On Formation of Carbonaceous Impurities in Flame Spray Synthesis of Maghemite Nanoparticles

Tischendorf, Ricardo; Duschik, Kristina; Dupont, Sophie; Fröde, Fabian; Pitsch, Heinz; Kneer, Reinhold; Schaper, Mirko; Schmid, Hans-Joachim

doi:https://doi.org/10.5194/ar-2023-14

Preprints

https://doi.org/10.5194/ar-2023-14

Preprints

03 Nov 2023

| 03 Nov 2023

Status: this preprint was under review for the journal AR but the revision was not accepted.

On Formation of Carbonaceous Impurities in Flame Spray Synthesis of Maghemite Nanoparticles

Ricardo Tischendorf, Kristina Duschik, Sophie Dupont, Fabian Fröde, Heinz Pitsch, Reinhold Kneer, Mirko Schaper, and Hans-Joachim Schmid

Abstract. Particles generated by spray flame synthesis (SFS) can exhibit impurities, with surface adsorbates and soot being the best-known examples. Unfortunately, the extent to which the formation of these impurities can be characterized, and how their formation can be avoided is not fully understood yet. In order to contribute to the understanding of the formation of carbonaceous impurities, we investigated their formation using a standardized burner type (SpraySyn burner) while synthesizing maghemite by the gas-to-particle conversion. Two approaches were followed to characterize the formation of soot and surface adsorbates in this paper. Firstly, fabricated powders were analyzed by complementary powder analysis. In this term, the detection of soot was negative, but powders contained carboxylates and carbonates, which were bound to particle surfaces. The methodological procedure used allowed a reliable quantification of those adsorbates, and it was possible to normalize their relative mass fractions (up to ~16 %) to the powders' respective specific surface areas. The normalization approach yielded a surface loading values of 0.84–0.85 mg m^-2. The methodical procedure and the data provided can be used in the future to denote/compare SFS materials in sensitivity studies. Secondly, particles were thermophoretically extracted from the process in situ and subsequently characterized by transmission electron microscopy (TS-TEM). Interestingly, particles extracted at 5 cm height above the burner (HAB) (representing the center of the visible flame) showed amorphous core-shell structures and amorphous aggregates, indicating a coexistence of soot alongside maghemite particles. Via reducing the sampling time of the TS system to 1.5 ms samples were taken either from flame pulses (a state of high flame activity) or from flame flickering (lower flame activity). A comparison of these samples showed that heterogeneous particle structures formed predominantly in flame pulses. As samples extracted at 15 cm HAB neither showed amorphous coating nor amorphous aggregates, it was indicated that soot fractions oxidize between 5 and 15 cm HAB.

Received: 24 Oct 2023 – Discussion started: 03 Nov 2023

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Ricardo Tischendorf, Kristina Duschik, Sophie Dupont, Fabian Fröde, Heinz Pitsch, Reinhold Kneer, Mirko Schaper, and Hans-Joachim Schmid

