“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, Small, 2011, DOI: 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.
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.