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Nanoparticle Salad: A general route to Metal Oxide Nanoparticles using Green Chemistry

“Green Nanochemistry: Metal Oxide Nanoparticles and Porous Thin Films from Bare Metal Powders” Engelbert Redel, Srebri Petrov, Ömer Dag , Jonathon Moir, Chen Huai, Peter Mirtchev, and Geoffrey A. Ozin, Small2011DOI: 10.1002/smll.201101596

Advocates for green chemistry and nanotechnology have both promised technological solutions to society’s great challenges. Some of the barriers to widespread adoption of nanotechnology have been outlined by Jim Hutchison, and many of these barriers can be addressed by green chemistry. In particular the two issues that the current paper addresses are the excessive waste and the potential hazards associated with the metal precursors.

Ozin and his group take “a metal powder and add aqueous hydrogen peroxide and a small amount of acetic acid (10:1 ratio) under ambient conditions and, in one simple step, colloidally stable dispersions of nanoparticle (NP) metal oxides of the respective metals form with diameters in the range approx. 3–8 nm.”

This approach is elegant in its simplicity. Peroxide and acetic acid act respectively as the oxidizing and capping agents under ambient conditions to make nanoparticles. The simplicity of this approach makes me want to run into the lab and try these reactions. If they work well, I am considering working them into our undergraduate labs.

OK, so let’s take a closer look at what makes this simple approach so appealing. The first thing that jumps out is the impressive  scope of this reaction. Table 1 shows the diversity of metal precursors that can be used in this reaction. Of particular interest is the fact that they were able to successfully make both binary and ternary metal oxides by simply combining stoichiometric amounts of the precursor metals. Mixed-metal oxides find application in many areas of materials science and are drawing particular interest for their unique catalytic behavior.

Table 1: Reaction Summary

Metal Nanoparticle Composition STEM (a)[nm] Size Range(a)[nm] PXRD (b)[nm]
Mo MoO3 3.6 ± 0.5 2.5 – 4.1 4 – 5 (S1)
W WO3 3.8 ± 0.3 2.0 – 4.7 4 – 4.5 (S2)
Ni NiO 3.1 ± 0.4 2.2 – 3.7 amorphous
Co Co3O4 6.4 ± 2.7 4.5 – 8.3 amorphous
Fe Fe2O3 3.4 ± 0.5 2.7 – 4.5 3 – 3.5 (S4)
Zn ZnO2 (ZnO) 3.9 ± 0.4 3.1 – 5.2 3 – 4.0 (S5)
Mg MgO2 (MgO) 4.3 ± 0.9 3.2 – 5.7 4.5 – 5 (S6)
Mg+Co MgCo2O4 21.4 ± 5.2 12 – 27 22 ± 4 (S7)
Mg+Zn MgZn2O4 3.5 ± 0.4 2.8 – 4.6 amorphous
Fe+Co+Mo Fe0.3Co0.7MoO4 3.1 ± 0.5 2.3 – 4.3 2.8 – 3.2 (S8)

(a) Sizes were determined through the analysis of High resolution Transmission Electron Microscopy (HR-TEM) and Scanning Transmission Electron Microscopy (STEM). The average, standard deviation, and size ranges for these particles have been reported.

(b)  Powder X-ray Diffraction (PXRD) was used in conjunction with Rietveld refinement and the Scherrer Equation to determine the nanoparticle composition, crystalline phase and size.

In addition these reactions necessitate very little workup. The authors report that the particles were simply filtered through a 0.7 um filter and either stabilized with PEG or used as made! This is great given the large amount of solvent often used in nanoparticle purification.

Finally, the combination of peroxide and acetic acid as the oxidant ends up working particularly well. They attribute this to the formation of peroxyacetic acid which is a strong enough oxidizing agent to work on a wide range of metal precursors. As the authors note, the formation of this strong oxidizing agent and subsequent reaction with the metals can be a very exothermic reaction and should be done carefully.

So what is the catch? Well I do have a few questions that despite their 30 pages of supplemental information, I would still like answered.

1) What is the yield of these reactions? I have always felt that nano-scientists should start reporting yields. I certainly expect this reaction to be quantitative, but is it?

2) They cool these reactions because of the exothermic oxidation reactions, but then they let them stir for long times, “overnight to several days”. I wonder what is the effect of temperature or concentration during the subsequent reaction period. Materials scientists are often interested in size control, does this method provide size control?

Overall this was an excellent article and I’ll post an update after I give these reactions a try.


4 Comments

  1. Courtney says:

    Any mention of whether nanoparticles made by this method have similar properties to those made by less green methods?

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  2. Marty Mulvihill says:

    Excellent question, the researchers used these particles to make thin films. In this application they work better then other nanoparticles that have more complex coatings. On the other hand, I have seen synthesis procedures with better control over size and shape of the particles which may be necessary for certain applications.

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  3. Wouldn’t it be ‘greener’ to make nanoparticles straight from a salt rather that from a powder of the reduced metal? A lot of energy goes into making those metals and then you used H2O2 to oxidize them back to the oxidation states you find in nature. So going straight from an ore to a nice thin film or whatever seems like it would really be ideal.

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    • Marty Mulvihill says:

      Thank you for the comment, this is a great point.

      It is certainly worth while to consider the embedded energy of the precursor metals. In some cases the salts will have lower embedded energy, but this is not always true. Most Mo salts are actually prepared from the metal and many of the high purity salts that you would buy from Aldrich are also obtained from their respective metal precursors.

      Additionally, I don’t think that the authors base their “greenness” claim on energy usage. I think the benign oxidants and solvents used for this general procedure along with ease of purification support the authors greenness claim, regardless of the embedded energy of the precursor salts.

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