Hi Dear

Use different surfactants or ionic liquids while in the preparation of metal oxide nanoparticles that will control the size of nanomaterials for long time

If you are asking about the specific methods

Then aqueous chemical growth method is quite suitable for the metal oxide nanoparticles' preparation

Better to use polymer encapsulation if you are looking for biomedical applications. Otherwise, pH, long-chain fatty acids, and surfactants also can be used to stabilize metal oxide NPs.

Thanks all for your answers. To add more details, we are using the aqueous chemical growth method with addition of alkali (10M NaOH) to a solution of the nickel salt; all other steps are as documented in the two references posted previously. Ideally we are aiming for a particle size of 10-20nm on average, several times smaller than our current average

Leaford Nathan Henderson

Small sizes are favored by low concentrations of the Ni salt. Your NaOH seems very strong at 10M and I suspect you'll be losing particles in any subsequent washing cycle. Do you have a subsequent calcination step to get NiO from Ni(OH)2? Please provide more information about your DLS test. Did you dilute the sample for measurement? If so, how? Also, how are you filtering your continuous phases? One would expect to filter to 0.02 μm or better if one is aiming for 10 - 20 nm (nano)particles.

Alan F Rawle thanks for your answer! Yes, we grind the Ni(OH)2 in a mortar, and then calcine for 2h at 270 degrees C. Subsequent to this, for DLS measurements, we dispersed the particles in DI water at 20mg/ml, which was then diluted to about 100-300 ppm for the DLS measurements. Regarding the alkali strength, we followed the approach from the articles linked above; however, I'll definitely look into using a lower concentration of the NaOH and see if that helps

Leaford Nathan Henderson The decomposition reaction takes place at a temperature of 230 - 360°C (https://chemiday.com/en/reaction/3-1-0-9129), so you may be a little low. Are you using TGA to confirm full decomposition and verification of the Ni(OH)2 and NiO phases with XRD? Dilution in DI water is about the worst thing you can do in this situation - alters the pH and removes any protective ions and surfactants encouraging aggregation and agglomeration. You need to dilute in the mother liquor (from filtration of centrifugation of the original continuous phase that your NiO resided in).

Are you interested in the dichotomy of 'Yes, we grind the Ni(OH)2 in a mortar' with 'nanoparticles'? I am... Can we be confusing crystallite size with particle size?

A true nano system (< 100 nm) will be transparent (no turbidity or settling) and will be darkish or maybe a little green. NiO is quite dense (~ 6.7 g/cm3) and a Stokes' Law calculation (to show what is likely to settle or remain in suspension) would be a useful exercise.

In the past, I have made supported Ni (note not NiO where my experience is more limited) catalysts (from reduction of Ni precursors) which were sterically stabilized by incorporation on (inert?) substrates.

Dear Leaford Nathan Henderson for some potentially useful information about the preparation of particularly small NiO nanoparticles please have a look at the following article:

Ultra-Small NiO Nanoparticles Grown by Low-Temperature Process for Electrochemical Application

Article Communication—Ultra-Small NiO Nanoparticles Grown by Low-Tem...

This paper is freely available as public full text on RG. The synthetic method reported here appears to be rather straightforward.

Good luck with your work!

Leaford Nathan Henderson Frank T. Edelmann

Thank you to Frank for posting another paper relevant to the question. Sometimes one needs experience to be able to critique a paper and (unfortunately) I have to do so...

• The article refers to small NiO nanoparticles prepared in powder form when it's quite clear that these are fused aggregates of crystallites (see Figure 1a, for example). 'Particle size' is not the correct term to be using. I would have more support if a BET specific surface area (SSA) was provided to indicate a SSA > 60 m2/cm3 and thus a D[3,2] < 100 nm. I am reasonably happy with their statement 'Using the diffraction peak corresponding to (200) plane, the synthesized NiO nanoparticles showed an ultra-small crystalline size of 5.43 nm' even though we know that there must be a distribution of crystallite sizes and I am dubious of the second decimal place
• 'The obtained black precipitates as products were collected by centrifugation' indicates that the formed size of the particles was not < 100 nm otherwise they would have remained in (transparent) colloidal suspension. A simple Stokes' Law calculation would show this. I also would appreciate some justification that the material didn't contain significant Ni(OH)2 although XRD should be able to show the difference. Such a statement would allay my fears here
• Figure 1c shows the fused aggregates and a plot (not in line with the recommendations of ISO9276-2 where the horizontal axis should be logarithmic ideally). The horizontal axis is labeled 'particle size' when it should be crystallite size. There are no separate, discrete, independently transportable 'particles' of 5 or 6 nm in the micrograph
• There is a maximum of 10 or 11 counts (!) at ~ '5.5 - 6.0 nm' when 10000 particles would need analysis for a 1% standard error (SE) on the mean as the SE is proportional to 1/n2. Seems the particles have not been randomly selected and there are not enough measurements for a 'distribution'
• Figure 1d shows an interatomic spacing on the (200) plane to 3 decimal places. A hydrogen atom has a diameter of ~ 0.07 nm so this single spacing (not an averaged from say 100 spacings) is somewhat misleading
• The statement is made 'The crystallite size of synthesized ultra-small NiO was accounted by well-known Debye–Scherrer equation'. There is no Debye-Scherrer equation as indicated in many posts on RG. This means that the reviewers have not caught this fundamental faux pas. See: https://en.wikipedia.org/wiki/Scherrer_equation and https://www.researchgate.net/post/How_do_I_calculate_nanocrystallite_size_by_Debye-Scherrer_equation_using_XRD

For some more detail please see these webinars (registration needed):

Dispersion and nanotechnology

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W190528DispNanoMat

Adhesion and cohesion

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W180725AdhesionCohesion

Dear Alan F Rawle many thanks for taking a critical look at the suggested paper. To me it is rather surprising (if not frightening) to learn that there are so many flaws in this article. After all, the Journal of the Electrochemical Society has a fairly good impact factor of 4.316. The editor should have sent the paper to you for reviewing! 😎

Frank T. Edelmann

Thank you for your comment immediately above. I may be over critical of the paper but there were some questions I would have had as a reviewer. Also if a paper is reviewed by scientists skilled in other fields then things that are obvious to some are not seen by others.

However, when one sees 'Debye-Scherrer equation' it casts doubts (perhaps incorrectly) on the entire paper. Use of too many decimal points would have been deplored by our physics teacher at school - simple things here. Some basic questions (do we actually have NiO or significant Ni(OH)2) go unanswered. There are some apparent general 'confusions' in the paper (e.g. particle/crystallite size; numbers of particles for statistical validity) that could have been dealt with in the text that would have provided more confidence.

However, if the material 'works' for the application it was intended and the numbers can be used to relate to that performance, then my words and comments are meaningless. I would always support the production of better materials and appropriate techniques to be able to relate that performance to (stable and repeatable) measured values and properties.

Frank T. Edelmann

The other (belated) point I would make is related to the question: 'What are the best methods of controlling size of chemically precipitated metal oxides (e.g. NiO)'?. Typically speaking oxides are never precipitated in aqueous media - a (pseudo-) hydroxide is formed and this need heat treatment (calcination) to form an oxide. Invariably powders end up as a set of fused aggregates and loose agglomerates. If they were really 'nano' - < 100 nm, then they'd not be able to be retained in a container: thermal air currents and similar would release them into the surroundings. Also with small metals - pyrophoric often - need to be retained in liquid suspension with protective surface to preserve both the metallic state and the fineness...