs on TiO2 surfaces. In this context, the oxide affinity of certain metals, e.g. Au vs. Cu or Ni, could be systematically investigated, including many of the already presented data in a condensed way.
The individual points are explained in more detail below, with the page (P) and line (L) indicated.
1)
Introduction
The spark ablation section is well described; however, a detailed motivation for coating TiO2 NPs with spark generated particles is missing. Topical literature regarding this aspect is missing, too. While the introduction covers many interesting topics a clear focus of the presented work at the end of the introduction is missing. i.e. which open research questions should be addressed here. This goes beyond the listing of the performed investigations and covers rather the coherence of the results presented here.
2)
P 4, L 104-108: Collecting small particles by diffusional deposition leads to a size-biased representation of the particle, which is specially agravated for small nanoparticles as outlined in the MS. However, also the amount of deposited particles differs substantially from the number of particles in the aerosol. Therefore, the question needs to be discussed how the fraction of nanoparticles sampled on the TEM grid (and on the Teflon filters) in relation to the total aerosol particles is determined. It is surprising that the number of deposited particles and aerosol particles are so close to each other (e.g. Fig. 4). Which approach was taken to quantify the fraction of deposited particles? Please explain this point in more detail.
3)
How does a quantitative NP sampling work on Teflon filters with 2 µm pore size and NPs being smaller than approx. 50 nm?
4)
P 5, L 126: What is the motivation for depositing agglomerate NPs on a solid TiO2 film and the subsequent sputter coating with a 100Â nm Au layer? It is not clear why such a thick platinum layer needs to be applied. Does this not lead to a significant loss on resolution?
5)
P 6, L 151ff: The idea of employing the circularity to define the spherical particles is very good. This approach could make an important contribution to the issue about the first nucleating clusters and their further fate. This approach is certainly a highlight of the MS and would deserve some more evaluation. Why was the limit of 90% also applied for sphericities staring well below 1.0 (cf. Fig. A1, e.g. Pt for 1.3 s residence time)? Is the fact that some particles start already with sphericities well below 1.0 an indication that in fact they consist of much smaller units, i.e. atomic clusters? In Fig. 3 for Pt 3 residence times were analyzed with respect to size distribution. Why was the results for 26 s not also included in Fig. A1 in the Pt column?
6)
The authors state that the coagulation time influenced the primary particle size of the respective materials. How is that possible when primary particle formation is completed approx. 100 µs after the spark discharge? And how can the sphericity of the Pt increase with longer residence time (from 1.3 s to 2.2 s) in Fig. A1?
7)
P 7, L 155 and Table I: The authors refer to “primary particle size” and “agglomerate particle size”. Please include a detailed description which size (primary vs. agglomerate) is shown in the size distributions. What equivalent diameter is presented here?
8)
P 7, L 169-171: The authors state that Au NPs might form oxides as a consequence of using N2 5.0 with oxygen impurities. This argument holds certainly for Cu and Ni. Pt NPs from spark ablation can exhibit thin oxide layers on the particle surface; however, for Au, an oxidation is impossible under the mentioned experimental circumstances. This fact supports, in turn, the relatively large primary particle size of <6Â nm for Au NPs with a circularity of C=1 that is mentioned by the authors.
9)
Besides the low melting point, especially the absence of oxidation of the surface of Au NP contributes to the strong necking and coalescence growth of Au primary particles/clusters. On the other hand, oxygen-sensitive materials such as Ni and Cu experience a so called "pinning effect" by adsorption/reaction of the surface with oxygen (e.g. Seipenbusch et al. J. Aerosol Sci. 34, 2003). Such a pinning effect can increase the activation energy for coalescence from typically 50 kJ/mol (clean metal surface) to about 80 kJ/mol (partly oxidized metal surface, e.g. Ni). This point should be more elaborated here.
10)
P 8, L 193-195: Regarding the generally poor charging of very small NP it is surprising that losses due to electric fields should accumulate to such an important amount. Are the substantial losses in the SDG chamber not rather related to the high particle diffusivity?