Status: closed

RC1:
'Comment on ar-2023-14', Anonymous Referee #1, 05 Dec 2023

The manuscript deals with the formation of impurities of maghemite nanoparticles during spray flame synthesis (SFS). The resulting particles are characterized by ex-situ methods and investigated in situ at two heights above the burner by thermophoretic sampling (TS) and subsequent TEM analysis. From the comparison of the two approaches, it is deduced that amorphous core-shell structures and amorphous aggregates are present at low heights above the burner (HAB = 5 cm), which indicate the coexistence of soot and maghemite NP. This phenomenon is more pronounced for the pulsation state of the flame than for flickering states. Surprisingly, these amorphous soot structures are neither found higher in the flame (HAB = 15 cm) nor in the powder deposited on a filter outside the hot temperature region of the flame. Instead, substantial amounts of adsorbed carboxylates and carbonates are found in the deposited powder. From this, the authors conclude that the soot is oxidized between 5cm and 15cm HAB.
General comments:
The manuscript contains many interesting and valuable results and aspects, which are of interest to the aerosol community, such as the appearance and disappearance of soot at low flame heights. The uniform coverage with carboxylates and carbonates, independent of the precursor concentration, is also astonishing and should be followed up. Unfortunately, the structure oft he manuscript is very confusing due to excessively long insertions, such as for the TS method, which makes the reading flow unclear. The discussion of the spectra is also not very fluent, for each peak reference is made to the literature, which does not contribute significantly to improved insights. Individual passages, such as the overlong explanation of the D and G bands in Raman spectra, are also unnecessary. This basic knowledge can be summarized in one sentence and it is sufficient to provide a reference to further literature. It is therefore recommended that the manuscript be radically shortened and made more compact. Only after such focusing is the work suitable for publication. Detailed comments are listed below.
Specific comments:
In general, simple facts are presented in an excessive manner. In particular, the detailed explanations of TS-TEM sampling are unnecessarily broad and long, so that the flow of reading comes to an almost complete standstill without any direct added value. This part, called a mini-review, seems to be neither necessary nor helpful for the orientation of the manuscript and should be published in a separate publication, e.g. as technical note.
Also other parts of the manuscript should also be shortened in the interests of a denser and more targeted presentation of the data. For example, the collection time of 40ms at 5cm HAB proves to be far too long, so that it is subsequently reduced to 1.5ms. However, one sentence is sufficient to explain this and it does not require an entire page including Fig. 10.
Page 6, Line 169-170: „… influence of changing the INN concentration is rather small.“ This is not such a surprise when changing the precursor concentration only by a factor of 2. Why was it not varied by an order of magnitude?
P 8-16: The TS-TEM system is discussed in great detail and classified historically. All these explanations are not necessary to present the three main points to be considered in the design. It would also suffice to discuss the experimental results on spring deflection and the influence of slider mass in the detailed discussion of performance. Models that show the same trends but deviations from the experimental results should be included in a separate paper on TS technology. Surprisingly, however, the central question of the representativeness of the drawn sample for the NP in the flame is not addressed. There are several papers on the representativeness of TS samples for perpendicular flow. Is this also the case for parallel flow? That would be a valuable addition.
P 18, L 389-393: While crystalline areas are relatively easy to recognize in TEM micrographs, this is much more difficult for amorphous areas, especially for very small structures. Given the moderate quality of the TEM micrographs, I would be cautious with statements such as "no evidence of amorphous particles or coatings". In my opinion, the images do not provide this. This also applies to the later statement (P 18, L 409): "However, no graphite traces were detected in the powder by TEM.“ and to the statement (P 19, L 424): „Even if the powder samples appear to be pure by TEM microscopy, …“
P 21, Fig. 8: What is the second curve in the TGA spectrum? Supposedly, the derivation of the mass curve. In the figure, „phase transfer“ should be replaced by „phase transformation“. Later (P 22, L 496), it is stated that „the purification is much more efficient in air“. However, in Fig. 8 at the end temperature (800°C) the mass loss in air is about 20%, while in argon it is about 30%, which does not agree with the statement above. What are the resaons for this discrepancy?
P 22, L 489: The phase transformation deduced from the DCS spectrum at 495°C from maghemite to hematite is in contradiction to the behavior of the powder mass. A change from maghemite (rho_p = 4.86 g/cm3) to hematite (rho_p = 5.26 g/cm3) should be accompanied by a mass increase. However, this is not observed in the TGA curve.
P 22, L 502: „… was much higher than the sample from 0.2 M INN.“ Regarding the total mass losses in the TGA measurements, the mass loss is only 16% higher, which may not correspond to „much higher“.
P 22, L 508-511: What layer thickness does the found value of 0.85 mg/m2 correspond to? Why should there not be more or less adsorbates if precursor concentration is changed? Could this indicate a transport limitation or a self-terminating effect, e.g. after the deposition of one monolayer?P 24, L 560-561 and L 565: This was said before. Please do not repeat previously made statements again and again. P 24-25, L 560-579: The only important information of this paragraph is that the exposure time of 40ms is too long and should be reduced. Plaese remove this paragraph.
P 29, L 656-658: Unfortunately EELS was not done here. It could have been an important piece in confirming the conclusions drawn here.
Technical corrections:
P 3, L 73: „… it is noteworthy <to mention> that carbon…“
P 6, L 157-158: strange sentence: „… influencing the formation of disperse particle properties or impurities.“ What is the formation of particle properties?
P 7, L 191: „… sufficient <low> temperature range …“
P 11, L 268: The „2“ is missing in the paragraph numbering
P 14, L 322: „The drilling hole is highlighted in Fig. 2b.“ This cannot be seen there.
P 19, L421: „… concentration, which indicates a larger …“ (not a point before „which“)
P 19, Fig. 7a: Was the INN measured with ATR-FTIR in a dry state? If so, please indicate.
P 21, L 484: The number given in the text (227 and 273°C) do not agree with the ones given in Fig. 8 (225 and 270°C).
P 24, L 552: „… were in absolut agreement to TEM examinations…“ Given the quality of the data, “absolute agreement” may be somewhat exaggerated. P 28, L 613-615: Would the evaporation of carboxylates and carbonates not also be expected in the UHV of the TEM?
P 29, L 636: „Assuming that the change <of> the precursor concentration…“

Citation: https://doi.org/10.5194/ar-2023-14-RC1
- AC3: 'Reply on RC1', Ricardo Tischendorf, 03 Feb 2024
  
