HRTEM study of freeze-dried and untreated synthetic ferrihydrites: consequences of sample processing

Two-line ferrihydrite samples were synthesized following conventional proce dure. Detailed characterizations of freeze-dried and untreated samples - pr epared from direct inclusion of the fresh precipitates in hydrophilic resin - were made by high resolution transmission electron microscopy to investi gate in detail their structural organization and to compare the two types o f preparations. Only highly disordered chain-like aggregates of 2-5 nm diameter size nano-p articles were revealed within the untreated samples. Conversely, in freeze- dried samples, domains with different degrees of order were recognized, fro m poorly ordered 2-line ferrihydrite to 3-line ferrihydrite particle aggreg ates that revealed lattice fringes. Associated with these aggregates, a goe thite phase with a modified crystal morphology was also observed. These HRTEM observations revealed that freeze-drying techniques induce arti facts by disturbing ferrihydrite aggregates while direct inclusion in hydro philic resin preserves the primary organization of such poorly ordered phas es.