4. Chemical characterization

#### 4.1. Elemental chemical composition

The elemental chemical composition was determined using an energy-dispersive X-ray spectroscopy (EDS) system, integrated to the SEM equipment, which employs a Si-Li detector (Oxford Pentafet, mod. 7582). From the EDS spectra recorded for energies ranging 0.12–12 keV, the elemental quantification, as well as the Ca/P and the (Ca + O)/P ratios, has been determined and summarized in Table 4.

4.2. Functional groups detected

metric stretch but for the HPO4

the HdP bond in the HPO4

5. Optical properties

≤ k ≤ 650 cm<sup>1</sup>

the PO4

For the identification of the functional groups present in samples, the diffuse reflectance spectra were obtained using a FTIR spectrophotometer (Shimadzu, IR Affinity-1S), operating in the attenuated total reflection mode, and the spectroscopic wave number ranging 4000 cm<sup>1</sup>

The signals centered at 958 and 1018 cm<sup>1</sup> are typical of asymmetric stretching of PdO bond of

and 1460 cm<sup>1</sup> correspond to stretching of the CdO bond, proper of the inorganic carbonate group. Finally, the signals at 2121 and 1988 cm<sup>1</sup> are related to the symmetrical stretching of

quantification and the X-ray diffraction patterns, confirming the formation of calcium-deficient hydroxyapatite. In addition, the presence of carbonate signals supports the hypothesis that the

To investigate on the optical properties of the CDHA samples (particularly the optical band gap), two experimental techniques were employed here: the UV–Vis spectroscopy and the

carbon content is due to the atmospheric CO2 absorbed during the synthesis.

Figure 6. FTIR spectra of samples: (a) CDHA\_A, (b) CDHA\_B, (c) CDHA\_C, and (d) CDHA\_D.

<sup>3</sup> functional group, while the signal at 875 cm<sup>1</sup> corresponds also to the PdO asym-

. At next (Figure 6), the FTIR spectra are displayed identifying the detected signals.

<sup>2</sup> functional group. The signals in the neighborhood of 1408

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<sup>2</sup> functional group. These results agree with the EDS elemental

The results of the elemental quantification by EDS, together with the pH behavior during the synthesis, are indicative that the mass flow (controlled through the drip rate) significantly affects the reaction kinetics during the formation of hydroxyapatite, influencing the efficiency of ion exchange, promoting calcium and oxygen vacancies. The values of carbon content in the samples are consistent with the observed surface morphology and textural properties at different drip rates, suggesting the hypothesis that the presence of carbon in the samples could be due to the absorption of atmospheric CO2.


Table 4. Quantification of the elemental chemical composition of the samples.

Sol-Gel Synthesis of Calcium-Deficient Hydroxyapatite: Influence of the pH Behavior… http://dx.doi.org/10.5772/intechopen.76531 87

Figure 6. FTIR spectra of samples: (a) CDHA\_A, (b) CDHA\_B, (c) CDHA\_C, and (d) CDHA\_D.

#### 4.2. Functional groups detected

been summarized in Table 3. From their shape, the isotherms shown in Figure 5 can be classified as type V isotherm, which indicates unrestricted multilayer adsorption, characteristic of mesoporous materials. The shape of the hysteresis loops is indicative of ink-bottle-shaped

) Pore size diameter (Å) Total pore volume (cm3

∙g<sup>1</sup> )

The textural properties agrees well with the SEM observations, correcting the initial visual impression on the pore size on the surface of the samples. With the exception of the CDHA\_A sample, the pore radius size, as well as the total pore volume, decreases with increasing drip rate.