  The authors would like to express their severe gratitude to the referee for his valuable comments. We regret that the submitted version of our manuscript was not focused appropriately: On the one hand, we did not focus on our findings/intensions enough. On the other hand, we provided too much text/information regarding i) our TS-sampling procedure and ii) regarding basic knowledge (e.g., Raman references). We completely reworked the manuscript and drastically shortened it in order to comply with the referees’ suggestions. Thereby, the manuscript has improved tremendously. In detail, we respected the referees’ suggestions the following way.
  Reviewers' specific comments:
  Reviewer: In general, simple facts are presented in an excessive manner. In particular, the detailed explanations of TS-TEM sampling are unnecessarily broad and long, so that the flow of reading comes to an almost complete standstill without any direct added value. This part, called a mini-review, seems to be neither necessary nor helpful for the orientation of the manuscript and should be published in a separate publication, e.g. as a technical note.
  Author: We removed this section, and we are preparing a technical note with this content to be submitted to Review of Scientific Instruments as suggested by the Reviewer.
  Reviewer: Also, other parts of the manuscript should also be shortened in the interests of a denser and more targeted presentation of the data. For example, the collection time of 40 ms at 5 cm HAB proves to be far too long, so that it is subsequently reduced to 1.5 ms. However, one sentence is sufficient to explain this and it does not require an entire page including Fig. 10.
  Author: Information regarding the TS-TEM experiments we drastically shortened in the new manuscript. Now, in the Materials & Methods section, we solely present most important information about the TS-TEM experiments at ~one page (no subchapters and solely including one Figure). In the Results & Discussion section, we excluded information regarding the mentioned experiment (5 cm HAB, 40 ms). Thus, we solely focused in this section on samplings from 15 cm HAB, and from 5 cm HAB using 1.5 ms.
  Reviewer: Page 6, Line 169-170: „… influence of changing the INN concentration is rather small.“ This is not such a surprise when changing the precursor concentration only by a factor of 2. Why was it not varied by an order of magnitude?
  Author: This is a very important issue. And admittedly, we have missed to explain the choice of the INN concentration in the first manuscript draft. Our intension was to generate powder samples with different specific surface areas (SSA), to prove the idea that it’s possible to relate the relative mass of surface adsorbate depositions to the SSA-value. However, we tried to avoid a considerable influence on the flame chemistry and on physical processes which take place in the flame and cause a change in formed adsorbates. We name one possible reason how flame physics could be affected by increasing the INN concentration by a magnitude. In particular, droplet explosions are highly related to the precursor concentration. Considering this phenomenon, one has to expect increasing the INN concentration to e.g., 1 M INN should have a considerable effect on the flame physic and chemistry and hence also a change in formed carboxylates/carbonates could be expected in this case.
  Reviewer: P 8-16: The TS-TEM system is discussed in great detail and classified historically. All these explanations are not necessary to present the three main points to be considered in the design. It would also suffice to discuss the experimental results on spring deflection and the influence of slider mass in the detailed discussion of performance. Models that show the same trends but deviations from the experimental results should be included in a separate paper on TS technology. Surprisingly, however, the central question of the representativeness of the drawn sample for the NP in the flame is not addressed. There are several papers on the representativeness of TS samples for perpendicular flow. Is this also the case for parallel flow? That would be a valuable addition.
  Author: To our best of knowledge, a detailed study considering the representativeness for TS-TEM experiments for samplers using parallel flow does not exist. However, when a perpendicular flow is used for sampling the collection is influenced by inertia and thermophoresis. In our parallel flow arrangement only thermophoretic forces are responsible for particle deposition. Since thermophoretic sampling velocity is only affected by particle size due to the Cunningham slip correction via the Knudsen number, the particle size dependency is rather small (Talbot et al. 1980). Therefore, it is presumed that thermophoretic sampling leads to a high representativeness of the sampled particles. However, further investigations to quantify this should be done in the future.
  Reviewer: P 18, L 389-393: While crystalline areas are relatively easy to recognize in TEM micrographs, this is much more difficult for amorphous areas, especially for very small structures. Given the moderate quality of the TEM micrographs, I would be cautious with statements such as "no evidence of amorphous particles or coatings". In my opinion, the images do not provide this. This also applies to the later statement (P 18, L 409): "However, no graphite traces were detected in the powder by TEM.“ and to the statement (P 19, L 424): „Even if the powder samples appear to be pure by TEM microscopy, …“
  Author: We mitigated our wording accordingly. And we agree, our TEM-examinations were accompanied by uncertainties regarding the presence of carbon depositions. Now, we highlight TEM examinations were not used to derive a ‘secure proof’. Instead, we explain, TEM observations gave an initial indication that carbon was not abundant. This claim was later evaluated using TGA-DSC-MS.
  Reviewer: P 21, Fig. 8: What is the second curve in the TGA spectrum? Supposedly, the derivation of the mass curve. In the figure, „phase transfer“ should be replaced by „phase transformation“. Later (P 22, L 496), it is stated that „the purification is much more efficient in air“. However, in Fig. 8 at the end temperature (800°C) the mass loss in air is about 20%, while in argon it is about 30%, which does not agree with the statement above. What are the reasons for this discrepancy?
  Author: We adapted the Figure accordingly. Additionally, we explain why final-end-masses are different under air and argon. Since oxygen was limited under argon atmosphere, oxygen was most probably extracted from the Fe2O3-phase to oxidize carbonates/carboxylates. This phenomenon was also found in an earlier study. For instance, Grimm et al. conducted TGA-experiments on SFS-made maghemite particles under different atmosphere and observed identical tendencies.
  Reviewer: P 22, L 489: The phase transformation deduced from the DCS spectrum at 495°C from maghemite to hematite is in contradiction to the behavior of the powder mass. A change from maghemite (rho_p = 4.86 g/cm3) to hematite (rho_p = 5.26 g/cm3) should be accompanied by a mass increase. However, this is not observed in the TGA curve.
  Author: Here, we do not agree with the statement. Indeed, the density changes due to the phase transformation from maghemite to hematite. However, the samples mass is not affected this way. In TGA-measurements a certain sample mass is deposed on a sample holder. When reaching the transformation point, crystallographic properties change. Regarding the manufactured iron oxide phase, a transformation from the α-structure to the γ-structure was indicated. But, in fact, the chemical stoichiometry (Fe2O3) stayed unaffected. Thus, the material density changed (because crystallographic properties changed), rather than the sample mass.
  Reviewer: P 22, L 502: „… was much higher than the sample from 0.2 M INN.“ Regarding the total mass losses in the TGA measurements, the mass loss is only 16% higher, which may not correspond to „much higher“.
  Author: We agree, using terminologies like ‘much higher’ can be ambiguous. We carefully revised our document for comparable wordings, and we mitigated it accordingly. In the particular case mentioned here, we use the term ‘considerable’ now.
  Reviewer: P 22, L 508-511: What layer thickness does the found value of 0.85 mg/m2 correspond to? Why should there not be more or less adsorbates if precursor concentration is changed? Could this indicate a transport limitation or a self-terminating effect, e.g. after the deposition of one monolayer? P 24, L 560-561 and L 565: This was said before. Please do not repeat previously made statements again and again. P 24-25, L 560-579: The only important information of this paragraph is that the exposure time of 40ms is too long and should be reduced. Please remove this paragraph.
  Author: We removed the paragraph concerning the sampling at 5 cm HAB using 40 ms. Additionally, we tried to avoid repetitions. Admittedly, we do not know how to calculate a corresponding, theoretical layer thickness. From current state, we think the particle surface is covered entirely by water and carbonates/carboxylates. Latter species are attached on the particle surface by complex binding. Hence, oxygen-atoms are connected to Fe from the particle phase. In terms of carboxylates (COO--R), we think molecular rests R are present with a variety of structure/length. In this regard, the mass which is bound on surfaces should depend on several parameters. For instance, the mass should be sensitive to the surface area, as well as on the molecular mass of the carboxylate Rests, R. We did not expect the INN concentration should affect the formation of carbonaceous depositions, as the flame enthalpy and the flame stoichiometry is not very sensitive to the change from 0.1 M to 0.2 M INN.
  Reviewer: P 29, L 656-658: Unfortunately, EELS was not done here. It could have been an important piece in confirming the conclusions drawn here.
  Author: We agree having EELS-data (elemental maps considering the core-shell structures) would be very valuable. In fact, we plan to work on the TS-TEM topic in more detail in future and we intend to use statistical particle analytics by quantitative image-post-processing. In this regard we will also apply elemental-particle-mapping using EELS.
  Reviewers' technical corrections:
  Reviewer: P 3, L 73: „… it is noteworthy <to mention> that carbon…“
  Author: The wording was corrected accordingly.
  Reviewer: P 6, L 157-158: strange sentence: „… influencing the formation of disperse particle properties or impurities.“ What is the formation of particle properties?
  Author: Unclear wording was removed.
  Reviewer: P 7, L 191: „… sufficient <low> temperature range …“
  Author: The wording was corrected accordingly.
  Reviewer: P 11, L 268: The „2“ is missing in the paragraph numbering.
  Author: The “2” was inserted.
  Reviewer: P 14, L 322: „The drilling hole is highlighted in Fig. 2b.“ This cannot be seen there.
  Author: Since the material & method section was shortened considerable, the figure was removed.
  Reviewer: P 19, L421: „… concentration, which indicates a larger …“ (not a point before „which“)
  Author: The wording was corrected accordingly.
  Reviewer: P 19, Fig. 7a: Was the INN measured with ATR-FTIR in a dry state? If so, please indicate.
  Author: We adapted the text in this regard: ‘To evaluate whether precursor feed residuals were owing to these features or not, we also characterized the feed components (INN, EtOH, 2-EHA) by ATR-FTIR (INN measured in dry-state).’
  Reviewer: P 21, L 484: The number given in the text (227 and 273°C) do not agree with the ones given in Fig. 8 (225 and 270°C).
  Author: We adapted the numbers.
  Reviewer: P 24, L 552: „… were in absolut agreement to TEM examinations…“ Given the quality of the data, “absolute agreement” may be somewhat exaggerated. P 28, L 613-615: Would the evaporation of carboxylates and carbonates not also be expected in the UHV of the TEM?
  Author: We mitigated the wording in this regard.
  Reviewer: P 29, L 636: „Assuming that the change <of> the precursor concentration…“
  Author: The wording was corrected accordingly.
  
