*3.1. X-ray Diffraction (XRD)*

Figure 1a shows the X-ray diffraction patterns of the PEEK/Ag0.04, PEEK/Ag0.08 and PEEK/Ag0.12 samples with a coating layer. The semicrystalline structure of the PEEK polymer is revealed with the 2θ = 16◦, 23◦ and 30◦ positions, which are assigned to the (110), (220) and (211) facets, respectively [31–33]. The diffraction peaks found at 38◦, 44◦ and 64◦ corresponding to the (111), (200) and (220) crystal planes of the face-centered cubic structure of the silver (FCC), which are related with JCPDS (No. 89-3722), confirmed the successful reduction process of the Tollens reagent with D-glucose to obtain the first coating layer of the silver nanoparticles in the polyetheretherketone films [22,34,35]. Figure 1b shows the X-ray diffraction patterns of the samples coated with two layers of silver. The crystallinity of the PEEK film is preserved with the deposition of the second layer of silver on the surface of the polymer since there is no decrease in the intensity of the peaks. Similarly, the intensity of the characteristic metallic silver signals is maintained and increased with the second coating process without detriment to the metallic surface.

**Figure 1.** X-ray diffraction (XRD) patterns of polyetheretherketone (PEEK)/Ag0.04, PEEK/Ag0.08 and PEEK/Ag0.12 samples (**a**) with one coating layer and (**b**) with two coating layers.

The average sizes of the crystalline domains (L(111)) of the silver nanoparticles deposited on the PEEK polymer surface were calculated with the Debye–Scherrer formula. The resulting values are listed in Table 1.

$$L\_{(111)} = \frac{k\lambda}{\beta \cos \theta},\tag{4}$$

where *k* is the shape factor (0.9), λ is the X-ray wavelength, β is the full width at half maximum (FWHM) in radian, and θ is the Bragg angle in radians corresponding to the most intense (111) diffraction peak [36,37].

**Table 1.** Data derived from XRD and transmission electron microscopy (TEM) analyses.


FWHM: Full width at half-maximum of the XRD peak.

#### *3.2. Fourier Transform Infrared (FTIR) Spectroscopy*

The Figure 2 shows the infrared spectrum of PEEK uncoated and coated with all samples obtained. Figure 2a shows the infrared spectra of PEEK polymer substrates with 1 coating layer. The Fourier transform infrared (FTIR) spectrum of the original PEEK film shows a band at 1647 cm−<sup>1</sup> of the conjugated ketone stretch (C=O), the bands at 1587, 1480 and 1410 cm−<sup>1</sup> corresponding to stretching vibrations of the conjugated carbons on the chains' aromatics. The signal located at 1305 cm−<sup>1</sup> is characteristic of the flexion between the ketone group and the adjacent carbons. In 1275 cm−<sup>1</sup> the stretching signal of the ether group is shown. The bands between 1215 and 1100 cm−<sup>1</sup> are attributed to the deformation flexion in the plane of the C–H bond. The signal at 1008 cm−<sup>1</sup> can be attributed to the stretched vibrations of the diphenylether bonds of the p-substituents on the aromatic ring (Ar–O–Ar). From 950 to 765 cm−<sup>1</sup> corresponds to the flexure of deformation outside the plane of the C–H bond [38–40].

**Figure 2.** Fourier transform infrared (FTIR) of PEEK/Ag0.04, PEEK/Ag0.08 and PEEK/Ag0.12 samples (**a**) with one coating layer and (**b**) with two coating layers.

The main signals of the polyetheretherketone are present in all the samples obtained with a single layer, because the silver deposited in the polymer is small and does not cover the entire film. The signal in 3310 cm−<sup>1</sup> corresponds to OH groups of NaOH remanent molecules, after the synthesis process of the AgNPs. The OH groups are not water molecules, since the films formed were completely dried. Likewise, in 3060 cm−<sup>1</sup> the absorption band corresponding to the stretch of the C–H bond of the aromatic ring carbon is evidenced, which loses intensity in the whole spectrum of the coated films, this may be due to the interactions that are generated between the silver nanoparticles and the PEEK [41]. On the other hand, there are no signs of nitro groups (N–O), so it can be said that there are no residual nitrates after the process of synthesis and drying of the formed films.