The elemental chemical composition was determined using an energy-dispersive X-ray spectroscopy (EDS) system, integrated to the SEM equipment, which employs a Si-Li detector (Oxford Pentafet, mod. 7582). From the EDS spectra recorded for energies ranging 0.12–12 keV, the elemental quantification, as well as the Ca/P and the (Ca + O)/P ratios, has been determined and

The results of the elemental quantification by EDS, together with the pH behavior during the synthesis, are indicative that the mass flow (controlled through the drip rate) significantly affects the reaction kinetics during the formation of hydroxyapatite, influencing the efficiency of ion exchange, promoting calcium and oxygen vacancies. The values of carbon content in the samples are consistent with the observed surface morphology and textural properties at different drip rates, suggesting the hypothesis that the presence of carbon in the samples could

Sample Na (at.%) C (at.%) O (at.%) P (at.%) Ca (at.%) Ca/P (Ca + O)/P CDHA\_A — 9.78 54.58 13.87 21.77 1.57 5.50 CDHA\_B — 7.40 56.91 14.56 21.13 1.45 5.36 CDHA\_C — 6.53 57.69 14.79 20.99 1.42 5.32 CDHA\_D 0.21 8.59 57.32 14.42 19.46 1.35 5.32

pores, associated to poor network connectivity effects [12].

∙g<sup>1</sup>

CDHA\_A 101.860 203.2 0.434 CDHA\_B 91.856 212.9 0.464 CDHA\_C 96.178 181.9 0.405 CDHA\_D 92.403 117.2 0.391

4. Chemical characterization

Sample Specific surface area (m<sup>2</sup>

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Table 3. Textural properties of the studied samples.

4.1. Elemental chemical composition

be due to the absorption of atmospheric CO2.

Table 4. Quantification of the elemental chemical composition of the samples.

summarized in Table 4.

For the identification of the functional groups present in samples, the diffuse reflectance spectra were obtained using a FTIR spectrophotometer (Shimadzu, IR Affinity-1S), operating in the attenuated total reflection mode, and the spectroscopic wave number ranging 4000 cm<sup>1</sup> ≤ k ≤ 650 cm<sup>1</sup> . At next (Figure 6), the FTIR spectra are displayed identifying the detected signals.

The signals centered at 958 and 1018 cm<sup>1</sup> are typical of asymmetric stretching of PdO bond of the PO4 <sup>3</sup> functional group, while the signal at 875 cm<sup>1</sup> corresponds also to the PdO asymmetric stretch but for the HPO4 <sup>2</sup> functional group. The signals in the neighborhood of 1408 and 1460 cm<sup>1</sup> correspond to stretching of the CdO bond, proper of the inorganic carbonate group. Finally, the signals at 2121 and 1988 cm<sup>1</sup> are related to the symmetrical stretching of the HdP bond in the HPO4 <sup>2</sup> functional group. These results agree with the EDS elemental quantification and the X-ray diffraction patterns, confirming the formation of calcium-deficient hydroxyapatite. In addition, the presence of carbonate signals supports the hypothesis that the carbon content is due to the atmospheric CO2 absorbed during the synthesis.

#### 5. Optical properties

To investigate on the optical properties of the CDHA samples (particularly the optical band gap), two experimental techniques were employed here: the UV–Vis spectroscopy and the photoacoustic spectroscopy (PAS) techniques. The UV–Vis technique provides the diffuse reflectance, characterized by the Kubelka-Munk function F(R), as function of the wavelength (equivalent to the photon energy) of the excitation beam. Since the F(R) function depends linearly on the ratio between the optical absorption and the scattering coefficients, an empirical estimation of the band gap, Eg, is possible by extrapolation of the linear region of the Tauc plots [13]. However, the UV–Vis technique is quite sensitive to light scattering effects, so, an over estimation of E<sup>g</sup> value is frequent. On the other hand, the PAS technique directly provides the optical absorption spectra, being less disturbed by light scattering effects than other optical spectroscopic techniques, because the PAS signal is generated only by the internal heat diffusion in the sample, as result by the optical absorption and the non-radiative thermal relaxation mechanisms [14, 15].

5.2. PAS absorption spectra

the optical band gap energies (Figure 9).

Table 5, for purposes of comparison between techniques.

(Figure 8).

