2.1. Hydrothermal Synthesis of Hematite

FeCl₃·6H₂O, p.a, 99% (Merck KGaA, Darmstadt, Germany), and ammonia solution (Chimreactiv SRL, Bucharest, Romania) were used for hydrothermal synthesis of iron oxides.

In a first step, FeCl₃·6H₂O was dissolved in distilled water to obtain a solution whose iron concentration was determined by the Inductively coupled Plasma—Optical Emission Spectroscopy (ICP-OES) method. For the precipitation of the iron oxide precursors, a 25% ammonia solution was added dropwise under magnetic stirring. A brown precipitate with alkaline pH was obtained. Thus, obtained suspension was washed with water to remove the by-products.

In the second step, the washed suspension (iron oxide precursor) was transferred to the autoclave and subjected to hydrothermal synthesis at 200 °C for 3 h. SAM autoclave (Romania) endorsed with cooling system, was used to prepare samples at pressures of 20–100 bar (experimental conditions: working volume 0.3 L; pressure created inside the stainless steel vessel of the autoclave using argon gas; temperature 200 °C), while HP Systems autoclave (Bordeaux, France) was used in the case of sample obtained at 1000 bar (experimental conditions: working volume 1 L; isostatic pressure; temperature 100 °C). After hydrothermal treatment, the nanostructured powders were dried by lyophilization using a Martin Christ Alpha 1–2 LD Plus freeze dryer (City, US State abbrev. if applicable, Country). Experimental parameters of the investigated samples and chemical analysis results are presented in Table 1. Fe content was determined using a chemical quantitative method, according to STAS 1574/3-90.

Synthesis conditions of iron oxide nanostructures prepared in this work and Fe content of the investigated samples, according to quantitative chemical analysis.