Osmium and OsO x nanoparticles: an overview of syntheses and applications

Precious metal nanoparticles are key for a range of applications ranging from catalysis and sensing to medicine. While gold (Au), silver (Ag), platinum (Pt), palladium (Pd) or ruthenium (Ru) nanoparticles have been widely studied, other precious metals are less investigated. Osmium (Os) is one of the least studied of the precious metals. However, Os nanoparticles are interesting materials since they present unique features compared to other precious metals and Os nanomaterials have been reported to be useful for a range of applications, catalysis or sensing for instance. With the increasing availability of advanced characterization techniques, investigating the properties of relatively small Os nanoparticles and clusters has become easier and it can be expected that our knowledge on Os nanomaterials will increase in the coming years. This review aims to give an overview on Os and Os oxide materials syntheses and applications.

Molecules comprising one of few atoms or precious metals stabilized by ligands in complexes have been largely investigated for use as catalysts or in medical applications 5 .More recently, nanomaterials made of several hundreds or thousands of metal atoms have been investigated for their unique properties 6 relevant for medicine 4 , chemical synthesis and catalysis 7 , sensing 8 , water/air purification 9 , optics 10 , electronics 11 , building and construction 12 , to name only a few examples.
For precious metals, a trend in the literature is to focus on Au, Ag, Pt, Pd, Ru or Rh nanoparticles and nanomaterials.Figure 1 shows the results from a search on the Web of Science (WOS) database (Clarivate Analytics) with different keywords including 'metal' and 'nanoparticles'.These results show the number of references returned for different combination of keywords and metals.A clear trend is that the least studied precious metals are iridium (Ir), rhenium (Re) and osmium (Os) -assuming that the number of references returned for each search gives an indication of the importance of the related research area.This can be explained by the fact that these metals are among the least available on Earth 13 .The focus here is on the least studied material: Os.
Os is the densest metal and has been mainly studied for its mechanical properties 14,15 .However, Os nanomaterials also show promising features for applications in catalysis and medicine 16 .There is, to the best of our knowledge, no review on Os nanoparticles.Os nanoparticles are partially covered in a very recent review witch mainly focuses on Ru and Rh and catalytic applications 2 .In addition to its natural scarcity, the relatively limited amount of work on Os nanomaterials can be inferred to the typically smaller size (<2 nm) for Os nanoparticles compared to other precious metals, for most syntheses

Amendments from Version 1
This new version addresses the comments from the reviewers, including in particular regarding the Theory section and the Discussion section, towards a broader opening on challenges and opportunities in studying Os based nanomaterials.
Any further responses from the reviewers can be found at the end of the article reported 17 .This small size makes the nanomaterials challenging to characterize.In addition, the relative limited number of reports on Os can be related to the fact that Os easily get oxidized to OsO x materials such as OsO 4 , a highly toxic compound 18 .Nevertheless, OsO 4 has been commonly used as a staining agent in microscopy 19 and in catalysis 20 .Os complexes and clusters have been used as model systems over many years, for instance in the work of Professor Gates 21 .Based on the knowledge already available on Os complexes, it is expected than the interest and knowledge on Os nanomaterials will grow in the coming years.This review proposes an overview of Os nanoparticles syntheses and applications.Rather than a detailed discussion of selected work, the aim is here to give a broad view of work reported to date on Os nanoparticles, as illustrated in Figure 2.
Discussion/analysis of the recent literature Formation mechanism It is expected that understanding the formation mechanism(s) of nanomaterials will be a key to develop more controlled syntheses 22 .This in turn will lead to nanomaterials designed with tuned properties to best match the requirements for a given application.Certainly, materials like Au nanoparticles have been intensively investigated and a relatively clear picture of the nanoparticle formation has been proposed 23,24 .Nevertheless, several questions remain to understand and control how atoms of metal in a complex form larger nanomaterials, e.g. even for the case of well-studied metals like Pt 25 .It can be observed that metals for which the synthesis can easily be followed by simple techniques, such as ultraviolet-visible spectroscopy (UVvis) for Au or Pt, have been more intensively studied.It is therefore tempting to explain the relatively limited knowledge on Os nanoparticles by the challenging characterization of the related materials.Importantly, the risk of forming the toxic OsO 4 18 is also a bottleneck in the investigation of Os nanoparticles compared to Au or Pt.
A specific feature of Os nanomaterials is to lead to relatively small (<2 nm, see Table 1) nanostructures, regardless of the    synthesis approach used.For this size range, most characterization techniques, until recently, are not easily implemented to evaluate size, shape and structure or to follow the formation mechanism of Os nanomaterials.Recently, using a combination of complementary in situ X-ray diffraction (XRD), quick X-ray absorption fine-structure (QXAFS) and X-ray photoelectron spectroscopy (XPS) performed at synchrotron facilities, the formation at high temperature of PdOs nanoparticles from [Pd(NH 3 ) 4 ][OsCl 6 ] was studied 44 .Such advanced studies are much needed to better understand the formation of nanomaterials but remain scarce for Os and Os based materials.Another example is the use of X-ray total scattering with pair distribution function (PDF) analysis, also requiring access to synchrotron facilities, where Os x Cl y intermediates structures were suggested for the formation of Os nanoparticles in a colloidal approach 42 .Despite a relatively poor understanding on how Os nanomaterials form, and few reports focusing on the formation mechanism of Os nanoparticles, a range of successful syntheses have been reported and are illustrated in Table 1.

