The pH Effect on the Growth of Hexagonal and Monoclinic Hydroxyapatite Synthesized by the Hydrothermal Method

In the present work, a study of H effects on the microstructure and morphology of hydroxyapatite synthesized by the hydrothermal method is reported. The synthesis was realized with pH values of 10, 9.6, 9, 8, and 7. To know the particle size distribution, growth habit, chemical composition, and crystalline phases present, SEM-EDS, XRD, AFM, and FTIR spectroscopy techniques were used and completed with Rietveld analysis. The obtained results showed an important effect of H on the morphological and crystallographic characteristics of the hydroxyapatite, demonstrating that the shape and nanoparticle size, as well as the number of crystalline phases, can change depending on the pH during the synthesis. It was observed that there is an increase in the formation of the monoclinic phase and a decrease of the hexagonal phase when the pH value diminishes from 9.6 to 7. The crystallite size also decreases from 46.69 to 19.56 nm. An explanation of the role of pH on the final characteristics of the hydroxyapatite was related to the amount of H and OH ions, and it was included in this work.

For this reason, different chemical-processing methods to synthesized HAp have been developed, such as solid-state, mechanochemical, chemical precipitation, hydrolysis, sol-gel, hydrothermal, and emulsion. Also, there are hightemperature methods such as pyrolysis and combustion by the synthesis of biological sources [8,9]. The use of different precursors based on sources of calcium and phosphate has also generated a world of possibilities for obtaining HAp.
In past decades, works about the synthesis of HAp by hydrothermal [10] and chemical precipitation method have been reported [11], in which the dissolution stages, crystal growth, and aging time are carried out at high temperature (above the boiling point) and pressure inside of a reactor/autoclave [12]. Nowadays, the hydrothermal method synthesis presents a greater dissolution of the precursors and more homogeneous mixtures; in addition, the pressure exerted by the equipment generates structures of smaller size and minimal porosity. Also, this method has an advantage because when temperature (>350°C) and pressure (>1 atm) inside the container increases, the water behaves like an amphoteric substance. This facilitates the formation of more stable acid or base solutions.
On the other hand, the synthesis conditions must also be considered, such as the precursor's concentration, reaction temperature, reaction time, and pH, since these variables strongly influence the shape and size of the HAp (mainly in the nanometric scale). These last characteristics may dramatically increase the HAp applications, for example, as cancer cells inhibitor as well as in different biomedical and industrial applications, such as a catalyst for Michael type addition, lasers host materials, fluorescence materials, ionic conductors, and gas sensors [13][14][15][16][17].
For this reason, recent studies have focused on the synthesis of hydroxyapatite with specific size and shape, in which synthesis parameters such as the kind of precursors, concentration, reaction time, reaction temperature, and pH are closely monitored. Specifically, the initial pH (ipH) is a determinant in the final morphology of structures; if the ipH increases or decreases, it is possible to appreciate a morphology change; then, sphere, rollers, needle, wire, leaves, spherulites, belts, etc. can be formed [2,[18][19][20][21][22][23][24][25]. Besides, the size of the formed structures can also change drastically. This is because the pH is a supersaturation indicator since it influences the ions balance; by varying it, different OHconcentrations are generated, and as a consequence, the Ca 2+ , PO 4 3 , (HPO 4 ) 2ion concentrations are also modified. It should be mentioned that a high concentration of OHcan facilitate the precipitation during the synthesis of HAp [26,27].
In this sense, the majority of scientific papers report the presence of hexagonal HAp at low temperatures, without reporting the presence of monoclinic phase. The explanation provided indicates that the monoclinic HAp is mainly obtained at higher temperatures than 1000°C [28]; however, there are few papers in which it has shown that monoclinic HAp can be obtained at lower temperatures [25]. Moreover, there are minimal morphological and crystallographic differences among them, then, the identification of monoclinic HAp is very difficult. G.Ma and X. Y. Liu [29] had to use electron microscopy and electron diffraction techniques to show the presence of monoclinic HAp. Regardless of the applications of the synthesized HAp, it is believed necessary to use several tools to be sure of the presence of hexagonal, monoclinic, or both crystalline phases in the final product of the Hap synthesis. The aim of this work was to study the H + effects on the microstructure and morphology of hydroxyapatite synthesized by the hydrothermal method; emphasis was placed on determining the possible presence of hexagonal, monoclinic, or both crystalline phases. To carry out this study, SEM-EDS, XRD, AFM, and FTIR spectroscopy techniques were used. As it will be shown, it was identified the presence of monoclinic HAp; that is very interesting because the synthesis was made at low temperature. For this reason, a reasonable explanation based on the pH effects on the final hydroxyapatite characteristics by hydrothermal method was included [26][27][28][29].