  Citation: https://doi.org/10.5194/ar-2023-14-AC3
RC2:
'Comment on ar-2023-14', Anonymous Referee #2, 27 Dec 2023

This is an important topic that has not been addressed in depth in the “flame synthesis” community. A 30-page long manuscript written in small font size (~12’000 Words, twice as many as a standard research article) and approx. 120 references raise high expectations with regard to a detailed analysis of the subject.
Unfortunately, 50 to 70% of the text is not relevant for the topic suggested by the title. This text seems to be an excerpt from a thesis in draft version rather than a manuscript of a research paper. For instance, seven and a half pages on thermophoretic sampling (without significantly advancing the original work of Dobbins and Megaridis, 1987) have nothing to do with “Impurities in … Maghemite Nanoparticles”. A three-line description in the Methods section would suffice. If the authors want to share their study of thermophoretic sampling, they should do so in a separate manuscript devoted to that topic rather than diluting the essence of the study here.
A scientific article must be concise and straight to the point with a discussion based on pertinent literature. The contribution of every sentence and every word to the overall message of the article has to be evaluated. If there is no contribution, this word or this sentence should be removed. The reader of a scientific article expects a condensed essence and not a diluted blah-blah (with my apologies for the latter word). This includes the reviewer who works on a voluntary basis and does not get paid for going through 30 pages.
The authors must do this evaluation of relevance for every single word of their text and may submit a drastically shortened and sharpened version as a new manuscript. Once this has been done, we can talk about the scientific content. Btw, based on the list of author contributions it seems that none of the senior authors have worked on the manuscript or reviewed it.