Dry syntheses
As opposed to wet chemical syntheses detailed below, where the formation of Os nanoparticles proceeds in the liquid phase, a range of high temperature dry syntheses are reported for Os nanoparticles.Typically a support material is needed to stabilize the nanoparticles 2 .An overview of different syntheses is proposed and an example of synthesis is the thermal decomposition of Os precursors 45 .Pyrolysis leads to different nanoparticle size depending on the ligand structures of the precursors 27 and needs to be performed at relatively high temperature, e.g.300 °C, when the precursor is an Os carbonyl complex 26 .Hydrogen (H 2 ) reduction is also an option 29 .Magnetron sputtering has been reported for Os films 46 .Alternative methods include wet incipient impregnation 47 , freeze drying 48 or atomic layer deposition (ALD) of Os films and particles 28 .However, in this last approach and in this specific study, where osmocene and molecular oxygen were used as precursors, the challenge was the formation of highly toxic OsO 4 at high temperature.

Wet chemical syntheses
Wet chemical or colloidal syntheses are very popular synthetic approaches to obtain a range of nanomaterials directly relevant for multiple applications 1,7,49 .The formation of nanoparticles proceeds in a solvent via the reduction of a metal complex in an oxidized state in the presence of a reducing agent 50 , followed by the growth of the nanoparticles 51 .In most cases, the syntheses do not require a support material.This is an advantage to truly investigate support and loading-related effects in catalysis since the nanoparticle formation and control over the nanoparticle size and other properties is independent of the presence of a support 52 .Typically, the syntheses are performed in presence of a range of additives like surfactants to stabilize the nanoparticles.
In methanol-water in presence of PVP, sub-nanometer nanoparticles are obtained 17 .Ionic liquids are also suitable to obtain nanoparticles for instance from the metal carbonyl precursor Os 3 (CO) 12   55,56   .The reaction of OsO 4 in aqueous solution of cetyltrimethylammonium bromide (CTAB), 2,7-dihydroxynaphthalene (2,7-DHN) and NaOH, leads to nanoparticles circa 1-3 nm 36 .Adjusting the concentration of CTAB, different morphologies made of individual nanoparticles, chain-like or aggregated clusters were obtained.Chains of Os nanoclusters are also obtained using ascorbic acid (AA) as a reducing and capping agent in an aqueous medium to lead to nanoparticles in the size range 1-1.5 nm with properties suitable for surface-enhanced raman spectroscopy (SERS) 35 .
Os nanoparticles are typically small (<2 nm) across different syntheses 34 .This therefore questions the actual need to stabilize the small nanoparticles.Developing surfactant-free colloidal syntheses, although it is challenging, is possible 57 .Surfactant-free nanoparticles with a more accessible surface to reactants are directly relevant for catalysis.Surfactant-free nanoparticles are also more simply modified, for instance with dedicated ligands and molecules towards bio-medical applications.Examples of surfactant-free nanoparticles include the polyol synthesis 33 , typically performed in alkaline ethylene glycol, or recently reported mono-alcohol synthesis 1 , performed in alkaline methanol or ethanol.In the latter case, it was actually shown that high precursor concentrations up to 100 mM 43 and even without the need for a base 42 , leads to the formation of small size <3 nm Os nanoparticles, see Figure 3.Such small size nanoparticles were obtained across a large parametric study investigating the time of synthesis from hours to weeks, nature and concentration of precursors, solvent composition and reducing agent (methanol or ethanol) as well as base concentration.