Materials and Methods
In our experiments, all the reagents were analytical grade and used without further purification: calcium hydroxide (Ca(OH) 2 ) was purchased from Sigma Aldrich (98% purity) and dibasic ammonium phosphate (NH 4 ) 2 HPO 4 ) supplied by Meyer brand (98.5% purity). Nitric acid (HNO 3 , 2 M concentration) and ammonium hydroxide (NH 4 OH, 30%) were purchased from Meyer Chemical Reactive, and they were used for pH control.
2.1. Synthesis of Hydroxyapatite. The hydroxyapatite was synthesized varying the pH (7, 8, 9, 9.6, and 10), taking into account that at lower pH it has a higher amount of H + ions, while at higher pH it has higher OHions [30]; the corresponding labeled samples were HAp-7, HAp-8, HAp-9, and HAp-10, respectively, keeping the synthesis temperature and the stoichiometric concentrations according to the chemical reaction presented in the following equation.
Firstly, 40 ml of NH 4 ) 2 HPO 4 (0.09 M) and 40 ml of Ca(OH) 2 (0.15 M) solutions were prepared independently (Figure 1(a)). Subsequently, they were mixed, adding the ammonium phosphate solution onto the calcium hydroxide solution dropwise with constant stirring for 10 minutes (Figure 1(b)); the pH was 9.6 (ipH). The final solution was added to a stainless-steel autoclave reactor and kept at 200°C for 24 hours (Figure 1(c)). Then, the obtained product was allowed to slowly cool at room temperature, the solids contained within the autoclave were removed, washed three times with deionized water, and filtered by vacuum filtration (Figure 1(d)). After that, the solid was dried in a drying oven at 80°C for 12 hours (Figure 1(e)). Finally, the powders were subjected in a calcination treatment at 500°C for 3 hours to remove other impurities (Figure 1(f)). The same methodology was followed for samples obtained at different pH. The pH 10 has been achieved by adding (dropwise) 25 ml of ammonium hydroxide while the mixture of ammonium phosphate and calcium hydroxide was maintaining constant stirring ( Figure 1(b)). For the pH of 9, 8, and 7, nitric acid was added in volumes of 15 ml, 30 ml, and 45 ml, respectively.

Characterization.
The structural and elemental characterization was performed using a scanning electron microscope (SEM) JEOL, model JSM5900-LV, equipped with an X-ray energy dispersive spectrometer (EDS). For XRD analysis, a Bruker D8 Discover diffractometer was used (with a CuKα = 1:5406 Å radiation source) operating at 40 kV and 40 mA. Diffraction patterns were collected in a 2θ range from 10°to 70°with an incremental step size of 0.03°. The X-ray diffraction patterns were subjected to a Rietveld analysis using a Topas 2 software; R 3e factor and χ 2 were parameters used to indicate the accuracy of the refinement of XRD patterns. FT-IR analysis was performed in a Bruker Vertex 70 brand device with an attenuated total reflection (ATR); the analysis was in a spectral range of 4000-400 cm -1 . The 2 Journal of Nanomaterials topology analysis was carried out in an atomic force microscope (AFM) and Asylum Research model Cypher. The particle size distribution was determined using the ImageJ software in a total of 300 particles per sample.