To record the absorption spectra of the samples, a homemade PAS measurement system was used for such goal, for a wavelength ranging 206 nm ≤ λ ≤ 288 nm and for a modulation frequency f = 17 Hz. A schematic drawing of the experimental setup is presented as follows

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The continuous beam, emitted by the 200 W Hg Arc lamp (Newport, Mod. 66,483) optimized for UV, passes through a monochromator (Newport, mod. Cornerstone 130 1/8 m) to obtain a quasi-monochromatic excitation beam. The continuous excitation beam was then modulated by a mechanical chopper (Stanford Research Systems, mod. SR-540), impinging into the optical window of the PAS measurement cell (MTEC, mod. 300). The PAS signal (S, Δϕ) was then filtered and amplified by a lock-in amplifier (Stanford Research Systems, mod. SR 830), using the modulation frequency as reference, to be storage for its further analysis. From the absorption spectra, the Tauc plots of the samples were constructed for the empirical determination of

The optical band gap calculations, from UV–Vis and PAS measurements, are reported in

As can be seen from the above results, as the drip rate gets higher, the energy band gap also increases, with the one exception of the CDHA\_D, and as it was expected, there is an overestimation on the optical band gap calculations from UV–Vis data. Nevertheless, in both cases (and for all samples) the empirical determination of E<sup>g</sup> agrees with the reported values for hydroxyapatite from UV–Vis measurements and density functional theory (DFT) calculations [16, 17]. Using atomistic calculations, Santos and Rezende [18] conclude that the formation of the most probable defects in hydroxyapatite always involves calcium and oxygen vacancies, in agreement to the DFT calculations reported by de Leeuw et al. [19] and de Leeuw [20]. Based on the previous works in the EDS quantification, the increasing of the E<sup>g</sup> is

Figure 8. PAS measurement system. Here, S and Δϕ are the amplitude and the phase shift of the PAS signal, respectively.

#### 5.1. UV–Vis diffuse reflectance spectra

A UV–Vis spectrophotometer (Agilent, mod. Cary-100) was employed to measure the F(R) spectrum of the synthesized samples, ranging the wavelength from 200 nm ≤ λ ≤ 250 nm, correcting the obtained spectrums by measuring the background signal. In Figure 7, the Tauc plots, from the F(R) values, are exhibited considering the samples as indirect band gap materials.

Figure 7. Tauc plots of (a) CDHA\_A, (b) CDHA\_B, (c) CDHA\_C, and (d) CDHA\_D samples. The red line shows the extrapolation of the linear region.

#### 5.2. PAS absorption spectra

photoacoustic spectroscopy (PAS) techniques. The UV–Vis technique provides the diffuse reflectance, characterized by the Kubelka-Munk function F(R), as function of the wavelength (equivalent to the photon energy) of the excitation beam. Since the F(R) function depends linearly on the ratio between the optical absorption and the scattering coefficients, an empirical estimation of the band gap, Eg, is possible by extrapolation of the linear region of the Tauc plots [13]. However, the UV–Vis technique is quite sensitive to light scattering effects, so, an over estimation of E<sup>g</sup> value is frequent. On the other hand, the PAS technique directly provides the optical absorption spectra, being less disturbed by light scattering effects than other optical spectroscopic techniques, because the PAS signal is generated only by the internal heat diffusion in the sample, as result by the optical absorption and the non-radiative thermal relaxation

A UV–Vis spectrophotometer (Agilent, mod. Cary-100) was employed to measure the F(R) spectrum of the synthesized samples, ranging the wavelength from 200 nm ≤ λ ≤ 250 nm, correcting the obtained spectrums by measuring the background signal. In Figure 7, the Tauc plots, from the F(R) values, are exhibited considering the samples as indirect band gap materials.

Figure 7. Tauc plots of (a) CDHA\_A, (b) CDHA\_B, (c) CDHA\_C, and (d) CDHA\_D samples. The red line shows the

mechanisms [14, 15].

88 Powder Technology

extrapolation of the linear region.

5.1. UV–Vis diffuse reflectance spectra

To record the absorption spectra of the samples, a homemade PAS measurement system was used for such goal, for a wavelength ranging 206 nm ≤ λ ≤ 288 nm and for a modulation frequency f = 17 Hz. A schematic drawing of the experimental setup is presented as follows (Figure 8).

