Similarly, a carbonate stretch vibration was also observed on HAp
Captal® R at 1420 cm-1. On the other
hand, hydroxyl (OH) peaks that correspond to OH liberation and OH
stretch were only detected on the HAp Captal® R at 631
and 3574 cm-1 respectively (Supplementary S1), however
were absent in all our test groups at different temperatures. This
observation gives an indication of the OH group substitution in the
lattice, which would be in that case substituted with F- ions, and the
coupling to F-in our study was enough to mask the OH group agreed with
the observation seen in previous studies.38,39 In
contrast, Chen et al.34 used a wet chemical method,
where they changed the pH, while keeping the temperature constant; they
found that with increasing pH, more
CO32- and OH- have
been incorporated into the lattice, on the contrary, our study did not
find similar trend as a function of temperature. On the other hand, at
the high temperature group (90ᵒC), we observed the presence of
CO32-, this can be explained by the
inclusion of CO2 in the solutions at the high
temperature. Wang et al,40 reported that the relative
intensity of CO32- vibration compared
to phosphate vibration increase with time, whilst, in our study, the
time constant was kept constant, therefore it will be useful in the to
vary the reaction time to test this hypothesis, as this part is beyond
the scope of our experiment.
S-XRD Characterization
The Miller indices (hkl) of all samples were investigated as a function
of temperature, suggesting the presence of HAp, as its main
characteristic Bragg peaks were seen as 002, 211, 112 and 300 (Figure
2a). All synchrotron XRD patterns at different temperatures;
37ᵒC, 70ᵒC and
90ᵒC demonstrate the presence of mixed phases within
the samples. As the temperature increased, the diffraction patterns
tended to show more similarities to the control group. For example, our
test groups exhibit the presence of the typical high intense HAp peaks
such as 002, 211, and 300. Those peaks indicate the presence of HAp
(purple tick marks- Figure 2a-d), which were well-fitted to our
calculated data. On the other hand, changes in the relative intensities
between 102 and 210 were observed, in comparison to the control sample.
This observed change in the relative intensities seemed to be higher at
the 37ᵒC, followed by 70ᵒC, and
90ᵒC respectively. This change may be attributed to
the presence of calcium fluoride (CaF2) (light blue tick
marks – Figure 2b-d). Furthermore, the broadening of 222 reflection of
HAp seemed to be inversely proportional with the increased temperature,
as more evident at both 37ᵒC and
70ᵒC. Those previous observations indicate the
presence of a different phase along the HAp, which is the calcium
fluoride (CaF2). This is due to the diffraction
reflection from 111 plane of CaF2, which overlaps with
the reflection of 102 plane of apatite, causing the relative peak
variation with 210 plane. As well as the reflection from 220 plane
overlaps over the 222 apatite plane leading to the broadening of the
associated peak. Therefore, the presence of CaF2 at all
temperatures was confirmed as calculated in Table 2, however, with
different ratios. Shapkin et al.41 studied the
formation of FAp from both the intermediate amorphous phase and
CaF2, as a function of time using synchrotron X-ray
diffraction. They observed similar overlapping events of apatite and
CaF2, as reported in the current study.