Preparation of Superparamagnetic Fe₃O₄ Nanoparticles via Co-precipitation Method

Magnetic nanomaterials are one of the research hotspots in today's materials science. They show great potential in high-density magnetic recording, ferrofluids, magnetic sensors, microwave materials, catalysis, and environmental remediation. Among them, Fe₃O₄ nanoparticles have attracted considerable attention due to their simple synthesis, low cost, and excellent properties. At the nanoscale, Fe₃O₄ changes from ferrimagnetic (in bulk) to superparamagnetic behavior due to the size effect. These particles are easily magnetized and demagnetized.

The co-precipitation method is a commonly used technique in materials synthesis and is widely applied for the preparation of nano-sized Fe₃O₄. One representative example of this approach can be found in Alfa Chemistry's description of Fe₃O₄ nanoparticle synthesis via co-precipitation, which outlines its practical advantages and process simplicity.

In this method, a precipitating agent is added to a solution containing multiple cations. After thorough mixing, the metal ions precipitate completely, and the resulting precipitate is thermally decomposed to form fine powders. Co-precipitation is well-suited for the preparation of complex oxides, as it minimizes the introduction of harmful impurities. The resulting products exhibit high chemical homogeneity, fine particle size, narrow size distribution, and defined morphology. Moreover, the equipment required for this method is simple, making it favorable for industrial-scale production.

According to the order of reagent addition, the co-precipitation method can be classified into three types: sequential co-precipitation, reverse co-precipitation, and concurrent co-precipitation (see Figure 1).

In sequential co-precipitation, the precipitating agent is added dropwise to the mixed metal salt solution. Since the precipitant is not in excess at this stage, it can easily lead to product inhomogeneity.

In reverse co-precipitation, the mixed metal salt solution is added to the precipitating agent. Because the precipitant is always in excess, the resulting product tends to be more uniform.

In concurrent co-precipitation, both the precipitant and the mixed metal salt solution are added simultaneously. This ensures that the ion concentrations remain consistent throughout the precipitation process, leading to particles with minimal differences in composition, properties, size, and distribution.

In this experiment, the sequential co-precipitation method was adopted: sodium hydroxide was added dropwise to a mixed solution of divalent and trivalent iron ions. Under the action of hydroxide ions, Fe²⁺ and Fe³⁺ co-precipitated and dehydrated to form Fe₃O₄.

Fig. 1. Schematic Diagram of Several Common Co-precipitation Methods

Instruments and Reagents

Instruments: Beaker (100 mL), balance, glass rod, neodymium magnet

Reagents: Sodium citrate, sodium hydroxide, ferrous chloride (FeCl₂), ferric chloride (FeCl₃)

Experimental Procedure

1. Preparation of the Precipitating Agent

(1) Prepare 20 mL of sodium citrate solution with a concentration of 0.1 g·mL^-1.

(2) Prepare 30 mL of sodium hydroxide solution with a concentration of 2 mol·L^-1.

(3) Mix 20 mL of sodium citrate solution with 20 mL of sodium hydroxide solution and stir thoroughly to obtain the precipitating agent.

2. Synthesis of Fe₃O₄ via Co-precipitation

(1) Add 0.381 g of FeCl₃, 0.975 g of FeCl₂, and 20 mL of water to a beaker, and stir until fully dissolved.

(2) While stirring, add the prepared precipitating agent dropwise until the pH exceeds 11. Continue stirring for 10 minutes, then let the mixture stand for 5 minutes. Decant the well-dispersed portion to obtain crude Fe₃O₄ sample A.

(3) Prepare a second batch using the same reagents as in step 1. This time, add NaOH solution directly (instead of citrate-containing precipitant) dropwise until pH exceeds 11. Stir for 10 minutes and let stand for 5 minutes. Decant the well-dispersed portion to obtain crude Fe₃O₄ sample B. Compare the quantity and dispersion of sample B with sample A to assess the effect of sodium citrate.

3. Separation of Fe₃O₄

(1) Place the crude Fe₃O₄ sample into a beaker, then place a neodymium magnet on the side of the beaker and let stand for 10 minutes.

(2) Attach the magnet to the beaker wall, carefully decant the supernatant, and retain the magnetic particles adhering to the inner wall. Wash the product 3 times with deionized water using the same procedure.

(3) Dry the product in a vacuum oven for 1 hour, weigh the product, and calculate the yield.