The FTIR spectra of PEEK films with two layers of coating are shown in Figure 2b. The absorption bands below 3000 cm−<sup>1</sup> of the PEEK polymer are lost, and new characteristic signals of the silver nanoparticles appear which increase the intensity with the proportion of silver deposited, confirming an optimal distribution of silver on the surface of the polymer. The absorption band at 1070 cm−<sup>1</sup> is characteristic of the presence of AgNPs [42,43]. The bands at 1650 and 1400 cm−<sup>1</sup> are of the ketone bond and of the conjugated carbons in the aromatic ring, respectively, which are empty spaces of the polymer where no silver was deposited. In the IR spectrum of the PEEK/Ag0.12 sample with two layers, the characteristic signals of the organic compounds are not present, and this is only the characteristic of the interaction of the silver with the polymer, demonstrating the total coating of the surface area of the polymeric substrate [44].

#### *3.3. Transmission Electron Microscopy (TEM) Analysis*

The transmission electron microscopy images of PEEK polymers coated with two layers silver nanoparticles are shown in Figure 3. The analysis of the TEM images showed a distribution of the size of the crystals by means of the ImageJ software; this is shown in Figure 4. The results derived from the statistical analysis corroborated the average sizes of the silver crystals deposited in the polymer and evidenced by the X-ray diffraction patterns using the Debye–Scherrer equation.

**Figure 3.** TEM images of (**a**) PEEK/Ag0.04, (**b**) PEEK/Ag0.08 and (**c**) PEEK/Ag0.12. The samples were coated with two layers of AgNPs.

**Figure 4.** Distribution of the particle sizes determined from the TEM data at 50 nm of the obtained samples with 2 layers. (**a**) PEEK/Ag0.04, (**b**) PEEK/Ag0.08 and (**c**) PEEK/Ag0.12.

The comparison among particle sizes obtained by XRD and TEM are presented in Table 1. The experimental results shown that the silver nanoparticles with the polymer were coated on average by less than 30 nm. Based on previous studies [45], particle dimensions play an important role in the antibacterial activity of the coating and, therefore, this must be monitored in the final coating assembly. In the same way, the synthesis of silver nanoparticles using the Tollens reagent and an economic reducing agent, such as D-glucose, allowed control of the size and shape of the nanoparticles deposited in the PEEK [46]. In the same way, when different concentrations of AgNO3 are used, the increase in the size of the synthesized nanoparticles is related to the amount of ammonia. By increasing the concentration of silver in the different samples, the amount of ammonia required for the reaction is also increased, thus favoring the stabilization of the complex [Ag(NH3)]+. This phenomenon decreases the amount of Ag+ species, which causes the decrease of stable silver nuclei in the reduction process with glucose, and induces the formation of large particles in the growth stage [47–49].

Figure 5 shows the transmission electron microscopy images of the PEEK/Ag0.12 sample cut with ultramicrotome at different magnifications. In Figure 5a,b the obtaining of uniformly sized silver nanoparticles adhered to the PEEK polymer is corroborated by electrostatic forces generated by the high charge of the reduced silver in the synthesis process [17,50]. Likewise, the AgNPs form agglomerates of regular size, which extend throughout the PEEK substrate surface. Figure 5c shows the resistance to friction applied by the diamond tip in the cut with ultramicrotome on the polymer coated with silver nanoparticles. The cut is made in a transversal way allowing the PEEK film (thickness 6 μm) and the metallic silver adhered to the polymer by electrostatic interactions [51] to be observed.

**Figure 5.** TEM images of the sample PEEK/Ag0.12 with two layers. (**a**) 100 nm; (**b**) 200 nm; (**c**) 1 μm.

### *3.4. Thermogravimetric Analysis (TGA)*

Thermogravimetric analysis (TGA) of all samples is shown in Figure 6, in which it is clear that PEEK as a thermoplastic polymer exhibits a thermal decomposition above 500 ◦C. The amount of silver deposited is determined from the difference of pure PEEK and modified PEEK thermograms [52,53]. The temperature at which the difference was taken was 675 ◦C, since at this temperature the polymer is decomposed as shown in Figure 6 and as reported in the literature [54]. In this sense, for the polymers coated with a single layer of silver nanoparticles in concentrations of 0.04, 0.08 and 0.12 mol/L the amount of silver was 5.9%, 8.12% and 10.8% silver, respectively.

