Scope: water/alcohol dispersion of nano α-Fe₂O₃ powder and evaluation of dispersion quality (TEM/SEM observation)
Assumptions: the purpose is to evaluate dispersion quality and primary particle morphology; particle size is nano-scale. For high-solids slurry preparation, increase the solids content and adjust dispersant dosage accordingly.
Pre-wetting → pH adjustment / dispersant addition → ultrasonic dispersion → low-speed centrifugation (take supernatant) → EM sample preparation (TEM / SEM) → observation & evaluation
Medium: water (recommended) or anhydrous ethanol. Water is easy to pH-tune and suits electrostatic stabilization; ethanol dries faster, which helps sample preparation
Starting concentration: 0.5–1 mg/mL (for EM observation); high-concentration systems can be concentrated afterwards
The isoelectric point (IEP) of α-Fe₂O₃ is about pH 7 (varies 6.5–8.5 with particle size and preparation method)
Shift the system away from the IEP: pH 9–10 (NaOH / ammonia) or pH 3–4 (HCl / nitric acid), so that the zeta potential exceeds |30 mV| and sufficient electrostatic repulsion is obtained.
|
Type |
Examples & dosage |
Notes |
|
Inorganic polyelectrolyte |
Sodium hexametaphosphate (SHMP), 0.1–0.5% |
Inexpensive; adsorption provides electrostatic stabilization; note the introduction of Na/P |
|
Organic polymer |
Sodium polyacrylate, PVP, PEG, 0.1–1% |
Steric stabilization, suits higher concentrations; excess dosage forms a film upon drying and interferes with EM imaging |
|
Nonionic surfactant |
Triton X-100, Tween-80, trace amounts |
Aids wetting and reduces surface tension |
|
Surface modification |
KH-550 silane coupling agent (hydrolyze before grafting) |
If hydrophobicity or downstream grafting is needed, modify first, then disperse |
Probe sonicator: power 300–600 W, pulsed mode (3 s on / 2 s off), 5–15 min total, ice-bath coolingBath sonicator: low power density, only suitable for preliminary pre-dispersion
Key point: ultrasonic heating intensifies Brownian motion and causes re-agglomeration — an ice bath is mandatory; excessive sonication may fracture particles or induce phase transformation
Low-speed centrifugation (2000–4000 rpm, 5 min) to remove residual large agglomerates; use the supernatant for observation
Record zeta potential and DLS particle size (incl. PDI) in parallel as supporting evidence of dispersion quality
Dilute the dispersion to ~0.01–0.05 mg/mL (slightly translucent to the eye)
Drop 5–10 μL onto a carbon-film copper grid; wait ~1 min
Blot excess liquid with filter paper; dry naturally or under vacuum; avoid heat drying (thermal convection induces agglomeration)
At high magnification, capture lattice fringes / SAED patterns to distinguish α-Fe₂O₃ from γ-Fe₂O₃
Drop onto a clean silicon wafer / aluminum foil and dry; or dip a small amount of powder onto conductive tape directly
α-Fe₂O₃ is poorly conductive — sputter-coat with Au or C (5–10 nm) to prevent charging
Low magnification (×1k–10k): overall uniformity, presence of large agglomerates
High magnification (×50k–200k): primary particle morphology, degree of monodispersity
Measure particle size distribution with ImageJ (≥100 particles), report D10/D50/D90
|
Phenomenon |
Cause |
Countermeasure |
|
Particles form networks/clumps after drying |
Drying-induced agglomeration (capillary force) — not a problem of the dispersion itself |
Lower the concentration; vacuum/freeze drying; add trace surfactant |
|
Film-like matter / hazy background in EM images |
Excessive dispersant forming a film upon drying (same mechanism as the TiO₂ dispersion film-formation case) |
Reduce dispersant dosage; wash 2–3 times by centrifugation + deionized water to remove excess dispersant |
|
Particle size increases after sonication |
Overheating-induced re-agglomeration |
Ice bath + pulsed sonication |
|
Abnormal morphology (rods / irregular shapes) |
Contamination with γ-Fe₂O₃ or inhomogeneous synthesis |
Confirm phase and purity with XRD |
|
Obvious magnetic attraction / agglomeration |
Sample contains magnetic γ-Fe₂O₃ / Fe₃O₄ (α-phase is only weakly ferromagnetic and should not strongly attract) |
Confirm with XRD; magnetically separate the magnetic fraction first |
|
Zeta potential near zero |
pH close to the isoelectric point |
Adjust pH away from the IEP, or increase dispersant dosage |
Sedimentation test: obvious settling within 1 h → poor dispersion; adjust pH / add dispersant
Zeta potential: |ζ| > 30 mV indicates good dispersion
DLS size and PDI: PDI < 0.3 indicates a narrow size distribution