A recent work showed that face-centered cubic (fcc) nanoparticles instead of the expected hexagonal close packed (hcp) structure could be obtained by careful choice of the precursor, reducing agent and solvent, see illustration in Figure 4. -Iridium is the neighbor transition metal of Os and adopts the fcc structure.The difference in total energy between the hcp and fcc structures of Os is expected to be small and so it should be possible to obtain fcc Os nanoparticles.In presence of ethylene glycol and PVP using Os acetylacetonate (Os(acac) 3 ), fcc nanoparticles were obtained whereas hcp nanoparticles were obtained with OsCl 3 in water using NaBH 4 as reducing agent 41 .The change in structure is attributed to the role of the acac ligand that can stabilize the nearest-neighbor Os-Os bond length (ca.2.67 Å) in a close-packed plane of Os, that is close to the O-O length (2.74-2.93Å) of the acac ligand.This leads to nanoparticles with a different crystal structure.The question of whether or not this would happen is the synthesis was performed under exactly the same conditions (same precursor concentration, reducing agents and solvents) but only changing the precursor remains open.Size selected nanoparticles were obtained in a two-phase (water-toluene) approach from OsO 4 and tetrabutylammonium borohydride (TBABH 4 ): 1±0.2 nm, 10-30 nm, 22±2 nm and 31±3 nm nanoparticles were synthesized by changing the concentration ratio of the metal precursor and the amount of reductant 37 .
Barry et al. used Os atoms and complexes as their model system for various studies, e.g. to build up 3D nanocrystals to observe, study and quantify crystal growth at the atomic scale controlled in real time 68,69 , see the illustration in Figure 5.The experiments were conducted using the electron beam of a transmission electron microscope (TEM) and a micellestabilized complex of [Os(p-cymene)(1,2-dicarba-closo-dodecarborane-1,2-dithiolate)] 30 .The same precursor under microwave irradiation leads to supported Os nanoparticles circa 1 nm in diameter 31 .Os was used to show the temperature dependent nucleation and growth kinetics of precious metal nanocrystals supported on silicon nitride by aberration corrected TEM 32 .Barry et al. for that purpose used homemade Os complexes in that study.The growth rate was found to be dependent on the temperature (circa 2.5 times faster at 100 °C than at 20 °C).No effect of the temperature on the crystal structure of the nanocrystals was observed, ''although the sizes of the crystals (<2 nm) and the very small number of atoms per crystal render clear elucidation of the structures extremely difficult'' 32 .The challenging characterization of Os nanoparticles by routine equipment indeed remains a bottleneck.

Applications
Os nanomaterials found applications in a wide range of fields and a broad overview is proposed here.

Electrochemistry. OsO
x materials have been shown to be suitable for a range of electrochemical reactions including hydrogen evolution reaction (HER) 81 , oxygen reduction reaction (ORR) 82 or as direct borohydride polymer electrolyte membrane fuel cell anodes 83 .Freeze drying was used to obtain Os/Si nanowires and the corresponding nanoparticles by etching the Si nanowires 48 .In this comparative study with Rh, Pt, Pd, Re, Ru, Au or Ag nanocomposites, Os was found to give the higher activity for the HER, a small onset potential of -25 mV and long term stability 48 .Using magnetron sputtered Os it was found that the high activity of OsO x for the HER in acidic media was correlated with poor stability 84 .Nanoparticles based on Os are easy to de-alloy, e.g.Pt 2 Os to from quasi core-shell Os@Pt for ORR in acidic media 85 .This property can be used to develop high surface area materials by de-alloying, e.g. to develop improved porous-electrodes for the oxygen evolution reaction (OER), see Figure 6 86 .Os itself is expected to show very high activity for the OER but suffer from poor stability 84 .

