*3.1. Filler Materials*

The composite samples obtained by reacting dopamine with an aqueous dispersion of CeO2 nanoparticles were first characterized by ATR-FTIR spectroscopy to prove the PDA formation. In Figure 1 the ATR spectrum of Sample 3 (see Section 2.2) is compared with the spectra of CeO2 and PDA. The main bands of PDA are recognized in the spectrum of the composite. Specifically, the bands centered at 1596 and 1510 cm−<sup>1</sup> can be attributed to (C=C) and (C–N) stretching modes, respectively, and confirm the presence of

aromatic amine species in the coating. The band at ca. 1600 cm<sup>−</sup>1, as well as the feature at 1723 cm−1, are assigned to C=O quinone groups. All these peaks increase in intensity as dopamine concentration increases in the reacting mixture, which indicates the increasing PDA concentration in the coatings [49–52].

**Figure 1.** ATR spectra of CeO2, PDA and PDAx@ CeO2 samples.

PDAx@CeO2 materials were further characterized by NEXAFS to get information about the PDA oxidation state. All samples show similar features in C K-edge spectra (Figure 2). The main feature appears in the π\* region, at about 286 eV, and is attributed to C1s –π\*C=O transitions [53]. A couple of features around 289 eV are indicative of the N–containing ring (C=C π\* and C–N σ\* excitations), confirming the molecular structure integrity. The large and broad feature at about 300 eV in σ\* spectral region is associated with C1s – σ\*C=O excitations. C K-edge spectra suggest an abundance of C=O functional groups in the examined samples.

**Figure 2.** C K-edge spectra measured for Sample 1 (**top**), Sample 2 (**middle**) and Sample 3 (**bottom**).

As with C K-edge spectra, O K-edge spectra (Figure 3) are similar for the three measured samples. The energies of the features in O K-edge spectra and proposed assignments

are summarized as follows: the sharp and intense peak centered at 530.8 eV is attributed to the transition of 1 s electrons of C=O groups to antibonding molecular orbitals π\*C=O, while the small feature around 534 eV is indicative for transitions of 1 s electrons of hydroxyl-like O atoms to π\*O–C and 3 s/σ\*O–H [54]. As for the σ\* region, features around 540 and 544 eV are associated with O1s C–O and C=O σ\* transitions, respectively.

**Figure 3.** O K-edge spectra measured for Sample 1 (**top**), Sample 2 (**middle**) and Sample 3 (**bottom**).

Since in NEXAFS data analysis the so-called *building block approach* can be successfully applied [55] (i.e., the NEXAFS spectrum of a complex molecule or sample can be built by summing up the contribution arising by the different functional groups, weighted for their abundance in the sample), the presence of strong features diagnostic for carbonyl groups, and only weak contributions arising by hydroxyls, suggests that the polymer is mainly in the oxidized state.

The PDA content in the PDAx@CeO2 composite samples was determined by thermogravimetric analysis. The weight loss curves for Samples 1, 2 and 3, as well as for bare CeO2, are displayed in Figure 4. While CeO2 does not present any appreciable loss, the curves of the composites show a small weight loss up to 100 ◦C, due to the water loss, and a second loss above 200 ◦C arising from PDA decomposition, which increases with increasing the dopamine concentration used for the polymerization reaction. Based on the second weight loss, the PDA content in anhydrous PDAx@CeO2 turned out to be 10.2 wt% (Sample 1), 24.7 wt% (Sample 2) and 49.5 wt% (Sample 3); these samples will be hereafter indicated as PDA10@CeO2, PDA25@CeO2 and PDA50@CeO2, respectively.

The morphology of CeO2 and the PDAx@CeO2 composites was investigated by TEM. The pictures of Figure 5 reveal that PDA can coat the cerium oxide surface forming an irregular layer without affecting the shape and the dimension of the pristine CeO2 particles, which in all cases lies around 10 nm. In particular, as the amount of PDA in the composite increases, the thickness of the coating becomes more evident reaching a thickness of some nanometers for the highest PDA content.

**Figure 4.** Weight loss curves for bare CeO2 and for PDAx@CeO2 samples.

**Figure 5.** TEM images of (**a**) PDA10@CeO2, (**b**) PDA25@CeO2 and (**c**) PDA50@CeO2.

To use the PDAx@CeO2 materials as fillers of Aquivion composite membranes, we checked that the PDA coating is not soluble in the solvent (propanol) used for membrane preparation. To this aim, 0.05 g of PDA was dispersed in 20 mL of propanol and the mixture was kept under stirring at room temperature for 2 h and then at 80 ◦C in a closed bottle for 2 h. After centrifugation, the solid was dried at 80 ◦C. The weight loss curve of the starting material (PDA13@CeO2) is coincident, within the experimental error, with the curve of the treated material (PDA13@CeO2 PrOH 80) thus indicating that PDA is not soluble under the conditions of membrane preparation (data not shown).

X-ray diffraction (XRD) patterns were collected to reveal possible structural modifications or changes in crystallinity induced by the PDA formation. Figure 6 shows that the position and the intensity of the peaks of bare CeO2 do not change in the PDA coated samples suggesting that the presence of the PDA coating does not affect the CeO2 crystal structure.

Moreover, in agreement with the TEM images, the particle size calculated using the Scherrer equation lies in the range from 9.9 to 11.2 nm.

The PDA coating of CeO2 turned out to be insoluble in propanol at 80 ◦C, which is the solvent for Aquivion 830: this allowed the PDAx@CeO2 materials to be used as fillers of Aquivion based composite membranes.

**Figure 6.** XRD patterns for pristine and coated CeO2.
