I have been attempting to create Fe3O4 particles using the coprecipitation method using a 2:1 FeCl3:FeCl2 ratio. Today we used 0.1M FeCl3, 0.05M FeCl2 dissolved in 20 mL of water. This was stirred and 80mL of 1.5M NH4OH was added. This was heated to 80-85C and remained at this point for 1 hour. After the hour, we let the solution cool and noticed a defined separation between the particles and the supernatant. We mixed this solution and poured into a different tube which we magnetically decanted. After the first two washes the supernatant was clear, but then we combined the separate tubes we were using, the supernatant always contained particles and they wouldn't fully separate again. We tested the supernatant and it had very little separation of any particles. We were also expecting a particle size less than 20 nm according to multiple studies using TEM but we found a particle size of 597nm using DLS. We definitely had enough NH4OH this time, as previously we witnessed what happened with too little and did not receive the same results. I need them to fully separate and give me an accurate particle size because I plan to coat with TEOS and need to verify the particles were coated. Thank you so much!
you may have changed the zeta potential of the particles in the suspension. Since you wash the system and ressuspension occur you may have an electric charge change. total precipitation occur at zero charge. As you have a DLS you can measure the zeta potential variation with pH.
Hi Dr. Derenzo,
Thank you for your response! I do believe that this is possible but no other group (to my knowledge) has reported this as an issue which is why I don't know why it's an issue for us. I wash with degassed DD water which I tested to a pH around 6.5 but when we tested the supernatant of the solution, it came out to be slightly more acidic. The only thing I could think is that with excess washes, the particles begin to oxidize in the solution and essentially drop the pH but I didn't know these particles were that sensitive to change. This also doesn't solve the issue of the size. Thank you!
The oxidation of magnetite in aqueous media is common. The oxidation process becomes faster in the more acidic media. The magnetization of the system is reduced as a result of the oxidation process. regarding this issue, please refer to CrystEngComm, 2017, 19, 246–255 (DOI: 10.1039/c6ce02421a) as a starting point.
Regarding the size issue, I think that DLS measured the secondary size of the particles, i.e. the size of their aggregates or agglomerates, while TEM measures the primary size.
The long range magnetic forces easily overcome any attempts to charge stabilize a system for magnetite. The short range (attractive) van der Waals forces are small in comparison. The only route to stabilize such systems is sterically with either a suitable polymer (50 KDa PEG or PEI) or on an inert (?) support such as silica or alumina. At the sizes measured by DLS you should see significant settling (carry out a Stokes'Law calculation to see what size of particle with density ~ 5.5 g/cm3 can remain in suspension). If your particle size was actually 20 nm you'd have a colored but transparent (not opaque or cloudy) system. The DLS result gives you the actual size of the material in suspension (aggregates and all) and not a hypothetical selective 'primary size'. This DLS size actually refelcts the transport properties of the particles and is a guide to the reasons for settling in terms of size alone. See also (open in IE only):
Nov 11th, 2008. Dispersion and nanotechnology
http://tinyurl.com/hpywsge
November 3rd, 2015 Adhesion and cohesion
http://tinyurl.com/zwb2wlh
You need continuous and vigorous stirring and drop wise addition of ammonium hydoxide otherwise particle size will be non uniform. Also volume and capacity of beaker matter for reaction.Use comparatively large volume beaker as reaction mixture.
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