**Figure 6.** Thermogravimetric analysis (TGA) of uncoated PEEK, PEEK/Ag0.04, PEEK/Ag0.08 and PEEK/Ag0.12 samples (**a**) with one coating layer and (**b**) with two coating layers.

The percentages of silver deposited in the polymers coated with two layers of silver nanoparticles were 16.1, 18.5 and 20.99% Ag for the concentrations of 0.04, 0.08 and 0.12 mol/L, respectively. Due to the aromatic structure of the PEEK, a higher amount of residues are obtained (more than 60%) after heating at to 700 ◦C [31]. This analysis contrasts with the results of X-ray diffraction and FTIR in terms of the proportional amount of silver deposited in the PEEK, but confirm the thermal stability of materials above 450 ◦C.

#### *3.5. Morphological Evolution and Particle Distribution by Scanning Electron Microscopy (SEM)*

The deposition and dispersion of AgNPs in the PEEK was evaluated by scanning electron microscopy at different resolutions. Figure 7 shows the SEM images of different PEEK films with a silver layer on the surface, in which a proportional increase in the amount of silver deposited in the polymer is evidenced by the concentration of AgNO3 used. Considering that the silver nanoparticles are conductive, the SEM measurements were performed without sample preparation, i.e., no gold or graphite coating was applied before sample measuring. Consequently, the black areas correspond to the polymer which is not conductive, whereas the white and gray points correspond to silver particles. The homogeneity and distribution of the AgNPs deposited on the polymer surface are quite similar to other synthesis methods; this was possible because experimental conditions allowed controlling the amount of deposited particles, and thus, the formation of agglomerates that increase the density of the solids. The SEM images are congruent with the TEM images shown in Figure 5, in which it is shown that the particles with size greater than 80 nm are composed of aggregates of smaller silver nanoparticles.

**Figure 7.** Scanning electron microscopy (SEM) images of the polyetheretherketone coated with one layer of silver nanoparticles in concentrations of (**a**) 0.04 mol/L, (**b**) 0.08 mol/L and (**c**) 0.12 mol/L.

Figure 8 shows SEM images of the PEEK/Ag0.04, PEEK/Ag0.08 and PEEK/Ag0.12 systems with two silver layers. The evolution of the PEEK coating is identified by increasing in the concentration of silver nanoparticles. The empty black areas shown in Figure 7 correspond to PEEK; these are homogeneously filled by silver particles, which cover most of the polymer surface as shown in Figure 8 (when two layers were applied). In the PEEK/Ag0.08 and PEEK/Ag0.12 systems, the appearance of small agglomerations is favored, which were formed by the greater amount of silver ions available in the reaction medium. In this sense, SEM micrographs correlate with the results of XRD, FTIR and TGA, in terms of increasing the proportion of silver on the surface of the polymer, showing the efficiency of the simple chemical reduction method in the deposition of metallic nanoparticles.

**Figure 8.** SEM images of the polyetheretherketone coated with two layers of silver nanoparticles in concentrations of (**a**) 0.04 mol/L, (**b**) 0.08 mol/L and (**c**) 0.12 mol/L.

#### *3.6. Energy-Dispersive X-Ray Spectroscopy (EDS) Microanalysis*

EDS microanalysis shown in Figure 9, was performed to determine the elemental composition of the PEEK/Ag0.08 system with a single and two layers of AgNPs. The analysis was performed on an area of 600 μm and 500 μm. The analysis was performed on the PEEK/Ag0.08 sample since it was the one that presented the most intense OH absorption band. According to this analysis, it is clear that the present synthesis method provides a high level of purity of the silver nanoparticles deposited in the PEEK polymer, since a residue such as NaOH is in a proportion less than 1%. In the same way, the elemental analysis does not evidence the presence of nitrogen atoms associated with remaining nitrate ions.

The weight percentage of silver shown by the EDS spectra is proportional to the result of TGA, since it is evident that the amount of silver increases with the second layer of silver nanoparticles deposited in the polymer. The difference between TGA and EDS analysis is that in the EDS, only a small

and surface portion of the sample is analysed, while in the TGA a more quantitative portion is taken, being a more relevant result.

**Figure 9.** Energy-dispersive X-ray spectroscopy (EDS) microanalysis of the sample PEEK/Ag0.08 with a single (**a**) and with two layers (**b**).
