Facile production of ultra-fine silicon nanoparticles

A facile procedure for the synthesis of ultra-fine silicon nanoparticles without the need for a Schlenk vacuum line is presented. The process consists of the production of a (HSiO1.5)n sol–gel precursor based on the polycondensation of low-cost trichlorosilane (HSiCl3), followed by its annealing and etching. The obtained materials were thoroughly characterized after each preparation step by electron microscopy, Fourier transform and Raman spectroscopy, X-ray dispersion spectroscopy, diffraction methods and photoluminescence spectroscopy. The data confirm the formation of ultra-fine silicon nanoparticles with controllable average diameters between 1 and 5 nm depending on the etching time.


1.
The demerits of this simplified process. The authors claim this route is simplify, safe and scalable. But how about the quality of the NPs? Specially, is it comparable with the quality of NPs prepared from similar routes from ref. 10 and ref.27? This should be clarified in the introduction part.

2.
For the chemicals used in the paper, I suggest the authors to use the chemical formulas instead of (or with) the chemical names used in the paper, which is easier to read. Besides, the concentration of the chemicals for synthesis should be given. For example, the concentrations of HF and HCl on page 3 lines 14-17 are missing.

3.
It is critical that the SEM of SiNP (Fig 2) are NOT nanoparticles. Worse yet, the FTIR, Raman, and EDS all show Oxygen in the final products. The product is NOT pure Si. Even the high resolution TEM (Fig.4c) show one Si NP locally, and the XRD show Si Peaks, the quality of the products is really suspicious.
Based on these, the authors should optimize the process of synthesis and at least present Nanoparticles of Si.
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Dear Dr Rummeli: Title: Facile production of ultra-fine silicon nanoparticles Manuscript ID: RSOS-200736 Thank you for your submission to Royal Society Open Science. The chemistry content of Royal Society Open Science is published in collaboration with the Royal Society of Chemistry.
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Once again, thank you for submitting your manuscript to Royal Society Open Science and I look forward to receiving your revision. If you have any questions at all, please do not hesitate to get in touch. ********************************************** RSC Associate Editor: Comments to the Author: Detail morphological and structural characterization of silicon nanoparticles along with detail analysis need to be added. Interesting article to be considered in this journal.

RSC Subject Editor:
Comments to the Author: ********************************************** Reviewers' Comments to Author: Reviewer: 1 Comments to the Author(s) The paper presents in a clear manner the efficient approach to the synthesis of silicon nanoparticles. The authors characterized the materials at every stage of the preparation and provided reasonable discussion of the results. The authors should take into account the following comment: the reason behind a decrease of the Si NPs size with increased etching time should be discussed.

Reviewer: 2
Comments to the Author(s) The authors present a procedure of the synthesis of ultra-fine silicon nanoparticles. I have the following questions: 1. The demerits of this simplified process. The authors claim this route is simplify, safe and scalable. But how about the quality of the NPs? Specially, is it comparable with the quality of NPs prepared from similar routes from ref. 10 and ref.27? This should be clarified in the introduction part. 2. For the chemicals used in the paper, I suggest the authors to use the chemical formulas instead of (or with) the chemical names used in the paper, which is easier to read. Besides, the concentration of the chemicals for synthesis should be given. For example, the concentrations of HF and HCl on page 3 lines 14-17 are missing. 3. It is critical that the SEM of SiNP (Fig 2) are NOT nanoparticles. Worse yet, the FTIR, Raman, and EDS all show Oxygen in the final products. The product is NOT pure Si. Even the high resolution TEM (Fig.4c) show one Si NP locally, and the XRD show Si Peaks, the quality of the products is really suspicious.
Based on these, the authors should optimize the process of synthesis and at least present Nanoparticles of Si.
Author's Response to Decision Letter for (RSOS-200736 We hope you are keeping well at this difficult and unusual time. We continue to value your support of the journal in these challenging circumstances. If Royal Society Open Science can assist you at all, please don't hesitate to let us know at the email address below.
Dear Dr Rummeli: Title: Facile production of ultra-fine silicon nanoparticles Manuscript ID: RSOS-200736.R1 It is a pleasure to accept your manuscript in its current form for publication in Royal Society Open Science. The chemistry content of Royal Society Open Science is published in collaboration with the Royal Society of Chemistry.
The comments of the reviewer(s) who reviewed your manuscript are included at the end of this email.

Response to Reviewers
We first wish to thank the Editor, their team and the Reviewers for their constructive efforts to improve our manuscript. We believe we were able to address all criticism and the new manuscript version is ready for publication. Below we address each of the Reviewer's comments (point by point). In addition, a revised manuscript with highlights indicating revisions is also provided.

