Here is my synthesis process:
ref(Adv. Mater. 2019, 31, 1904639)
Zha Yongchao Have you carried out a Stokes' Law calculation for your material? MnO2 is quite dense (~ 5 g/cm3) and it does not take a large size for it to settle quickly. Now, if your particles are hollow then the size must be quite large in line with the calculated density (mix of MnO2 and water rather than MnO2 and air?). What does electron microscopy (EM) show? The single particles can be too large (here the Stokes' Law calculation will help) or you have aggregation and agglomeration occurring increasing the size of the 'particle cluster'. Again EM will help here to understand the form of the precipitate. Change of pH can help here with stabilization (alters the zeta potential). Also, I am also wary of 'washing stages' especially in pure liquids (removes stabilizing ions, for example). What happens if you omit the washing cycles in your first stage?
Alan F Rawle Thank you for answering my questions! Attached is a transmission electron microscope photo I took of the manganese dioxide, and you can see that its particle size is around 50-60 nm, so it is unlikely that the sinking is caused by the individual particles being too large. So I am more inclined to think that the manganese dioxide is aggregating, and I am looking for the reason. And its zeta potential is only -8mV, I think this may be one of the reasons for its aggregation and sinking. As for Stokes' law, I didn't calculate it, and to be honest I didn't know that I could make a theoretical judgment by calculating it before. I will try it.
Zha Yongchao The EM pictures are vey useful. Visualization is very important. Yes, of course, aggregates and agglomerates will settle much more quickly than individual particles. The issue one (always) has with a sedimentation calculation is the estimate of the particle density and this is complicated by the 'hollow' nature of those particles. So, there will be a large margin of variation to consider in such a calculation. An estimate of the density can be made by considering the volumes of the solid phase and the 'filling' in the sphere (is this water or air?).
For particles this size then steric (rather than charge/electrostatic) stabilization is perhaps a better bet (and here zeta potential is practically irrelevant). You can see the fused solid bridges in your first EM picture above. Here is some background webinar material that may be useful (registration required):
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
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