X-Ray Diffraction (XRD)
. The X-ray diffractograms of hydroxyapatite samples obtained at different pH values (7, 8, 9, 9.6, and 10) are presented in Figure  To determine the quantity of the crystalline phase and crystallite size present in all samples, a Rietveld method was applied using the data obtained from X-ray diffractogram, for each sample. Table 1 presents the crystalline parameters calculated for each sample. It can be observed that for HAp synthesized at pH 9.6, the quantity of monoclinic and hexagonal phases is 82.64 and 17.36 weight percent (wt%), respectively. If the pH decreases, the presence of hexagonal phase has been drastically reduced and the monoclinic phase increased, as it can be observed in Table 1; an important reduction of the crystallite size is also observed. Considering that, the ideal value of goodness of fit in Rietveld refinement analyses (χ 2 = Rwp/Rexp) should be equal to 1, the values in Table 1 show that the quality of the analysis is good because the values obtained for χ 2 are between 1.02 and 1.27.
As it can also be noted in Table 1, the presence of hexagonal HAp in the sample prepared at pH 10 slightly increased with respect to the samples synthesized at pH's of 9, 8, and 7, but it is lower than the obtained sample synthesized at ipH. These results indicated an evident influence of the pH on the crystallinity and the crystallite size in HAp samples. A typical Rietveld refinement plot is given in Figure 3 to visualize the goodness of fit obtained between the measured and calculated intensities for the HAp sample synthesized at pH 7. Figure 4 shows SEM images of HAp samples synthesized at different pH and the corresponding particle size distribution. For the sample at pH 10, agglomerates between 0.15 and 2 μm in size are observed; these agglomerates are made up of nanoparticles. At pH 9.6 (ipH), agglomerates of anhedral form with dimensions between 0.13 and 1.38 μm are appreciated. For the sample at pH 9, agglomerates with a rough surface and "spongy" shape and sizes between 0.18 and 4.44 μm are observed. At pH 8, the micrograph shows agglomerates with a "spongy" surface and sizes between 0.18 and 3.83 μm. Finally, the micrograph associated with the sample obtained at pH 7 shows the presence of agglomerates with a rough surface and sizes between 0.14 and 2.80 μm. These SEM results clearly show the effect of the pH of the synthesis on the morphological characteristics of the hydroxyapatite. In order to know the Ca/P ratio, an EDS analysis was performed in 3 different areas per sample, obtaining the average weight percent. According to the results, in the HAp-7 sample, the Ca/P ratio was 1.88, while in all other samples the Ca/P ratio was 1.67.     3 Journal of Nanomaterials these particles present diameters in a range between 34 and 39 nm and lengths between 58 and 74 nm; however, nanoparticles with semispherical shape were also observed. In the sample with pH 9.6 (ipH), structure in rods form with lengths from 22 to 37 nm and diameters from 22 to 134 nm are observed.     Journal of Nanomaterials

Scanning Electron Microscopy (SEM).
Sample synthesized at pH 9 showed structures in rod form with lengths between 29 and 38 nm and diameters between 54 and 72 nm. In samples with pH 8, rod structures with lengths from 29 to 34 nm and diameters from 49 to 63 nm are distinguished. In comparison to the above, the corresponding micrograph of the Hap-7 sample, a decrease in particle size can be seen, which consists of particles with length from 17 to 21 nm and diameter from 28 to 38 nm. These results indicated that if the pH is lowering (by adding more nitric acid), the particle size decreased both diameter and length. In the discussion section, a model of growth in particle formation according to the variation in pH is graphically described.    Figure 6 shows the FTIR spectra of HAp samples, and Table 2 shows the compilation of the results obtained by this technique. In the figure, it is possible to observe the characteristic absorption bands of the hydroxyl (OH -) group and the radical PO 4 3of phosphate apatite, confirming the successful  Journal of Nanomaterials synthesis of HAp. The absorption band at 473 cm -1 corresponds to the (PO 4 ) 3-(ν 1 ) functional group. The absorption bands at 565 and 604 cm -1 correspond to the bond (ν 2 ) O-P-O and P-O (v 2 ), respectively, while the identified symmetrical stretch bands at 3432 and 3570 cm -1 correspond to the hydroxyl group (OH -). The absorption band in the 1400-1456 cm -1 region, which is observed for HAp-7, HAp-8, and HAp-9 samples, corresponds to (NO 3 ) 2stretch bands, which it was incorporated during the pH control in the synthesis. The broad bands at 3432 and 1642 cm -1 were attributable to the absorbed water, and it happens for HAp-9, HAp-8, and HAp-7 samples. It can be observed that when adding ammonium hydroxide to reach pH 10 and comparing with the pH 9.6 spectra (natural), an increase in the OHband at 637 cm -1 is observed; the same happens for the OHgroup (3570 cm -1 ). In the (PO 4 ) 3group, a slight increase in the bandwidth of the hydroxyl group corresponds to 566 cm -1 , it can be observed that pH was reduced by adding nitric acid to the natural pH, and compared to pH 9, 8, and 7 spectra, an increase in the bandwidth of the OHgroup (3573 cm -1 ) was observed, as well as a decrease in the bandwidth of the hydroxyl group (634 cm -1 ), for the (PO 4 ) 3group, an increase in the bandwidth corresponding to 565 cm -1 is observed.