I. RSC Associate Editor
Detail morphological and structural characterization of silicon nanoparticles along with detail analysis need to be added. Interesting article to be considered in this journal.
Response: We thank Editor for sharing this. We also believe this work can be interest to the scientific community. We agree some morphological detail and structural characteristics need to be improved. To this end, we have now conducted additional optical microscopy, SEM and TEM characterizations to better evaluate the material. In the revised manuscript, we have added a higher magnification optical micrograph confirming the powdery nature of the SiNP product

II. Reviewer #1
The paper presents in a clear manner the efficient approach to the synthesis of silicon

III. Reviewer #2
The authors present a procedure of the synthesis of ultra-fine silicon nanoparticles. I have the following questions: In this work, we present the production of ultra-fine silicon nanoparticles using a novel and economical technique using low-cost trichlorosilane (HSiCl3) as the feedstock and without the need for a Schlenk vacuum line, which simplifies the process and is safer. The application of trichlorosilane (HSiCl3) as a precursor allows one to obtain ultra-fine nanoparticles of controlled size, terminated by hydrogen bonding. This process is a scalable. The obtained final product matches the quality of Si nanoparticles prepared in similar synthetic routes by Veinot et al. [6,27] and Jaumann et al. [10]. Veinot et al. [27] obtained free-standing hydrogen-terminated silicon nanocrystals, as evidenced by the characteristic stretching and scissoring frequencies at ca. 2100 and 910 cm -1 , respectively from IR spectroscopic investigations. While, Jaumann et al.
[10] obtained crystalline silicon nanoparticles with a size of 2-5 nm. They also noticed the tendency for the nanoparticles to agglomerate. Both groups also showed a small presence of oxygen in the obtained final product, which was attributed to residues and limited surface oxidation after sample preparation [6, 10, 27].

Question 2:
For the chemicals used in the paper, I suggest the authors to use the chemical formulas instead of (or with) the chemical names used in the paper, which is easier to read.
Besides, the concentration of the chemicals for synthesis should be given. For example, the concentrations of HF and HCl on page 3 lines 14-17 are missing.

Response:
We thank very much the Reviewer's comments and suggestions. We agree that the use of chemical formulas will be easier to read. We updated the chemical formulas in the manuscript. We did already mentioned the concentration of the chemicals for synthesis (HF and HCl) on page 2 lines 46-48. We also added the concentration of HF and HCl on page 3 lines 14-17. For the Reviewer's convenience the introduced changes are presented below:

Procedure
Before starting the synthesis, a Pyrex flask with a magnetic stirrer was sealed with a septum and flushed with argon for 2h. 15 min before the end the flask was placed in an ice bath (salt was added to ice to help maintain a lower temperature) and cooled to a temperature of -20°C. After that, 20 ml of trichlorosilane (HSiCl3) was slowly added to a three-neck round-bottom flask. The flask was purged with argon continuously. The system was equipped with a simple exhaust to neutralize the evolving HCl through a NaOH solution filled gas-wash bottle. Then, 0.6 ml of pure ethanol was added. After 10 min, deionized (DI) water (15 ml) was very carefully injected while vigorously stirring into the cooled HSiCl3 through a septum. After 1h the ice bath was removed and the flask was allowed to warm to room temperature under an argon flow and was maintained for 3h to evaporate any side products away. The pre-dry final product -(HSiO1.5)n precursor was transferred to an oven and dried at 80°C under vacuum for at least 12h.
The white solid and dry precursor was then ground in agate mortar and then annealed at 1150°C in an argon atmosphere (flow: 1L/min) for 2h (heating rate: 50°C/min to 800°C, then 12.5°C/min).
In the etching procedure 1.5 g of silicon-silica composite obtain after annealing process was placed into a Teflon beaker with magnetic stirrer. 15 ml of dionized (DI) water and 3 ml of HCl (35-38%) were added. After 5 min stirring, 7.5 ml of HF (48-51%) was added. The entire solution was vigorously stirred for 25 min in the dark. The solution was filtered and rinsed two times with water (2 x 10 ml) and once with ethanol (10 ml). The obtained material was dried overnight in vacuum oven and store under argon atmosphere. The etching procedure is fully scalable.

Question 3:
It is critical that the SEM of SiNP (Fig 2) are NOT nanoparticles. Worse yet, the FTIR, Raman, and EDS all show Oxygen in the final products. The product is NOT pure Si.
Even the high resolution TEM (Fig.4c) show one Si NP locally, and the XRD show Si Peaks, the quality of the products is really suspicious.
Based on these, the authors should optimize the process of synthesis and at least present Nanoparticles of Si.

Response:
We thank Reviewer for highlighting this points. We would like to break down this question into three parts: -It is critical that the SEM of SiNP (Fig 2) are NOT nanoparticles.
We very much appreciate the Reviewer's comment. We apologize for any confusion. In the SEM photos we wanted to show how the material structure changes during the each stage of Si nanoparticles production at low magnification which gives an unclear message. In view of this we conducted additional optical and electron microcopy studies at higher magnifications.
In the revised manuscript, we have added a higher magnification optical micrograph confirming We have also prepared a new figure, Figure 5, in which additional TEM investigations reveal the SiNP nature, confirming they are crystalline and we also provide additional micrographs with false color highlighting the shape of the NPs. We have also added additional text in the manuscript. For ease they are provided below: Additional text: Panel C of figure 4 and figure 5 show a high magnification micrographs of silicon nanoparticles, they show clear lattice fringes with a d spacing ≈ 0.3 nm and d ≈ 0,24 nm corresponding to the (111) and (100) orientation of crystalline silicon [10,11,16,38]. The TEM studies also show that the SiNPs agglomerate easily, however careful examination of the particles reveals that they are not facetted and are somewhat spherical in form as, for example, shown in false colour in panels C and D of figure 5.