Citation: https://doi.org/10.5194/ar-2023-14-RC2
- AC2: 'Reply on RC2', Ricardo Tischendorf, 03 Feb 2024
  
  We want to apologize in case our first manuscript draft caused any inconvenience. Our initial aim was to provide information regarding both, the TS-TEM topic, and the formation of non-product species in SFS. However, as mentioned by the reviewer, the insertion of the TS-TEM method diluted the content/quality of the manuscript considerably. We totally agree with the reviewer and consequently reworked the manuscript completely. Now, the TS-TEM methodologies are explained on ~ 1/2 page. Additionally, we also revised and sharpened the rest of our manuscript (adapting the wording and the storyline) to not dilute our conclusions/findings. We would be pleased if the manuscript aligns now with the reviewers’ expectations. We are looking very much forward to your valuable comments regarding the scientific content of our study in the next step. We would be grateful for any comments to further improve the manuscript.
  
  Citation: https://doi.org/10.5194/ar-2023-14-AC2
AC1: 'Comment on ar-2023-14', Ricardo Tischendorf, 05 Jan 2024

We sincerely appreciate the reviewers for prooving our manuscript, and for giving very valuable comments to improve it. Both reviewers suggested to condense the manuscript by reducing the technical/theoretical information about the TS-TEM procedure. We want to carefully consider all requested corrections and all suggestions in detail in a second manuscript draft. There, the TS-TEM part will be reduced to its essential content.

Citation: https://doi.org/10.5194/ar-2023-14-AC1

Status: closed

RC1:
'Comment on ar-2023-14', Anonymous Referee #1, 05 Dec 2023

The manuscript deals with the formation of impurities of maghemite nanoparticles during spray flame synthesis (SFS). The resulting particles are characterized by ex-situ methods and investigated in situ at two heights above the burner by thermophoretic sampling (TS) and subsequent TEM analysis. From the comparison of the two approaches, it is deduced that amorphous core-shell structures and amorphous aggregates are present at low heights above the burner (HAB = 5 cm), which indicate the coexistence of soot and maghemite NP. This phenomenon is more pronounced for the pulsation state of the flame than for flickering states. Surprisingly, these amorphous soot structures are neither found higher in the flame (HAB = 15 cm) nor in the powder deposited on a filter outside the hot temperature region of the flame. Instead, substantial amounts of adsorbed carboxylates and carbonates are found in the deposited powder. From this, the authors conclude that the soot is oxidized between 5cm and 15cm HAB.
General comments:
The manuscript contains many interesting and valuable results and aspects, which are of interest to the aerosol community, such as the appearance and disappearance of soot at low flame heights. The uniform coverage with carboxylates and carbonates, independent of the precursor concentration, is also astonishing and should be followed up. Unfortunately, the structure oft he manuscript is very confusing due to excessively long insertions, such as for the TS method, which makes the reading flow unclear. The discussion of the spectra is also not very fluent, for each peak reference is made to the literature, which does not contribute significantly to improved insights. Individual passages, such as the overlong explanation of the D and G bands in Raman spectra, are also unnecessary. This basic knowledge can be summarized in one sentence and it is sufficient to provide a reference to further literature. It is therefore recommended that the manuscript be radically shortened and made more compact. Only after such focusing is the work suitable for publication. Detailed comments are listed below.
Specific comments:
In general, simple facts are presented in an excessive manner. In particular, the detailed explanations of TS-TEM sampling are unnecessarily broad and long, so that the flow of reading comes to an almost complete standstill without any direct added value. This part, called a mini-review, seems to be neither necessary nor helpful for the orientation of the manuscript and should be published in a separate publication, e.g. as technical note.
Also other parts of the manuscript should also be shortened in the interests of a denser and more targeted presentation of the data. For example, the collection time of 40ms at 5cm HAB proves to be far too long, so that it is subsequently reduced to 1.5ms. However, one sentence is sufficient to explain this and it does not require an entire page including Fig. 10.
Page 6, Line 169-170: „… influence of changing the INN concentration is rather small.“ This is not such a surprise when changing the precursor concentration only by a factor of 2. Why was it not varied by an order of magnitude?
P 8-16: The TS-TEM system is discussed in great detail and classified historically. All these explanations are not necessary to present the three main points to be considered in the design. It would also suffice to discuss the experimental results on spring deflection and the influence of slider mass in the detailed discussion of performance. Models that show the same trends but deviations from the experimental results should be included in a separate paper on TS technology. Surprisingly, however, the central question of the representativeness of the drawn sample for the NP in the flame is not addressed. There are several papers on the representativeness of TS samples for perpendicular flow. Is this also the case for parallel flow? That would be a valuable addition.
P 18, L 389-393: While crystalline areas are relatively easy to recognize in TEM micrographs, this is much more difficult for amorphous areas, especially for very small structures. Given the moderate quality of the TEM micrographs, I would be cautious with statements such as "no evidence of amorphous particles or coatings". In my opinion, the images do not provide this. This also applies to the later statement (P 18, L 409): "However, no graphite traces were detected in the powder by TEM.“ and to the statement (P 19, L 424): „Even if the powder samples appear to be pure by TEM microscopy, …“
P 21, Fig. 8: What is the second curve in the TGA spectrum? Supposedly, the derivation of the mass curve. In the figure, „phase transfer“ should be replaced by „phase transformation“. Later (P 22, L 496), it is stated that „the purification is much more efficient in air“. However, in Fig. 8 at the end temperature (800°C) the mass loss in air is about 20%, while in argon it is about 30%, which does not agree with the statement above. What are the resaons for this discrepancy?
P 22, L 489: The phase transformation deduced from the DCS spectrum at 495°C from maghemite to hematite is in contradiction to the behavior of the powder mass. A change from maghemite (rho_p = 4.86 g/cm3) to hematite (rho_p = 5.26 g/cm3) should be accompanied by a mass increase. However, this is not observed in the TGA curve.
P 22, L 502: „… was much higher than the sample from 0.2 M INN.“ Regarding the total mass losses in the TGA measurements, the mass loss is only 16% higher, which may not correspond to „much higher“.
P 22, L 508-511: What layer thickness does the found value of 0.85 mg/m2 correspond to? Why should there not be more or less adsorbates if precursor concentration is changed? Could this indicate a transport limitation or a self-terminating effect, e.g. after the deposition of one monolayer?P 24, L 560-561 and L 565: This was said before. Please do not repeat previously made statements again and again. P 24-25, L 560-579: The only important information of this paragraph is that the exposure time of 40ms is too long and should be reduced. Plaese remove this paragraph.
P 29, L 656-658: Unfortunately EELS was not done here. It could have been an important piece in confirming the conclusions drawn here.
Technical corrections:
P 3, L 73: „… it is noteworthy <to mention> that carbon…“
P 6, L 157-158: strange sentence: „… influencing the formation of disperse particle properties or impurities.“ What is the formation of particle properties?
P 7, L 191: „… sufficient <low> temperature range …“
P 11, L 268: The „2“ is missing in the paragraph numbering
P 14, L 322: „The drilling hole is highlighted in Fig. 2b.“ This cannot be seen there.
P 19, L421: „… concentration, which indicates a larger …“ (not a point before „which“)
P 19, Fig. 7a: Was the INN measured with ATR-FTIR in a dry state? If so, please indicate.
P 21, L 484: The number given in the text (227 and 273°C) do not agree with the ones given in Fig. 8 (225 and 270°C).
P 24, L 552: „… were in absolut agreement to TEM examinations…“ Given the quality of the data, “absolute agreement” may be somewhat exaggerated. P 28, L 613-615: Would the evaporation of carboxylates and carbonates not also be expected in the UHV of the TEM?
P 29, L 636: „Assuming that the change <of> the precursor concentration…“

