## 3 Results and Discussion
### Final Particle Characteristics of Samples Obtained by SpraySyn1 and SpraySyn2
#### 3.1.1 TEM and Raman Spectroscopy  
By examining the samples by TEM and Raman spectroscopy, information considering the carbon presence and statements about the iron oxide phases present are obtained. Since all samples showed identical Raman spectra and since TEM is utilized on a qualitative basis, Figure 4 solely corresponds to one sample (0.1 M INN, SpraySyn1). Figure 3: TS-TEM extraction principle. Images captured by high-speed monitoring without flame. The TEM grid (magenta) is adhered to an inner slider (dotted line) and is exposed to the environment by passing through a recess (width = 7 mm). As shown exemplarily in Figure 4a, examining the synthesized samples by TEM evidences the presence of small-sized particles \(<\)10 nm. This implies that particles originate here predominately from the gas-to-particle pathway aligning with expectations. By proving arbitrary spots by Fast Fourier Transformation (FFT), clear diffraction patterns emerge (frame 3), indicating well-crystalline particles and the obtained d-spacing values match to \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) and/or Fe\({}_{3}\)O\({}_{4}\)[4; 5]. Regarding the particle purity, no indications for carbon depositions can be obtained by TEM as no core-shell structures and/or amorphous particles can be recognized. In case carbon would be present with some single wt.%, considerable core-shell structures should be recognizable in TEM observations [13]. Hence, the TEM examination indicated at very first glance that samples of high purity are obtained, which also matches with the experimental yield values indicating high reaction yields. Admittedly, some very small particles appear to be amorphous at first sight (frames 2, 4). However, by applying high resolutions in TEM, noticeable line diffractions become visible revealing crystalline particles (frames 5, 6). Since Fe\({}_{3}\)O\({}_{4}\), \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\), and \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) (hematite) exhibit individual signals in the low wavenumber region \(<\)1000 cm\({}^{-1}\)[49], Raman spectroscopy allows the qualitative identification of phase types in Fe\({}_{3}\)O\({}_{y}\) samples. As exemplary shown in Figure 4b, three strong signals exist at \(\sim\)350, \(\sim\)510, and \(\sim\)710 cm\({}^{-1}\) revealing a predominant presence of \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) rather than Fe\({}_{3}\)O\({}_{4}\) or \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\). Noteworthy, all powder spectra also exhibit a shoulder at \(\sim\)650 cm\({}^{-1}\), which can also be attributed to the \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) in general. However, since Fe\({}_{3}\)O\({}_{4}\) attains its strongest signal at this position, the data does not entirely discard the presence of any traces of Fe\({}_{3}\)O\({}_{4}\) from a very critical point of view. Given the fact that carbon yields Raman active features in the high wavenumber region, \(>\)1000 cm\({}^{-1}\)[7; 13; 19; 20; 50], we expected that the obtained spectra will also provide information about carbon depositions. Noteworthy, as mentioned above TEM examinations gave no indications for carbon depositions. However, TEM observations are also very point-selective, which is accompanied with a certain experimental uncertainty. In case carbon is present, Raman spectra can display two distinct bands. At the one hand the so-called D-band can be found at \(\sim\)1300 cm\({}^{-1}\), which is sensitive to atomic disorders, and is typically pronounced for amorphous carbon in consequence. At the other  
Figure 4: Exemplary TEM images (**a**) and Raman data (**b**). Both techniques suggest \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) as product. Shown sample: SpraySyn1, 0.1 M INN in EtOH + EHA. Raman references: Fe\({}_{3}\)O\({}_{4}\), \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\), and \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) according to [PERSON] et al. [49] and carbon bands according to [PERSON] and [PERSON] [19]. hand, carbon can exhibit the so-called G-band, typically located at \(\sim\)1590 cm\({}^{-1}\).