*3.2. XPS Analysis*

The surface chemistry of the PEEK substrate, and CaP modified PEEK (HA10, HA60, HA300, and HA600) were all examined by XPS. The XPS analysis of the PEEK substrate, as shown in Figures 3 and 4 (the wide Energy Survey Scan (WESS) and high resolution scans of the C1s and O1s regions, respectively) reveals that the uppermost surface (<10 nm) consisted of C and O only. Table 5 shows the C and O concentrations (Atomic Concentration %) measured at 88.02 ± 0.37 and 11.98 ± 0.37 respectively, and an O/C ratio of 0.14 for the PEEK substrate. This compares favourably with an O/C ratio of 0.16/1 for PEEK as highlighted in the literature [2,23–25].

**Figure 4.** High-resolution peak fitted XPS spectra for as-received PEEK. (**a**)C1*s* and (**b**)O1*<sup>s</sup>*.

Peak fitted high-resolution C 1*s* and O 1*s* spectra of PEEK are illustrated in Figure 4. The O 1*s* envelope (Figure 4a) revealed two peaks, the most intense peak attributable to the ether, O–C group, at 533.2 eV (O2) (73%) whilst the peak located at 531.1 eV (O1) was found to be indicative of the carbonyl group, C=O, (27%) [25,26]. The C 1*s* core level spectrum, as shown in Figure 4b has been peak fitted using four peaks, of which the highest intensity peak has been noted at 285.0 eV (C1), C–H and C–C bonds [27–29]. A further peak was noted at 286.6 eV (C2) which is known to correspond to the binding of C and O atoms, C–O ether bonding. A shoulder at the higher B.E of 287.5 eV (C3) is indicative of aromatic C bound to O, O=C carbonyl bonds [24,25]. The theoretical composition of the different C bonding peaks were found in literature to be approximately 68.5% (C1), ××.7% (C2) and 5.8% (C3) [24,25,28], with the relative proportion of the three types of C bonding found within study to be very similar at 67% (C1), 21% (C2) and 6% (C3), as shown in Table 5. A low intensity peak at a 291.8 eV, 5% (C4), has been attributed to a shakeup satellite, occurring due to the presence of π–π\* transitions [24,25,28]. No peaks representative of HA were observed on the PEEK substrate. Wide Energy Survey Scans and high-resolution XPS spectra of CaP as-deposited thin films onto PEEK, for (HA10) and (HA600) are presented in Figures 5–8.

**Figure 6.** High-resolution peak fitted XPS spectra for the as-deposited HA10 thin film. (**a**)C1*<sup>s</sup>*, (**b**)O1*<sup>s</sup>*, (**c**) Ca 2*p*, and (**d**)P2*p*.

**Figure 8.** High-resolution peak fitted XPS spectra for the as-deposited HA600 thin film. (**a**)C1*<sup>s</sup>*, (**b**)O1*<sup>s</sup>*, (**c**) Ca 2*p*, and (**d**)P2*p*.

Table 5 highlights the quantified result for the constituent elements of each of the high resolution O 1*s*,C1*<sup>s</sup>*, Ca 2*p*,P2*p* peaks including the Ca/P, and O/C ratios for each as-deposited surface. The C 1*s* envelope of the as-received PEEK HA10 thin film as shown in Figure 6a was observed as having three main peaks, representative of C–C/C–H, C–O, aromatic C=O, and CO3<sup>2</sup>− species at 285.0 (C1), 286.5 (C2), 287.8 (C3), and 289.5 eV (C4), respectively, with the sample having exhibited a peak at the higher B.E of 291.7 eV indicating the presence of a π–π\* shakeup. It is worth noting that 300 (HA300) and 600-min (HA600) samples did not contain the π–π\* shakeup peak. This can be observed in Figure 8a for the HA600 sample. The O 1*s* peak for the HA10 sample, illustrated in Figure 6b, exhibits two separate constituents, with the most intense observed at 531.0 eV (O1). This peak has been known to indicate O–P/O=P bonding and the less intense peak, at 532.7 eV (O2) has been attributed

to OH− species/O–C bonding. The high-resolution spectrum for a HA10 deposited thin film has revealed a Ca 2*p* envelope comprised of a well-resolved doublet with a B.E position for Ca <sup>2</sup>*p*3/2 being observed at 346.5 and Ca <sup>2</sup>*p*1/2 at 350.0 eV, as highlighted in Figure 6c. The fitted P 2*p* spectral envelope, as illustrated in Figure 6d, displayed B.E positions of 132.6 and 133.5 eV, which represents P–O (P1) and P–OH (P2) bonds, respectively.

The peak positions for the HA60, HA300, and HA600 samples were very similar to those of the HA10 thin film onto PEEK. As the deposition time increased, so too did the % AC of O 1*s*, Ca 2*p* and P 2*p*, while the concentration of each of the fine components of C 1*s* decreased, as can be seen in Table 5. The Ca/P ratio for each of the as deposited thin films were very similar being 0.83 ± 0.04, 0.84 ± 0.03, 0.85 ± 0.03, and 0.86 ± 0.04 for the 10, 60, 300, and 600 min surfaces, respectively. The O/C ratios for each of the as deposited thin films were 0.69 ± 0.02, 1.23 ± 0.05, 2.06 ± 0.08, and 4.00 ± 0.08 for each of the 10, 60, 300, and 600 min surfaces, respectively.


**Table 5.** Percentage of different of different elements and their ratios.