Citation: https://doi.org/10.5194/ar-2023-14-RC1
- AC3: 'Reply on RC1', Ricardo Tischendorf, 03 Feb 2024
  
  The authors would like to express their severe gratitude to the referee for his valuable comments. We regret that the submitted version of our manuscript was not focused appropriately: On the one hand, we did not focus on our findings/intensions enough. On the other hand, we provided too much text/information regarding i) our TS-sampling procedure and ii) regarding basic knowledge (e.g., Raman references). We completely reworked the manuscript and drastically shortened it in order to comply with the referees’ suggestions. Thereby, the manuscript has improved tremendously. In detail, we respected the referees’ suggestions the following way.
  Reviewers' specific comments:
  Reviewer: In general, simple facts are presented in an excessive manner. In particular, the detailed explanations of TS-TEM sampling are unnecessarily broad and long, so that the flow of reading comes to an almost complete standstill without any direct added value. This part, called a mini-review, seems to be neither necessary nor helpful for the orientation of the manuscript and should be published in a separate publication, e.g. as a technical note.
  Author: We removed this section, and we are preparing a technical note with this content to be submitted to Review of Scientific Instruments as suggested by the Reviewer.
  Reviewer: Also, other parts of the manuscript should also be shortened in the interests of a denser and more targeted presentation of the data. For example, the collection time of 40 ms at 5 cm HAB proves to be far too long, so that it is subsequently reduced to 1.5 ms. However, one sentence is sufficient to explain this and it does not require an entire page including Fig. 10.
  Author: Information regarding the TS-TEM experiments we drastically shortened in the new manuscript. Now, in the Materials & Methods section, we solely present most important information about the TS-TEM experiments at ~one page (no subchapters and solely including one Figure). In the Results & Discussion section, we excluded information regarding the mentioned experiment (5 cm HAB, 40 ms). Thus, we solely focused in this section on samplings from 15 cm HAB, and from 5 cm HAB using 1.5 ms.
  Reviewer: Page 6, Line 169-170: „… influence of changing the INN concentration is rather small.“ This is not such a surprise when changing the precursor concentration only by a factor of 2. Why was it not varied by an order of magnitude?
  Author: This is a very important issue. And admittedly, we have missed to explain the choice of the INN concentration in the first manuscript draft. Our intension was to generate powder samples with different specific surface areas (SSA), to prove the idea that it’s possible to relate the relative mass of surface adsorbate depositions to the SSA-value. However, we tried to avoid a considerable influence on the flame chemistry and on physical processes which take place in the flame and cause a change in formed adsorbates. We name one possible reason how flame physics could be affected by increasing the INN concentration by a magnitude. In particular, droplet explosions are highly related to the precursor concentration. Considering this phenomenon, one has to expect increasing the INN concentration to e.g., 1 M INN should have a considerable effect on the flame physic and chemistry and hence also a change in formed carboxylates/carbonates could be expected in this case.
  Reviewer: P 8-16: The TS-TEM system is discussed in great detail and classified historically. All these explanations are not necessary to present the three main points to be considered in the design. It would also suffice to discuss the experimental results on spring deflection and the influence of slider mass in the detailed discussion of performance. Models that show the same trends but deviations from the experimental results should be included in a separate paper on TS technology. Surprisingly, however, the central question of the representativeness of the drawn sample for the NP in the flame is not addressed. There are several papers on the representativeness of TS samples for perpendicular flow. Is this also the case for parallel flow? That would be a valuable addition.
  Author: To our best of knowledge, a detailed study considering the representativeness for TS-TEM experiments for samplers using parallel flow does not exist. However, when a perpendicular flow is used for sampling the collection is influenced by inertia and thermophoresis. In our parallel flow arrangement only thermophoretic forces are responsible for particle deposition. Since thermophoretic sampling velocity is only affected by particle size due to the Cunningham slip correction via the Knudsen number, the particle size dependency is rather small (Talbot et al. 1980). Therefore, it is presumed that thermophoretic sampling leads to a high representativeness of the sampled particles. However, further investigations to quantify this should be done in the future.
  Reviewer: P 18, L 389-393: While crystalline areas are relatively easy to recognize in TEM micrographs, this is much more difficult for amorphous areas, especially for very small structures. Given the moderate quality of the TEM micrographs, I would be cautious with statements such as "no evidence of amorphous particles or coatings". In my opinion, the images do not provide this. This also applies to the later statement (P 18, L 409): "However, no graphite traces were detected in the powder by TEM.“ and to the statement (P 19, L 424): „Even if the powder samples appear to be pure by TEM microscopy, …“
  Author: We mitigated our wording accordingly. And we agree, our TEM-examinations were accompanied by uncertainties regarding the presence of carbon depositions. Now, we highlight TEM examinations were not used to derive a ‘secure proof’. Instead, we explain, TEM observations gave an initial indication that carbon was not abundant. This claim was later evaluated using TGA-DSC-MS.
