I am trying to synthesize SrHfO3:Eu2+ nanoparticles. To reduce from Eu3+ to Eu2+, we calcinate it under Ar/H2 conditions but it ends giving a black coloured product. We need this for luminescence and expecting a white coloured product. How do I avoid this?
You may first characterize your product and find if some unwanted compound is arising. In general, many compounds get black during calcination due to burning of some surface layers, specially if carbon is present (in your case it is, as you are using organic molecules). However, if you grind the product, you may find the bulk colour is actually different, only the surface has become dark.
If oxygen is present, the effect may increase. So, find out the degree of purity of your Ar/H2 environment. If the oxygen content is less than 50 ppm, then the effect should diminish. Also you may optimize the calcination temperature for best results.
Calcination in 2 steps might help. The first step calcination should be in the presence of O2 (ambient atmosphere) and low temperature (typically 500-700 °C) to get rid of organic moieties and avoid excessive particle growth. The second step is the calcination in reducing atmosphere to obtain the Eu2+. A more detailed process can be finely tuned by investigating the thermal degradation of the precursors at both oxidative and reducing atmosphere (TG, DSC, ...).
Best wishes,
José
In my experience, argon tends to adsorb onto surfaces and make it difficult to achieve equilibrium with the atmosphere. Of course, this only happens at not very high temperatures, somewhere below 700 C. However, I would advise you to replace argon with nitrogen evaporated from a liquid state. Another way is drying in vacuum in a closed volume in the presence of a getter. In your case, a small amount of metallic hafnium may be a good getter. I would put phosphorus pentoxide on the cold end of the reactor, keeping it at room temperature or even colder.
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