Other applications.
Os nanomaterials are less studied than other precious metals for medical applications 60 or pollution management 37 .However, Os nanoparticles found recent applications in sensing.Os nanoparticles protected by heparin as the protecting/stabilizing agent were used as a heparinase sensor 40 .Bovine serum albumin is an efficient protective shell to give Os nanoparticles an antifouling property regarding various  ions (e.g., Hg 2+, Ag + , Pb 2+ , I − , Cr 6+ , Cu 2+ , Ce 3+ , S 2− , etc.), saline (0−2 M), or protein (0−100 mg/mL) conditions.A colorimetric sensor was developed for H 2 O 2 detection with improved properties compared to Au or Pt based sensors 87 , see the illustration in Figure 7.Other examples include glucose and pyruvic acid detections 54 , folic acid detection 88 colorimetric sensors for heavy metal ions discrimination (Cu 2+, Ag + , Cd 2+, Hg 2+, and Pb 2+ ) 89 .Os nanoparticles also show SERS properties 36,37,76 .

Theory
Less work has been performed on Os nanoparticles than Ir 90 or Pt 25 nanoparticles but some theoretical work can be found in the literature 70,[91][92][93] .For instance, Os was suggested to be a suitable catalysts for ammonia production 94 .While being less investigated than Ir, Os xn clusters were studied by density functional theory (DFT) for instance in light of their interaction with MgO for n=4,5 91 .By analogy with what is available for Ir or Pt, it can be expected that theoretical work will be valuable to clarify why small size nanoparticles are easily obtained, which might be related to the formation of 'magic number' nanoparticles with specific sizes 95 and/or sintering resistance properties 96 .Equaly theoretical work could be relevant to explore further the properties of Os based nanomaterials, in particular towards improved stability.

Os in multi-metallic nanomaterials
In addition to the examples already mentioned above, for instance in Figure 6, various alloyed nanoparticles have been reported such as IrOs 14 , PtOs 97 , OsB 2

98
, in particular for their improved mechanical properties.PdOs nanoparticles were reported as catalyst for carbon nanotube synthesis 26 .Other examples include NiOs 4 reported for the improved hydrogenation of cinnamaldehyde 99 , PtOs for the methanol oxidation reaction 100 , CuOs for the methanol oxidation reaction and ORR 101 or OsTe nanorods for cancer therapy 102 .

Discussion
A range of Os nanomaterials can easily be obtained by various syntheses methods, see Table 1.In particular, a range of surfactant-free syntheses are well documented and are expected to lead to Os nanoparticles with improved properties in fields of applications like catalysis and sensing.However, the characterization of the small (<2 nm) nanoparticles obtained in most cases remains one of the bottlenecks in the study of Os nanomaterials.Relatively complex characterization techniques (not routine) are needed such as high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) 41 , or synchrotron based measurements 41 , see Figure 4.For instance, X-ray diffraction technique will lead to large Bragg peaks for such small nanoparticles and most TEM equipment will not easily characterize such small nanomaterials.Also, the size range around 1 nm is at the limit of most small angle X-ray scattering (SAXS) equipment.However, recent progress in the characterization of nanomaterials 103 , and in particular the increasing availability of high resolution TEM or techniques like X-ray total scattering with pair distribution function (PDF) analysis 42,104 , are well suited to characterized nanocrystals.Recent advances in these techniques are expected to bring new insights into Os nanomaterial formation.The knowledge gained will be the key for improving syntheses of nanomaterials towards more functional materials.There is a regain of interest on Ir and Ir oxide nanoparticles, in great part due to high expectations on Ir as a potential catalyst for OER 105 .Ir and Os chemistry are relatively similar in the sense that they both easily lead to small size nanoparticles and clusters.This makes them ideal candidates to study nanoparticle formation and to focus on nucleation phenomena since the nanoparticle growth is moderate.
In addition, the Os materials obtained are relevant for a range of applications.In particular, high expectations are on new or improved applications in catalysis and medicine.An example of emerging opportunity is for instance the possibility to investigate the different catalytic properties of fcc or hcp Os nanoparticles, see Figure 4, largely unexplored to date.Finally, it is expected that the interest on iridium 90 will trigger increasing interest in Os nanoparticles, which in turn will enable further exploration of Os chemistry.However, for long term applications recycling is an important issue to address 106 , in particular in light of the relatively poor stability of Os in application like electrochemical energy conversion 84 .In this respect, the role and stability of Os in increasingly studied bimetallic 86 and even high entropy alloys 107 is also an opening area of research.

