Synthesis of Aqueous Ferrofluid Nanoparticles: Glass Rod

A ferrofluid is a liquid made of nanoscale ferromagnetic particles dispersed evenly throughout a carrier fluid. Ferrofluids become very strongly magnetized in the presence of a magnetic field. Here, Fe3O4 magnetite nanoparticles are produced by mixing Fe(II) and Fe(III) salts together in a basic solution.

Safety:

• Wear eye protection
• Chemical gloves recommended
• Ferrofluids can be messy. The particular ferrofluid you will prepare will permanently stain almost any fabric.

Procedure:

Step 1. Check that the iron solutions are good since Fe(II) slowly reacts with O2 to become Fe(III). Verify the solution colors!

1 M FeCl3 in 2 M HCl (This solution should be brown and can be prepared ahead of time.)

2 M FeCl2 in 2 M HCl (This solution should be lime green and needs to be freshly prepared before the experiment.)

Step 2. Add 4.0 mL of 1M FeCl3 and 1.0 mL of 2M FeCl2 solution to a 100 or 150 mL beaker.

Step 3. Continue stirring with a glass rod throughout the dropwise addition of 50 mL 1.0 M aqueous NH3 solution over a period of about 5-10 minutes, adding approximately 1 mL every 10 seconds. Avoid addition that is faster than the solution can be mixed, but also avoid addition that is so slow that the particles grow large. CAUTION: Although 1 M NH3 is fairly dilute, NH3 is a strong base. At the end of the 5-10 minutes stirring session, the solution needs to be jet black.

Step 4. Let the magnetite settle. You can speed the settling process by putting a strong magnet (like a neodymium magnet) under the container.

Step 5. Decant (pour off) and discard the clear liquid without losing a substantial amount of solid. This works best if you keep a strong magnet under the container.

1. Transfer the solid to a weighing boat with the aid of a few squirts from a wash bottle.
2. Use a strong magnet (like a neodymium magnet) to attract the ferrofluid to the bottom of the weighing boat.
3. Pour off and discard as much clear liquid as possible, again keeping the strong magnet under the weighing boat.
4. Rinse with water from a wash bottle and decant the rinse as before.
5. Remove the strong magnet. Add 1-2 mL of 25% tetramethylammonium hydroxide. Use caution as this is a very caustic chemical.
6. Gently stir with a glass rod for at least a minute to suspend the solid in the liquid. Use a strong magnet (like a neodymium magnet) to attract the ferrofluid to the bottom of the weighing boat. Pour off and discard the dark liquid. Move the strong magnet around and again pour off any liquid. If the ferrofluid does not spike, slowly move the strong magnet away from the ferrofluid to decrease the strength of the magnetic field, or substitute neodymium magnet for a weaker magnet. Continue pouring off any liquid.
7. What happens when you move a magnet under the ferrofluid? If you are using a very strong magnet you might get more interesting results by varying the distance of the magnet below the boat.
8. In case of an accident, ferrofluid can be removed from the surface of magnets using a dilute acid.

Option: Obtain the powder x-ray diffraction spectrum (2θ = 25-65°) of the ferrofluid on a glass slide (the aluminum holder is under the slide as a template.) Measure the peak width for several peaks as shown at right. Diffraction peaks are a result of constructive interference of X-rays reflected by crystal planes. The more planes the sharper the peak so the smaller the crystallite size the broader the diffraction peak. Estimate the particle diameter,

where λ is the X-ray wavelength, Δθ is the peak-width at half-height (FWHM) and 2θ is the peak location. (For even better results use the peak width of the sample minus the peak width of the same peak in a bulk sample as the peak width.)

Questions:

1. Were you able to prepare ferrofluid and observe spiking in the presence of a magnet? Pick one spike in the middle. How many spikes were immediately around that spike and how were they arranged? (You might draw a picture viewed from above. If you did not observe spiking with your sample, you will need to inspect a sample that did spike.)
2. Examine the starting FeCl2 and FeCl3 solids used to prepare magnetite (leave the solids in their sealed containers.) How do they respond to a magnet? Which is magnetic?
3. What is the purpose of the surfactant? Explain chemically how the surfactant works to keep the nanoparticles suspended.
4. What evidence do you have for the formation of nanoparticles?

Materials:

Stock solutions for 50 preparations

• 2 M HCl (21 mL conc HCl in 250 mL water) for making iron solutions.
• 2.0 M FeCl2(H2O)4 in 2 M HCl. (Each student needs 1 mL. Dissolve 19.9 g in 50 mL 2 M HCl. Verify color as you weigh out the solid. This material dissolves readily but the solution reacts with oxygen and should be freshly prepared.)
• 1.0 M FeCl3(H2O)6 in 2 M HCl (Each student needs 4 mL. Dissolve 54.1 g in 200 mL 2 M HCl. This material often dissolves slowly.)
• 1.0 M NH3 in water. (Each student needs 50 mL. Dilute at least 200 mL of concentrated ammonium hydroxide with water to 3.0 L. It may be possible to use household ammonia; its concentration varies and an initial estimate would be 1.0 L household ammonia diluted to 3.0 L.) Open containers of ammonia will smell bad and their concentration will decrease leading to poor results.
• 25% tetramethylammonium hydroxide in water (commercially available from Aldrich, Fisher, etc.). A strong, fishy, amine odor indicates hydrolysis products which may interfere with the synthesis. In this case adding an additional 1-2 mL may help. Caution: This is a very caustic chemical.

Equipment:

• A way for students to measure iron solutions (one buret dispensing each iron solution works well; individual pipets could also be used)
• 100 or 150 mL beakers
• A way to slowly add the ammonia solution (a buret or separatory funnel or eyedropper)
• Glass rods for stirring
• Plastic weighing boats
• Wash bottles
• Disposable gloves
• Strong craft magnets
• Cow magnets
• Option: XRD spectrometer and glass slides

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