11)
P 8, L 206-208: The authors mention the ejection of micron sized particles during spark erosion and a subsequent deposition of those within the housing of the spark discharge generator. The observation of Tabrizi et al. is very interesting and is also found for other processes where metal surfaces are locally heated up by sparks or pulsed lasers. It is also reasonable to attribute a high mass loss to the deposition of large micron sized particles. However, since the significance of this mass loss channel is expected to depend very much on the material properties, a more direct confirmation of these droplet-based particles would improve the convincing power of the argument. For instance, samples from the SDG chamber could be taking a wipe sample and analyzing it with electron microscopy (SEM/EDX or TEM/EDX) for micronic spherical particles.
12)
P 12, L 281: The authors derived contact angles for Au and Pt on TiO2. Please show a TEM image with a magnification where the contact angle is outlined. Please increase also the size of the TEM micrographs shown in Fig. 5. The inserts are very hard to see.
13)
P 13/14/18, Fig.6, 8, B1: The coloring of the EDX maps is confusing. Please use a defined color for each element to distinguish, e.g., background from sputter layers from NPs from TiO2 support. Fig.6: The authors present dark field (DF) and bright field (BF) TEM(STEM) images. Please refrain from using BF images for Pt@TiO2 since Pt has a high contrast in HAADF, such as Au in Fig.6 top left.
14)
P 15, L 328: The authors mention a continuous layer of Au on the TiO2 substrate after deposition. This observation can be related to the oxygen affinity of the metals used. Due to the absence of oxygen in Au NPs, partial sintering and necking can be observed even at room temperature.
15)
P 19, L 370-371: The calculation of the corresponding aerosol concentration based on the TEM micrograph is still not clear (cf. comment above). How was this done?
16)
P 19, Caption of Fig. C1: The diffusion losses in laminar flow in tubes does not depend on the tube diameter! Therefore, this argument about the influence of the larger tube does not apply. There must be another reason for the similar losses at different residence times since the amount of loss depends critically on the duration.
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Minor comments:
- Check whole manuscript for typos
- Please be consistent with units, e.g., P 2, L 39 “20.000 Kelvin”; please write 20.000 K
- Please check all literature references for typos, e.g., P 2, L 47, P 12,L 273
- P 5, L 119: In front of "... m2 of the grid" seems something to be missing. Please correct it.
- P 12, L 273: The reference “(62)” is completely out of the normal referencing system, which relies on “name and year”. Please correct this.
- P 14, L 313: The sentence “This is a lower estimate of the coating” is not clear. Please be more specific.
Citation: https://doi.org/10.5194/ar-2025-2-RC1 -
RC2: 'Comment on ar-2025-2', Anonymous Referee #2, 16 Feb 2025
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Overall the work is written in a vague manner without direct means for relevant confirmations. This work also made the impression for reproducing already published works published years ago but not yet fully cited them. This “gave the chance” of this work to pretend to be the initial discoveries. The morphologies of the NPs demonstrate that this study did not yet optimize the system. Only switching on the generator and collecting them for subsequent analysis cannot add scientific values, particularly ignoring the large volume of literature, which reported almost the same line of argumentations. The authors also try to use artificially texted letters to mark the different metals without showing any experimental proofs. Raising such concern is mainly due to all their similar appearances but undistinguishable nature from the TEM results.Â
Large body of relevant literature was severely missing.
Primary particle was defined scientifically incorrect.
where the authors showed the 1-nm particles?
Melting point of the 4 metals cannot be directly used to evaluate their different particle sizes, as the impure surface of the particles also makes dramatic influences. Â
How the authors prove that the oxidation took place during NP production?
Why the mass was evacuated using micro molar? As the authors also claimed that the SDG can also deliver NPs of high quantity. How the amount of micro molar supports the aforementioned claim?
For the different production rates of the metals, previous study already clearly explained why. Please cite the relevant literature, not only saving the unnecessarily repeated work but also not making an impression of confined literature review.
The authors also argued that the mismatch of size distributions measured between the TEM analysis and SMPS data is mainly due to the low counting efficiency from the latter. How to explain the overestimation of diffusional deposition of smaller NPs for TEM analysis?
A very puzzled presentation for Fig. 5 onwards is to remove the Cu NPs. Could the authors provide any scientific reasons for such inconsistencies?
How the edges of the NPs were determined when using eq. 1 to calculate circularity?
How the authors separate the ones coated over TiO2 with those of self-coagulation?
Other mistakes also showed the carelessness and this can also give the reviewer for scientific suspect. Wrong spelling in page1, “Department od Environmental Sciences” should be “Department of Environmental Sciences”.
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