Conclusions
Despite a limited knowledge on the actual formation mechanism of Os nanoparticles, several approaches lead to simple syntheses of Os nanoparticles.The very small size circa 1-2 nm of most Os nanoparticles suggests that a range of reported syntheses probably can be simplified, e.g.avoiding the use of any surfactants or high temperature.Relatively high concentration of precursors can be used and still lead to small size nanoparticles which is a promising feature for future scaling.The obtained OsO x nanoparticles already proved to be relevant for a wide variety of applications in particular as active materials in catalysis or as templating agents.(Re) Emerging areas of application include chemical synthesis 79 , sensing 89 or medical applications 60 .

Comments to the Authors
In this work, Quinson and co-authors reviewed the recent advances in the syntheses and applications of Os and Os-based nanoparticles.They first summarized several important synthesis methods of Os-based nanomaterials.Then, several key applications of Os-based catalysts were introduced, such as chemical synthesis, electrocatalysis and medical applications, etc.This review is logical and well-organized, and provides a valuable reference for the future design of advanced Os-based nanomaterials.In view of the timely summary/highlight and importance of this work, I recommend its indexing, only after a minor revision to address the following concerns.
There are too many keywords.1.
The authors proposed that Os-based materials present unique features compared to other precious metals.What are these unique features?

2.
It is pointed out that the nanoparticles with very small sizes are difficult to characterize.However, I think this is no longer challenging, because many technologies have been used to characterize their structures, even some atomically dispersed catalysts, such as atomicresolution aberration-corrected scanning transmission electron microscopy.

3.
Some possible trends and challenges for future advanced research directions should be included at the end of this manuscript.

4.
In Figure 4, there are two-phase structures for Os nanoparticles (fcc and hcp).The influence of two kinds of Os on electrocatalysis should be mentioned.

Are the conclusions drawn appropriate in the context of the current research literature? Yes
Competing Interests: No competing interests were disclosed.

I confirm that I have read this submission and believe that I have an appropriate level of expertise to confirm that it is of an acceptable scientific standard.
Author Response 29 Jul 2022

Jonathan Quinson
I would like to thank the reviewer for his/her/their time to review the work and valuable comments.Answers to the comments are below: The number of 8 keywords is following the recommended numbers by the Open Research Europe guidelines: https://open-research-europe.ec.europa.eu/forauthors/article-guidelines/science-technology-and-medicine/reviews/#keywords. 1.
I understand that the reviewer refers here to the wording in the abstract.The wording is kept general to fit the abstract requirements.These unique features are somehow specified in the rest of the sentence 'catalysis or sensing for instance' and detailed all along the review.It is also stressed later that Os is one of the densest metal known.These unique features can be summed up as the different physicochemical properties highlighted in the review, such as small sizes, and the relevance for specific applications detailed in the Applications section, such as catalysis, medical applications, etc.

2.
The reviewer is right, and certainly how small a nanoparticle must be to start being 'challenging' to characterize depends on many factors.In particular, the type of equipment available to a research group will be a critical factor.However, it can be safely considered that the resolution of most transmission electron microscope (TEM) starts being limited for nanomaterials less than 1-2 nm.And certainly, high resolution TEM (HRTEM) and more advanced scanning transmission electron microscopy (STEM) are (fortunately) increasingly routine equipment.The increasingly availability of these characterization techniques is indeed expected to bring increasingly new insights into atomically structured materials.
The comments made here were maybe to be understood considering the historical development of precious metal nanomaterial studies.As reported in Figure 1, Ag and Ag nanomaterials have been the most studied.This is, I believe, in great part due to their well-defined plasmonic properties which makes it possible to study the nanomaterials (and even estimate size) by simple techniques such as UV-vis.Also, a range of synthetic methods allow to form nanomaterials with a size above 10 nm, relatively easy to characterize with most TEM equipment.In contrast Pt, Pd tend to lead to smaller nanoparticles, say 2-10 nm, across various synthetic approaches, and so are less simple to characterize.Further comes Ru, Rh, Ir and Os which tend to lead to even smaller nanoparticles.The rough relationship 'larger size correlates to larger number of research studies' might be in part due to the facility to study relatively larger nanoparticles.
I agree with the reviewer that the bottleneck related to characterization is maybe less challenging to date, but it still accounts for the fact that there are fewer studies on Ir or Os nanoparticles.Hence, there are high expectations on new discoveries on the related materials, now that this bottleneck is at least partially alleviated, as stressed in the second paragraph of the Discussion section.