  Reviewer: P 21, Fig. 8: What is the second curve in the TGA spectrum? Supposedly, the derivation of the mass curve. In the figure, „phase transfer“ should be replaced by „phase transformation“. Later (P 22, L 496), it is stated that „the purification is much more efficient in air“. However, in Fig. 8 at the end temperature (800°C) the mass loss in air is about 20%, while in argon it is about 30%, which does not agree with the statement above. What are the reasons for this discrepancy?
  Author: We adapted the Figure accordingly. Additionally, we explain why final-end-masses are different under air and argon. Since oxygen was limited under argon atmosphere, oxygen was most probably extracted from the Fe2O3-phase to oxidize carbonates/carboxylates. This phenomenon was also found in an earlier study. For instance, Grimm et al. conducted TGA-experiments on SFS-made maghemite particles under different atmosphere and observed identical tendencies.
  Reviewer: P 22, L 489: The phase transformation deduced from the DCS spectrum at 495°C from maghemite to hematite is in contradiction to the behavior of the powder mass. A change from maghemite (rho_p = 4.86 g/cm3) to hematite (rho_p = 5.26 g/cm3) should be accompanied by a mass increase. However, this is not observed in the TGA curve.
  Author: Here, we do not agree with the statement. Indeed, the density changes due to the phase transformation from maghemite to hematite. However, the samples mass is not affected this way. In TGA-measurements a certain sample mass is deposed on a sample holder. When reaching the transformation point, crystallographic properties change. Regarding the manufactured iron oxide phase, a transformation from the α-structure to the γ-structure was indicated. But, in fact, the chemical stoichiometry (Fe2O3) stayed unaffected. Thus, the material density changed (because crystallographic properties changed), rather than the sample mass.
  Reviewer: P 22, L 502: „… was much higher than the sample from 0.2 M INN.“ Regarding the total mass losses in the TGA measurements, the mass loss is only 16% higher, which may not correspond to „much higher“.
  Author: We agree, using terminologies like ‘much higher’ can be ambiguous. We carefully revised our document for comparable wordings, and we mitigated it accordingly. In the particular case mentioned here, we use the term ‘considerable’ now.
  Reviewer: P 22, L 508-511: What layer thickness does the found value of 0.85 mg/m2 correspond to? Why should there not be more or less adsorbates if precursor concentration is changed? Could this indicate a transport limitation or a self-terminating effect, e.g. after the deposition of one monolayer? P 24, L 560-561 and L 565: This was said before. Please do not repeat previously made statements again and again. P 24-25, L 560-579: The only important information of this paragraph is that the exposure time of 40ms is too long and should be reduced. Please remove this paragraph.
  Author: We removed the paragraph concerning the sampling at 5 cm HAB using 40 ms. Additionally, we tried to avoid repetitions. Admittedly, we do not know how to calculate a corresponding, theoretical layer thickness. From current state, we think the particle surface is covered entirely by water and carbonates/carboxylates. Latter species are attached on the particle surface by complex binding. Hence, oxygen-atoms are connected to Fe from the particle phase. In terms of carboxylates (COO--R), we think molecular rests R are present with a variety of structure/length. In this regard, the mass which is bound on surfaces should depend on several parameters. For instance, the mass should be sensitive to the surface area, as well as on the molecular mass of the carboxylate Rests, R. We did not expect the INN concentration should affect the formation of carbonaceous depositions, as the flame enthalpy and the flame stoichiometry is not very sensitive to the change from 0.1 M to 0.2 M INN.
  Reviewer: P 29, L 656-658: Unfortunately, EELS was not done here. It could have been an important piece in confirming the conclusions drawn here.
  Author: We agree having EELS-data (elemental maps considering the core-shell structures) would be very valuable. In fact, we plan to work on the TS-TEM topic in more detail in future and we intend to use statistical particle analytics by quantitative image-post-processing. In this regard we will also apply elemental-particle-mapping using EELS.
  Reviewers' technical corrections:
  Reviewer: P 3, L 73: „… it is noteworthy <to mention> that carbon…“
  Author: The wording was corrected accordingly.
  Reviewer: P 6, L 157-158: strange sentence: „… influencing the formation of disperse particle properties or impurities.“ What is the formation of particle properties?
  Author: Unclear wording was removed.
  Reviewer: P 7, L 191: „… sufficient <low> temperature range …“
  Author: The wording was corrected accordingly.
  Reviewer: P 11, L 268: The „2“ is missing in the paragraph numbering.
  Author: The “2” was inserted.
  Reviewer: P 14, L 322: „The drilling hole is highlighted in Fig. 2b.“ This cannot be seen there.
  Author: Since the material & method section was shortened considerable, the figure was removed.
  Reviewer: P 19, L421: „… concentration, which indicates a larger …“ (not a point before „which“)
  Author: The wording was corrected accordingly.
  Reviewer: P 19, Fig. 7a: Was the INN measured with ATR-FTIR in a dry state? If so, please indicate.
  Author: We adapted the text in this regard: ‘To evaluate whether precursor feed residuals were owing to these features or not, we also characterized the feed components (INN, EtOH, 2-EHA) by ATR-FTIR (INN measured in dry-state).’
  Reviewer: P 21, L 484: The number given in the text (227 and 273°C) do not agree with the ones given in Fig. 8 (225 and 270°C).
  Author: We adapted the numbers.
  Reviewer: P 24, L 552: „… were in absolut agreement to TEM examinations…“ Given the quality of the data, “absolute agreement” may be somewhat exaggerated. P 28, L 613-615: Would the evaporation of carboxylates and carbonates not also be expected in the UHV of the TEM?
  Author: We mitigated the wording in this regard.
  Reviewer: P 29, L 636: „Assuming that the change <of> the precursor concentration…“
  Author: The wording was corrected accordingly.
  