Discussion
SEM analysis shows the formation of agglomerates of nanoparticles; the size of these particles decreases as the pH value also decreases. The AFM characterization led to the following asseveration: by decreasing the pH value from 9.6 to 7, a decrement of the particle size was observed. However, for pH 10, an increase of the particle size was noted. FTIR results showed that, by decreasing the pH value, the monoclinic phase has twice as many atoms concerning the hexagonal phase, taking into account that the sample at pH 9.6 has a ratio of 17.36% of the hexagonal phase and 82.64% of the monoclinic phase, likewise at pH 7 a ratio of 2.58% is obtained and 97.42%, respectively. The bandwidth and crystallite size increase in hydroxyl and phosphate groups as the pH decreases, favoring the growth of the monoclinic phase.
The obtained results showed the effect of the pH on the morphological and crystallographic characteristics of the hydroxyapatite, demonstrating that the morphology, particle size, and the crystallinity can change depending on the pH during the synthesis. Table 3 shows a summary of the most important results of some recent scientific papers that used the hydrothermal method to synthesize hydroxyapatite. They all report the presence of the hexagonal crystalline phase with rod structure; none of them reported the presence of the monoclinic phase.
As it was mentioned above, in samples at pH 10 (OHions are predominant), rod structures with lengths from 58 to 74 nm and diameter from 34 to 39 nm were observed; in lower proportion, semispherical nanoparticles were also identified. At pH 9.6, rod structures of lengths from 22 to 134 nm and diameters from 22 to 37 nm were observed. When the pH values decreased from 9 to 7 (the concentration of OHdiminishes), the nanoparticle size also decreased. This behavior could happen because of the decrease in saturation of the solution as a result of the decrease in the concentration of OH -. Also, if the distribution of phosphate-formed species at low pH is changing, an increase in the protonation, as a result of adding nitric acid, is induced. It was also found that, if the synthesis was maintained at lower pH than 9.6, the nucleation and growth of crystallites decreases, and then, the formation of the monoclinic HAp phase is favored. Levels of pH lower than 9.6 cause an alteration of the electrical charge on the surface of the particles that are interacting, modifying the distribution of the groups of hydronium and hydroxyl ions. However, the H 3 O + and OHgroups are considered to be ions that can adjust their charge, although other ions can change the latter, in phosphate-rich solutions. The binding of HPO 4 2species results in a negative electrical charge, i.e., the adsorption of PO 4 3is facilitated, as it can be seen in the mechanism described in Figure 7.
It was observed that there is an increase in the formation of the monoclinic phase and a decrease of the hexagonal phase when the pH value diminishes from 9.6 (ipH) to 7, with the addition of nitric acid; the crystallite also decreases from 46.69 to 19.56 nm. This is related to the protonation that is carried out (that is, an increase of H + occurs), and as result, the H + is favoring the formation of the monoclinic  HAp, without the need to increase at higher temperatures, as it has been reported in some papers. That is, it obtained monoclinic HAp in the nanometric scale at a lower temperature. Unlike Ma and Liu [25] who used electron microscopy and electron diffraction techniques to identify the monoclinic HAp, in this research, it used XRD and the Rietveld method to identify and quantify the monoclinic HAp. It also showed that its abundance depends on pH synthesis.
Considering that the main difference between monoclinic and hexagonal Hap consists in the orientations of the hydroxyl groups; for the monoclinic HAp, all of the OH's in a given column are pointed in the same direction, and     Journal of Nanomaterials the direction reverses in the next column; while in the hexagonal HAp, the adjacent OH's point in the opposite direction as mentioned above [25,34]. This difference induced important modifications in their respective physicochemical properties. Because the monoclinic HAp is structurally more stoichiometric than the hexagonal HAp, the kinetic dissolution and the kinetics of diffusion along the OH-column region are expected to be different. Chemical stability and dielectric properties of HAp are modified because of the structural difference [25,35].
As it was mentioned above, the physicochemical properties could be changed depending on the quantity of the monoclinic or hexagonal phase in HAp. That is critically important if the synthesized HAp is going to be used, for example, as a coating material for metallic implants. However, these results also open a range of possibilities to design HAp samples for several applications such as cell activation, carrier in drug delivery systems, CO 2 , gas sensing, catalysis, and water treatment with much more efficient results.

Conclusion
The correlation between XRD, SEM-EDS, Rietveld method, and FTIR results showed an important effect of H + on the morphological and crystallographic characteristics of the hydroxyapatite, demonstrating that the shape and nanoparticle size, as well as the number of crystalline phases, can change depending on the pH during the synthesis. It was observed that there is an increase in the formation of the monoclinic phase and a decrease of the hexagonal phase when the pH value diminishes from 9.6 to 7; that means, the higher concentration of the monoclinic phase was at pH 7. This result offers a simple methodology to obtain monoclinic hydroxyapatite at a lower temperature of synthesis and without precursor residues; besides, it promotes new different applications due to the presence of this HAp crystalline phase with control of the shape and particle size. It was concluded that a decrease of pH from 9.6 to 7, using nitric acid, contributes to the formation of H + , which favors the majority formation of the monoclinic phase. Finally, it was demonstrated that it is necessary that the correlation of several characterization techniques to distinguish between the monoclinic and hexagonal hydroxyapatite if both are present in a HAp sample.

Data Availability
All the data results in our article, we can provide them if required by the reviewers.