Normally I would leave the literature alone on the production of ZnO — it has been explored on a number of occasions, and with several processes — including solution phase microwave. A recent publication (Journal of Materials 2013) intrigued my interest: this group detailed several parameters in their development of ZnO nanoparticles — and discussed at length the morphologies associated with the different conditions.

Let’s back up a bit — ZnO is a critical component in paints, plastics, ceramics and adhesives. It’s properties open the number of applications were it can be used. Hence the importance of looking into good ways to reproducibly make the material with the desired morphology. For this paper, the group utilized microwave radiation for the production of ZnO by studying the temperature effect, precursor variation, post-wash, base precursors, stoichiometry and power/irradiation time. Each contributes to the variation in products formed and provide some guidance for those interested in using mw for their research.

We are all familiar with the general advantages — rate acceleration, wide range of conditions, selective morphology formation, reproducibility of the instrumentation, easy-to-use and fast instrument optimization. However when it comes to inorganic microwave conditions the key performance factor relates to keeping the variable temperature gradient profiles to a minimum over the bulk of the reaction — and the microwave heating the solution and materials, rather than the vessel and relying on convection heating to transfer the heat…..serves as the prevailing reason for using the technology.

Before getting into some of the key parameters: the products we characterized by FE-SEM and XRD — pretty normal in the field.

Temp: Conditions were set to a range of 80-140C

Products formed at 80-100C were impure and contained a considerable amount of precursor starting materials. At 120-140C strong defraction peaks appear corresponding to wurtzite ZnO and a homogeneous distribution of particles from the conditions at 140C. Looking at the table 1 (Click on image to zoom) and characterizations 2, it is interesting to see differences in particle size at 140C depending on the amount of power that was administered — it was postulated that an increase in power could help reach the desired temperature quicker. While that is a good dose of handwaving, nucleation and crystal growth play a role in the formation of the the products so further studies would be needed to tweak out conclusively.

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Table 1: Microwave parameters

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100C, 120C and 140C

Particle size and precursor effect: For the zinc nitrate and zinc acetate, the distribution of particles is homogeneous whereas the ZnCl2 produced some aggregation and non-uniform distribution. Depending on ligand coordination or lack there of in the starting materials, the counter ion has a pronounced effect on the crystal growth — and this allows some control of over the type of ZnO desired…..Characterization 2

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ZnO with different starting materials with mw at 140C 600W

Base effect: Each of the bases used: NaOH, KOH and NH4OH provided different morphological products depending on conditions: the differences are attributed to the changes electrostatic properties of the hydrated ions Na+, K+ and NH4+ — and their effect on crystal growth on the surface (a good example is K+ producing more than one type of crystal depending on the concentration).

Power and irradiation time: Here is where I found the studies to be the most interesting — YOU HAVE TO READ! For time, there is a comparison at the 140C at 5, 10 and 20 min timepoints including the set-point temperature reached — so this is time at 140C. Looking at the Characterization 3, it is easy to see that some product formation happens during the ramp to temperature and more well defined particles are achieved at the late stage timepoint — or 10 minutes at 140C.

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Product once reached 140C, 5 min at 140C and 10 min at 140C

However, when looking at the power — the increase in power to reach 140c over the same time period produces less defined structures. Perhaps not so intuitive a result considering all of the parameters studied. From this it sounds like I should use less power, shallower ramp profile and hold the temp a bit longer to produce the ideal product. Give this a read, it certainly has me thinking. I firmly believe that the complexity of the reaction system and the mechanisms of nucleation and crystal growth makes this type of experimentation necessary in tuning desired product and morphology in nanoparticle design.

Enjoy!

 

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