  Citation: https://doi.org/10.5194/ar-2023-14-AC3
RC2:
'Comment on ar-2023-14', Anonymous Referee #2, 27 Dec 2023

This is an important topic that has not been addressed in depth in the “flame synthesis” community. A 30-page long manuscript written in small font size (~12’000 Words, twice as many as a standard research article) and approx. 120 references raise high expectations with regard to a detailed analysis of the subject.
Unfortunately, 50 to 70% of the text is not relevant for the topic suggested by the title. This text seems to be an excerpt from a thesis in draft version rather than a manuscript of a research paper. For instance, seven and a half pages on thermophoretic sampling (without significantly advancing the original work of Dobbins and Megaridis, 1987) have nothing to do with “Impurities in … Maghemite Nanoparticles”. A three-line description in the Methods section would suffice. If the authors want to share their study of thermophoretic sampling, they should do so in a separate manuscript devoted to that topic rather than diluting the essence of the study here.
A scientific article must be concise and straight to the point with a discussion based on pertinent literature. The contribution of every sentence and every word to the overall message of the article has to be evaluated. If there is no contribution, this word or this sentence should be removed. The reader of a scientific article expects a condensed essence and not a diluted blah-blah (with my apologies for the latter word). This includes the reviewer who works on a voluntary basis and does not get paid for going through 30 pages.
The authors must do this evaluation of relevance for every single word of their text and may submit a drastically shortened and sharpened version as a new manuscript. Once this has been done, we can talk about the scientific content. Btw, based on the list of author contributions it seems that none of the senior authors have worked on the manuscript or reviewed it.

Citation: https://doi.org/10.5194/ar-2023-14-RC2
- AC2: 'Reply on RC2', Ricardo Tischendorf, 03 Feb 2024
  
  We want to apologize in case our first manuscript draft caused any inconvenience. Our initial aim was to provide information regarding both, the TS-TEM topic, and the formation of non-product species in SFS. However, as mentioned by the reviewer, the insertion of the TS-TEM method diluted the content/quality of the manuscript considerably. We totally agree with the reviewer and consequently reworked the manuscript completely. Now, the TS-TEM methodologies are explained on ~ 1/2 page. Additionally, we also revised and sharpened the rest of our manuscript (adapting the wording and the storyline) to not dilute our conclusions/findings. We would be pleased if the manuscript aligns now with the reviewers’ expectations. We are looking very much forward to your valuable comments regarding the scientific content of our study in the next step. We would be grateful for any comments to further improve the manuscript.
  
  Citation: https://doi.org/10.5194/ar-2023-14-AC2
AC1: 'Comment on ar-2023-14', Ricardo Tischendorf, 05 Jan 2024

We sincerely appreciate the reviewers for prooving our manuscript, and for giving very valuable comments to improve it. Both reviewers suggested to condense the manuscript by reducing the technical/theoretical information about the TS-TEM procedure. We want to carefully consider all requested corrections and all suggestions in detail in a second manuscript draft. There, the TS-TEM part will be reduced to its essential content.

Citation: https://doi.org/10.5194/ar-2023-14-AC1

Ricardo Tischendorf, Kristina Duschik, Sophie Dupont, Fabian Fröde, Heinz Pitsch, Reinhold Kneer, Mirko Schaper, and Hans-Joachim Schmid

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Short summary

Although spray flame synthesis (SFS) is a promising approach to prepare nano materials with tailored properties, the impurity formation is not fully understood in SFS. To contribute to this topic, maghemite nanoparticles were prepared by SFS, and two complementary experimental approaches were used to investigate the evolution of soot and carbonaceous surface adsorbates. The methodological procedure and the data provided can be used to denote/compare SFS-materials more reliably in the future.


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