To investigate present phases in the structure, such as goethite, hematite and magnetite, the existence of O-H and Fe-O bonds were analyzed through Fourier transform infrared spectroscopy. (Fig.1) displays the infrared spectra of co-precipitated iron oxide samples. Characteristic Fe-O stretching between 400 to 100 cm-1 was detected in all samples. Specifically, at ~540 cm-1, iron oxide species were formed in the wide pH range between 5.5 to 11.5 (20–22).
In pH=5.5, peaks detected at 1195.6 cm-1, 1098.4 cm-1, 1036.3 cm-1, 997.01 cm-1, 875 cm-1 and 786 cm-1. Observed peaks at 1098.4 cm-1 and 1036.3 cm-1 correspond to water stretching, whereas 875 cm-1 and 786 cm-1 arise from –OH bonds (23,24). According to results, it could be understood that at pH=5.5, the Goethite phase was present in the composition. However, after the synthesis of iron oxide particles red precipitates were formed (Fig.1). From the color of the precipitates, it could be deduced that the phase also contains the hematite phase.
Powders synthesized at pH=7.5 have distinct peaks at 792.2, 893.6, and 3106.9 cm-1 corresponding to ν-OH, δ-OH, and γ-OH. Since the goethite phase has –OH stretching vibrations, Peaks indicate the goethite phase formation at pH=7.5 (24,25). In samples of pH 11.5 and pH 11.5 D, peaks at 3425.4 cm-1 and 1634.1-1638.4 cm-1 match with H-O-H and O-H stretching vibration (25). Moreover, both samples have shown identical peaks of Fe-O vibrations between 500 to 730 cm-1. According to the results, it can be concluded that at pH=5.5 both the hematite and goethite phases were present in the structure, while at pH=7.5, goethite was formed excessively.
X-ray diffraction was used to detect and distinguish Fe-O species of the synthesized powders, as shown in (Fig.2). From the results, only Fe3O4 phase was observed at pHs higher than 7.5
At higher pH ranges, 2θ degrees at 30.1°, 35.4°, 42.9°, 57.5°, 62.7° corresponds to diffraction planes of (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) of spinel structure of Fe3O4 was the only observed phase (26,27). The crystal configuration of spinel Fe3O4 is also referred to as (Fe3+)A(Fe2+Fe3+)BO4 where tetrahedral sites (site A) consist of eight Fe3+ surrounded by four O2-. At octahedral sites (site B) eight Fe2+ and eight Fe3+, each neighbored by six O2- (28). We also experimented with the rate of pH increase caused by NaOH addition to determine if there was any change in phase formation. Therefore, NaOH was added dropwise at pH11.5-D while at pH11.5 NaOH was added abruptly to the solution. No significant change was observed from the XRD data. However, at a pH of 7.5 a second phase, namely, goethite (FeOOH) started to form.
(Fig.3) The SEM images of synthesized at pH 11.5 and with 1% (w/w) PVP. In (Fig.3 a-b) formed particles are in the size of microns; however, in closer images (Fig.3 c-d) smaller particles have a size of less than 100 nm can be observed. This aggregate-like morphology may be due to the drying stage, where previously particles started to form hard aggregates in micron-scale.
The effect of PVP concentration on nanoparticle size was evaluated by SEM images and corresponding size distribution diagrams (Fig.4). the presence of nano-sized particles in the range of 50 to 120 nm was observed in all concentrations from 1% PVP to 5%.
With the rise of the pH value, Fe2+ and Fe3+ ions form metastable Fe3O4 nucleis’ which are high in free energy. To minimize Gibbs free energy and become stable, formed nucleis tend to grow. Growth occurs through various mechanisms such as Oswald ripening and aggregation (29). At the growth stage, either an electrostatic or steric barrier is needed to halt further particle growth at the nanoscale. Long polymeric chains with high electronegative functional groups, such as PVP, provide a protective shield where particle surfaces are coated with it (1).
For this reason, stabilizers in particle synthesis are mandatory to obtain nano-sized particles. From the SEM images, one can deduce that the particles were formed in nano-size and after that, all the solvent is evaporated at the drying stage. The evaporation of the solvent breaks the interaction of PVP with Fe3O4 particles, resulting in aggregated morphology.
The magnetic properties of the Fe3O4 nanoparticle synthesized at pH 11.5 is studied using a vibrating sample magnetometer (VSM). Results have shown that synthesized particles did not show any magnetic remanence and thus exhibited superparamagnetic behavior with magnetic saturation of 41 emu/g (30).
In the inverse spinel structure of Fe3O4, Fe2+ ions align their spins parallel to Fe3+ ions in adjacent octahedral sites leading to a net magnetization (31). The bulk form Fe3O4 possesses a ferrimagnetic behavior (32). Ferrimagnetism, meaning that magnetic moments stay aligned even in the absence of a magnetic field due to multi-domain structure as shown in (Fig.6.). Though, in nano-scale every particle act as a single magnetic domain where all the magnetic spins point at the same direction. At a critical diameter, the coercivity of the particles is reduced and showed paramagnetic behavior called “superparamagnetism”. Thus, can only be magnetized in an external magnetic field (33). In literature, the critical size threshold for superparamagnetic behavior was found to be 20 nm (34). In this regard, our findings suggest that formed Fe3O4 aggregates were composed of smaller particles that act as a single domain. Thus, for particles to possess paramagnetic property, the crystallite size is important rather than the particles.