3.
A generic statement can be found in the Discussion section.This section has been re-4.
shaped into four paragraphs and not two, to best stress future directions of research on both fundamental aspects of nanoparticle formation (and the related characterization) and more applications.An example of an emerging new opportunity is given for catalysis, considering the comment (5) from the reviewer.
The reviewer is right, and this has been stressed better now in the revised version of the work: '' An example of emerging opportunity is for instance the possibility to investigate the different catalytic properties of fcc or hcp Os nanoparticles, see Figure 4, largely unexplored to date '' has been added.

5.
Competing Interests: No competing interests were disclosed.In this submission the authors presents a review of Os and OsOx nanoparticles focussing on their synthesis and applications.
Overall, the review is interesting and brings together work on a metal that receives much less attention than other precious metals.
I have some comments that I hope can enhance the submission.Some comments on aspects of the language: Introduction "along the years" can be replaced by "over many years" (or similar) Page 4 "is to easily lead to relatively small" 1.
Introduction: Is Os the densest metal or element?2.
Table 1 caption: add "synthesis" at the end of the caption text.Is there a chemical formula for the "Home-made" complex?

3.
Author mentions a challenge with the ALD of Os nanoparticles, that is formation of toxic OsO4.Did the ALD use an oxygen source?A H2 plasma could be one option to reduce to Os 4.

metal.
Page 7 mentions how different Os precursors can produce different structured nanoparticles -do the original papers have any discussion on the origin of this difference.For me stating "role of ligands that can stabilise a specific facet" is not sufficient.

5.
I may have missed it, but given that Os appears to make small (2 nm) nanoparticles, can the author make some discussion on the origin of this?

6.
The theory part is probably much too short to be of real value.The author can either remove it or expand to give some more details.

7.
In multimetallic systems, is the role of Os described in the literature?What value is there in making Os alloys for chemistry?Reviewer Expertise: First principles simulations, nanoparticles and nanostructures, surface chemistry I confirm that I have read this submission and believe that I have an appropriate level of expertise to confirm that it is of an acceptable scientific standard, however I have significant reservations, as outlined above.
The caption of Table 1 has been corrected in the new version by adding: ''Table 1. Examples of literature on osmium oxide (OsO x ) nanoparticle synthesis and applications''

3.
There is certainly a chemical forumal for the ''Home-made'' complex, given in the main text, but in light of the overview provided, we do not wish to complicate the Table and hope that interested readers will refer to the relevant literature.
These complexes are for instance: The comment from the reviewer was: ''Author mentions a challenge with the ALD of Os nanoparticles, that is formation of toxic OsO 4 .Did the ALD use an oxygen source?A H 2 plasma could be one option to reduce to Os metal.''This is a fair point raised by the reviewer.In the specific study referred to where ALD was used, oxygen was used as co-precursor (Hämäläinen J, Sajavaara T, Puukilainen E, et al.: Atomic Layer Deposition of Osmium.Chem Mater.2012;24(1):55-60, DOI: 10.1021/cm201795s).I am not familiar enough myself with the ALD requirements but the suggestion of the reviewer makes sense.I guess this is one of the many aspects of Os chemistry still to be explored.

