## 3 Results
### Phosphate Anion  
The interlayer phosphate anion is characterized by the P 2p3/1 peak with a BE of 133.0 eV. The corresponding P 2p1/2 is found at a BE value of 132.1 eV (Figure 5 left). The synthesis of the phosphate-containing LDH was performed at three different pH values (9.3, 11.9 and 12.5). No difference in the BE values was observed, but a significantly higher amount of phosphate was present in the interlayer space of the LDH in the lowest pH sample. The BE value is similar to some common phosphate minerals, such as monazite, autunite, vivianite and apatite, which have P 2p3/2 Be values around 133.2 to 134 eV [50]. On the other hand, the BE of H\({}_{3}\)PO\({}_{4}\) is slightly higher than observed here at 135.2 eV. The same is true for NaH\({}_{2}\)PO\({}_{4}\) with a BE of 134.0 to 134.2 eV [51]. [PERSON], [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] [22] reported that the P 2p was approximately 134 eV for LDH containing hexametaphosphate. At the same time, they observed a shift in the Mg 2p and Al 2p towards higher BE. They ascribed this shift in BE to the fact that the Mg\({}^{2+}\)/Al\({}^{3+}\) in the LDH hydroxide layers can extract electron density from the negatively charged oxygen atoms in hexametaphosphate [52]. [PERSON], [PERSON], [PERSON] and [PERSON] [52] found a BE shift in the Zn 2p of 0.04 eV, which is considered to be within the experimental error of XPS, while the Fe 2p showed a larger shift in BE of about 0.2 eV. In this study, no shift was observed for the Mg 2p, and there was a minor shift toward a higher BE value of 0.1 eV for the Al 2p. The O 1s is characterized by three peaks associated with the layer hydroxyl groups at 530.9 eV, interlayer water at 532.4 eV and the interlayer PO\({}_{4}\)\({}^{2-}\) at 531.6 eV (Figure 5 right). Again, the BE value for the oxygen in the phosphate anion is comparable to that found for various phosphate minerals, e.g., monazite (531.2 eV), vivianite (531.2 eV), ambygonite (531.6 eV) and apatite (531.0 eV) [50]. Similar BE values are also listed in the NIST database, with, e.g., AlPO\({}_{4}\) at 532.8 eV [51]. [PERSON], [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] [22] only reported a general position for the O 1s at about 532 eV but did not attempt to provide details fitting  
Figure 5: XPS high-resolution scan of P 2p and O 1s of LDH with phosphate as the interlayer anion. the different oxygen species present. [PERSON], [PERSON], [PERSON] and [PERSON] [52] fitted the O 1s with three bands with the O 1s OH at 530.1 eV, slightly lower than observed here. In contrast, though, the other two peaks are at very different BE values, with an extremely broad peak at 530.9 eV assigned to H\({}_{2}\)O, -O-Nand O-C, while a third peak at very low BE of 528.4 eV assigned to M-O. No explanation is given for what M is, but presumably, this is the P-O in the phosphate, as there are no M-O modes in the LDH hydroxide layer. These assignments do not seem to be correct as, generally, O 1s for PO4 are always found at much higher BE. LDHs intercalated with PO4\({}^{3-}\) have been shown to form different phases with very different basal spacings: 0.84 nm [20], 0.78 nm, 0.80 nm, 1.19 nm [17] and 1.11 nm [53], depending on the synthesis conditions such as Mg/Al ratio, pH, etc. This spread in basal spacings can be explained by different positions adopted by the PO4\({}^{3-}\) between the LDH hydroxide layers (perpendicular, inclined and planar). [PERSON], [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] [22] even indicate that the hexametaphosphate complex stays intact within the interlayer space of the LDH after exchange. Mid-infrared and Raman spectra have been reported in a limited number of papers. [PERSON] et al. [54] reported the IR-active bands in the 1050 cm\({}^{-1}\), 870 cm\({}^{-1}\) and 550 cm\({}^{-1}\) regions, which correspond to v\({}_{3}\), v\({}_{1}\), and v\({}_{4}\) vibrations [55; 56]. The PO4\({}^{3-}\) anion is tetrahedral with \(T_{d}\) symmetry; thus, there are four normal modes of vibration, all of which are Raman active.