The continuous beam, emitted by the 200 W Hg Arc lamp (Newport, Mod. 66,483) optimized for UV, passes through a monochromator (Newport, mod. Cornerstone 130 1/8 m) to obtain a quasi-monochromatic excitation beam. The continuous excitation beam was then modulated by a mechanical chopper (Stanford Research Systems, mod. SR-540), impinging into the optical window of the PAS measurement cell (MTEC, mod. 300). The PAS signal (S, Δϕ) was then filtered and amplified by a lock-in amplifier (Stanford Research Systems, mod. SR 830), using the modulation frequency as reference, to be storage for its further analysis. From the absorption spectra, the Tauc plots of the samples were constructed for the empirical determination of the optical band gap energies (Figure 9).

The optical band gap calculations, from UV–Vis and PAS measurements, are reported in Table 5, for purposes of comparison between techniques.

As can be seen from the above results, as the drip rate gets higher, the energy band gap also increases, with the one exception of the CDHA\_D, and as it was expected, there is an overestimation on the optical band gap calculations from UV–Vis data. Nevertheless, in both cases (and for all samples) the empirical determination of E<sup>g</sup> agrees with the reported values for hydroxyapatite from UV–Vis measurements and density functional theory (DFT) calculations [16, 17]. Using atomistic calculations, Santos and Rezende [18] conclude that the formation of the most probable defects in hydroxyapatite always involves calcium and oxygen vacancies, in agreement to the DFT calculations reported by de Leeuw et al. [19] and de Leeuw [20]. Based on the previous works in the EDS quantification, the increasing of the E<sup>g</sup> is

Figure 8. PAS measurement system. Here, S and Δϕ are the amplitude and the phase shift of the PAS signal, respectively.

6. Thermal properties

TTL signal), as is shown in Figure 10.

linear region in the semi-logarithmic f

summarized in Table 6.

depend on the modulation frequency as indicated by Eq. (3):

S ¼ A<sup>0</sup> �

For the determination of the thermal response of the samples, a homemade photoacoustic detection (PA) measurement system was employed, for a modulation frequency ranging 400 Hz ≤ f ≤ 4 kHz. The experimental setup is similar to the PAS measurement system but replacing the Hg arc lamp and the monochromator by a 405 nm laser diode (controlled by a

The excitation beam wavelength was chosen to avoid the contribution of the photogenerated charge carriers to the PA signal. To perform the PA measurements in the transmission configuration [21, 22], pills of powdered samples were obtained by compacting 100 mg of powdered sample. Considering the samples as optically opaque and thermally thick, the PA signal will

In Eq. (3), A<sup>0</sup> is an instrumental constant, f<sup>c</sup> is the so-called characteristic frequency of the sample, and α<sup>s</sup> and l<sup>s</sup> are the thermal diffusivity and thickness of the sample, respectively. For modulation frequencies where Eq. (3) is applicable, α<sup>s</sup> can be calculated from the slope of the

Although the values of the thermal diffusivities agree with the reported values for hydroxyapatite [7, 23, 24], there is no clear correlation with the synthesis drip rate nor the stoichiometry of the samples. This is because the different levels of compaction of the sample's pills affect the effective thermal properties. The nonlinear behavior at low modulation frequencies is

Figure 10. PA measurement system. Here, S and Δϕ are the amplitude and the phase shift of the PA signal, respectively.

<sup>f</sup> ; f <sup>c</sup> � <sup>α</sup><sup>s</sup>

π � l 2 s

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1/2 vs. f∙S plot (Figure 11). The results obtained have been

(3)

91

exp � ffiffiffiffiffiffiffiffi f =f <sup>c</sup> � � p

Figure 9. Tauc plots of (a) CDHA\_A, (b) CDHA\_B, (c) CDHA\_C, and (d) CDHA\_D samples. The red line shows the extrapolation of the linear region.


Table 5. Optical band gap values determined from UV–Vis and PAS measurements.

explained as an effect mostly due to a higher levels of calcium and oxygen (in the OH sites) vacancies, as the drip rate increases from 5 to 10 μl∙s 1 . For a higher drip rate (i.e., 17 μl∙s 1 ), it is possible that the oxygen vacancies occur at the phosphate sites as well at the OH sites. In such case, the DFT calculations predict the existence of energy levels inside the forbidden band, which explains why the band gap energy decreases for the CDHA\_D sample.