5.
The comment from the reviewer was: ''Page 7 mentions how different Os precursors can produce different structured nanoparticles -do the original papers have any discussion on the origin of this difference.For me stating "role of ligands that can stabilise a specific facet" is not sufficient.''The original paper (Wakisaka T, Kusada K, Yamamoto T, et al.: Discovery of face-centred cubic Os nanoparticles.Chem Commun.2020;56(3):372-374, 31808775, DOI: 10.1039/c9cc09192k) certainly does discuss this.The discussion has now be completed: by adding: ''A recent work showed that face-centered cubic (fcc) nanoparticles instead of the expected hexagonal close packed (hcp) structure could be obtained by careful choice of the precursor, reducing agent and solvent, see illustration in Figure 4. Iridium is the neighbour transition metal of Os and adopts the fcc structure.The difference in total energy between the hcp and fcc structures of Os is expected to be small and so it should be possible to obtain fcc Os nanoparticles.In presence of ethylene glycol and PVP using Os acetylacetonate (Os(acac) 3 ), fcc nanoparticles were obtained whereas hcp nanoparticles were obtained with OsCl 3 in water using NaBH 4 as reducing agent [41].The change in structure is attributed to the role of the acac ligand that can stabilize the nearest-neighbour Os-Os bond length (ca.2.67 Å) in a close-packed plane of Os, that is close to the O-O length (2.74-2.93Å) of the acac ligand.This leads to nanoparticles with a different crystal structure.''6.
To the best of my knowledge, there is no definite framework to explain why Os nanoparticles are easily obtained with small sizes, although it is a very clear trend in the literature ( A13(6):727-750, DOI: 10.1080/00222337908056685).Just like for other transition metals, it is very likely that 'magic number' nanoparticles are stable and can be obtained.The open question is the driving force probably differing for different metals that seem to lead to some of the magic clusters more than others (smaller one or larger ones depending on the metal, see Reference / comments 42 in Watzky, M.A.; Finke, R.G. Nanocluster size-control and "magic number" investigations, experimental tests of the "living-metal polymer" concept and of mechanism-based size-control predictions leading to the syntheses of iridium(0) nanoclusters centering about four sequential magic numbers.Chem.Mater.1997, 9, 3083-3095, DOI:10.1021/cm9704387).The best analogy to propose an explanation to the observed small size of Os nanoparticles is probably to compare with the neighbor element Iridium, for which it has been suggested that some small structures are so stable that the nanoparticles will be resistant to sintering ( The reviewer is right that the theory part is relatively scarce.Especially when comparing with what can be found on Iridium (Quinson, J. Iridium and IrO x nanoparticles: an overview and review of syntheses and applications.Adv Colloid and Interface Sci.2022, 303, 102643, DOI:10.1016/j.cis.2022.102643).In light of this 'lack' of theoretical studies, this section is now including comments along the line of the answer made on comment (7) from the reviewer: lack of explanation of the small size of the Os nanoparticles observed across the literature.This now reads as follows: ''Less work has been performed on Os nanoparticles than Ir [90: Quinson, J. Iridium and IrO x nanoparticles: an overview and review of syntheses and applications.Adv Colloid Interface Sci.2022, 303, 102643, DOI:10.1016/j.cis.2022.102643]or Pt [25: Quinson J, Jensen KMØ: From platinum atoms in molecules to colloidal nanoparticles: A review on reduction, nucleation and growth mechanisms.Adv Colloid Interface Sci.2020;286:102300.33166723, DOI: 10.1016/j.cis.2020.102300]nanoparticles but some theoretical work can be found in the literature [70, 91-93] .For instance, Os was suggested to be a suitable catalyst for ammonia production [94].While being less investigated than Ir, Os n clusters were studied by density functional theory (DFT) for instance in light of their interaction with MgO for n=4,5 [91].By analogy with what is available for Ir or Pt, it can be expected that theoretical work will be valuable to clarify why small size nanoparticles are easily obtained, which might be related to the formation of 'magic number' nanoparticles with specific sizes [95: Watzky, M.A.; Finke, R.G. Nanocluster size-control and "magic number" investigations, experimental tests of the "living-metal polymer" concept and of mechanism-based size-control predictions leading to the syntheses of iridium(0) nanoclusters centering about four sequential magic numbers.).The lack of consideration for Os can also come from the related toxicity and risk to form OsO 4 .Finally, a generally small number of reports, and even fewer theoretical reports as highlighted in the answer to comment (7), accounts for the scarcity of rational arguments to use Os in general, and in bi/multi metallic in particular.Nevertheless, the activity of Os for catalytic applications, e.g. for the OER, is very promising if the stability issues can be addressed.In this respect, the range of opportunities offered by developing Os based materials is still to be explored.In this regard, the emerging interest around high entropy alloys (Miracle, D.B.; Senkov, O.N.A critical review of high entropy alloys and related concepts.Acta Materialia 2017, 122, 448-511, DOI:10.1016/j.actamat.2016.08.081) might bring new highlights in the properties of Os-based nanomaterials and this is now stressed more in the manuscript with the relevant new references here mentioned: Finally, it is expected that the interest on iridium [90: J. Iridium and IrO x nanoparticles: an overview and review of syntheses and applications.Adv Colloid Interface Sci.2022, 303, 102643, DOI:10.1016/j.cis.2022.102643]will trigger increasing interest in Os nanoparticles, which in turn will enable further exploration of Os chemistry.However, for long term applications recycling is an important issue to address [106], in particular in light of the relatively poor stability of Os in application like electrochemical energy conversion- [84].In this respect, the role and stability of Os in increasingly studied bimetallic

9.
Competing Interests: No competing interests were disclosed.

Figure 2 .
Figure 2. Overview, aim and scope of this review into Osmium oxide (OsO x ).

