## 3 Results and Discussion
### Structural and Morphological Analysis
These results confirm that both materials are active under visible light irradiation, with FeWO\({}_{4}\) exhibiting a slightly narrower band gap, which may contribute to its higher photocatalytic performance. A lower band gap favors more efficient absorption of visible light and may enhance charge carrier generation. To complement the UV-vis results, PL spectra were recorded to investigate the presence of defects in the Fe\({}_{2}\)(MoO\({}_{4}\))\({}_{3}\) and FeWO\({}_{4}\) samples. Measurements were performed at room temperature over a wavelength range of 350-600 nm, as shown in Figure 2C,D. Both samples exhibited broad emission bands in the visible region. For Fe\({}_{2}\)(MoO\({}_{4}\))\({}_{3}\), the main emission band is centered between 450 and 500 nm, while for FeWO\({}_{4}\), it appears between 420 and 470 nm. According to the literature, these broad emissions originate from the recombination of charge carriers trapped at defect sites [29; 30]. The electronic characteristics and charge migration behavior were further analyzed by deconvoluting the emission bands using Gaussian fitting, revealing two peaks for Fe\({}_{2}\)(MoO\({}_{4}\))\({}_{3}\) and three for FeWO\({}_{4}\). Emissions in the red and yellow regions are typically associated with deeper defects, such as oxygen vacancies, whereas emissions in the blue region are linked to shallow defects caused by structural distortions in metal-oxygen bonds. Table 2 summarizes the main PL peaks, their relative intensities, and the corresponding defect types, offering insights into the recombination dynamics and defect structure of the material. The Fe\({}_{2}\)(MoO\({}_{4}\))\({}_{3}\) crystals are composed of interconnected [MoO\({}_{4}\)]-[FeO\({}_{6}\)]-[MoO\({}_{4}\)] clusters, as illustrated in Figure 1A, suggesting that [MoO\({}_{4}\)] clusters play a key role in the emission behavior. The PL results for Fe\({}_{2}\)(MoO\({}_{4}\))\({}_{3}\) are consistent with those reported by [PERSON] et al. [31], who observed a maximum emission at 435 nm. Deconvolution of the PL spectrum revealed two main components: one at 435 nm (68%), corresponding to a blue emission, and another at 489 nm (32%), associated with blue-green emission. The lower-energy 489 nm component suggests radiative recombination via deep-level defect detection. The  
states, such as oxygen vacancies or structural distortions, which act as trap sites for charge carriers. These results indicate that deep defects significantly contribute to the overall emission behavior of the material. As a result, the recombination process in Fe\({}_{2}\)(MoO\({}_{4}\))\({}_{3}\) is slower, as electrons and holes are first captured by these deep-level states before recombining. A schematic illustration is provided in the inset of Figure 2C, depicting the excitation of electrons (e\({}^{-}\)) to the conduction band (CB), followed by recombination through deep defect states before returning to the valence band (VB) via holes (h\({}^{+}\)). In contrast, FeWO\({}_{4}\) exhibits predominantly shallow-level defects, with deconvoluted PL components centered at 473 nm (30%), 441 nm (56%), and 421 nm (14%). According to the XRD pattern shown in Figure 1A, the FeWO\({}_{4}\) sample displays lower crystallinity, which correlates with the higher PL intensity observed at 441 nm. Typically, lower PL intensity is associated with reduced recombination of photoinduced charge carriers (e\({}^{-}\)/h\({}^{+}\) pairs), indicating more effective charge separation due to the compact interaction between distinct structural units. FeWO\({}_{4}\) consists of interconnected [WO\({}_{6}\)]-[FeO\({}_{6}\)]-[WO\({}_{6}\)] clusters, as illustrated in Figure 1A, suggesting that the [WO\({}_{6}\)] units play a key role in the PL behavior. The two high-energy (blue) emission components are attributed to the radiative recombination of self-trapped electrons localized on [WO\({}_{6}\)] clusters, while the lower-energy (blue-green) emission likely arises from shallow defect states or distorted [WO\({}_{6}\)] units. [PERSON] et al.