Figure 3 .
Figure 3. Example of small size Osmium (Os) nanoparticles.(a-b) transmission electron microscope (TEM) micrographs of Os nanoparticles obtained using water (66 volume %) and methanol (33 volume %) and 100 mM of (a) OsCl 3 and (b) H 2 OsCl 6 as precursors after a one-week reaction at 85 °C.The size analysis (c) suggests that the nanoparticles are (a) 1.6±0.4nm and (b) 1.7±0.3nm.Reproduced from 42 with permission from the Beilstein-Institut.

Figure 4 .
Figure 4. Tuning osmium (Os) nanoparticle structures by controlled synthesis.(Top) Schematic of the formation of face-centered cubic (fcc) or hexagonal close packed (hcp) Os nanoparticles depending on the precursor used.(Bottom) Synchrotron X-ray diffraction (XRD) patterns of Os nanoparticles synthesized using the Os(acac) 3 complexes (red) and (blue) OsCl 3 , and the simulations of fcc (upper black) and hcp Os (lower black).Reproduced from 41 with permission from the Royal Society of Chemistry.

Figure 5 .
Figure 5. Os nanoparticles as model system to study the formation and stability of nanomaterials.(a-d) Migration of small Os clusters and their coalescence (e.g.clusters in yellow and dark blue circles merge to give crystal in green circle) over a period 1-30 min; scale bars, 2 nm.(e) Nanocrystals after 60 min.(f) Example of an Os crystal of ca.1.5 nm, formed after 30 min of irradiation, scale bar, 1.5 nm.(g) Width of the clusters/crystals versus time.(h) Fast Fourier transform analysis of the nanocrystal shown in f.Reproduced from 68 with permission from Springer Nature.

Figure 6 .
Figure 6.Electrochemical properties of Osmium (Os)-based nanomaterials: Activity-conductivity relationships in de-alloyed thin-film and nanoparticles.(a) Comparison between oxygen evolution reaction (OER) polarization curves for polycrystalline Iridium (Ir), de-alloyed thin film (dtf) Ir 25 Os 75 and de-alloyed nanoparticles (dnp) Ir 50 Os 50 , indicating that conductivity limitations are observed for dnp-Ir 50 Os 50 at higher current densities (denoted as iR oxide ).Inset shows the corresponding cyclic voltamograph.(b) X-ray photoelectron spectroscopy (XPS) sputter etching experiments demonstrating that the dtf-Ir 25 Os 75 consists of an IrO x shell with Ir-metallic core, in contrast to dnp-Ir 50 Os 50 that consists entirely of IrO x .Schematic illustrates the impact of multiple oxide-oxide interfaces (present on dnp-Ir 50 Os 50 electrodes) on conductivity.(c) Change in activity-stability factor values with overpotential for dtf-Ir 25 Os 75 and dnp-Ir 50 Os 50 highlighting the importance of balancing activity-stability-conductivity properties of oxide materials for the OER.Reproduced from 86 with permission from Springer Nature.

Reviewer Report 20
June 2022 https://doi.org/10.21956/openreseurope.15760.r29491© 2022 Nolan M. This is an open access peer review report distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.Michael Nolan 1 Tyndall National Institute, University College Cork, Cork, Ireland 2 Tyndall National Institute, University College Cork, Cork, Ireland

8 .
Is the topic of the review discussed comprehensively in the context of the current literature?PartlyAre all factual statements correct and adequately supported by citations?YesIs the review written in accessible language?YesAre the conclusions drawn appropriate in the context of the current research literature?YesCompeting Interests: No competing interests were disclosed.
Hirai H, Nakao Y, Toshima N: Preparation of Colloidal Transition Metals in Polymers by Reduction with Alcohols or Ethers.J Macromol Sci Chem.1979; 7.