Effect of Surfactant in Formation of Hydroxyapatite Nano-Rods under Hydrothermal Conditions

1. Introduction

         Hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂; HA) nanoparticles have been of great interest because of their mineral components being similar to human hard tissues [1-3]. Synthetic HA has excellent biocompatibility and bioactivity, so it is used in reconstruction of damaged bone or tooth zones [4, 5]. The principal limitation in its clinical use as a load bearing implant is mechanical brittleness.

       Generally, the fracture strength and fracture toughness of ceramic materials are effectively improved by dispersing rod-like crystals and whiskers into the bulk materials. Thus, rod-like crystals and whiskers of HA sound to be useful as materials for improving the mechanical properties of synthetic biomaterials, etc. [6].

       It is well-known that the strength of ceramic fibers and whiskers is of size-dependence. As the diameter or length decreases, the strength of ceramic fibers and whiskers increases. Moreover, their physical properties such as fracture toughness and fracture strength, etc. depend on the crystal structure, composition and sizes [7].

     HA can be synthesized by many chemical-processing routes including solid-state reaction [8], co-precipitation, sol-gel synthesis, pyrolysis of aerosols, microemulsion and hydrothermal reaction [5, 9]. These methods, however, mostly prepare irregular forms of powder [5]. The most attractive method to synthesis nano-rods of HA is using surfactants [1, 10].

     In this study, HA nano-rods with uniform morphologies and controllable size have been successfully synthesized in the presence of cetyltrimethylammonium bromide (CTAB) as a cationic surfactant and polyethylene glycol as a non-ionic surfactant. Also influences of hydrothermal temperature on the morphology and size of hydroxyapatite nanoparticles have been investigated.

2. Material and methods

2.1. Materials

     The starting materials used in this study were calcium nitrate tetrahydrate (Ca(NO₃)₂.4H₂O) (Merck Prolabo 22 384.298), diammonium hydrogen phosphate ((NH₄)₂HPO₄) (Merck Prolabo 21 306.293), cetyltrimethylammonium bromide (CTAB) (102342 Merck) and polyethylene glycol (PEG400) (807485 Merck). All chemicals were analytical grade. The Ca/P molar ratio was equal to 1.67 (stoichiometric ratio of HA).

2.2. Methods

      The general procedure was the following: 0.03 mole of (NH₄)₂HPO₄ and 0.021 mole of CTAB were dissolved completely in 125 ml of deionized water. This solution was stirred with a magnetic stirrer for 30 min to ensure that the cooperative interaction and self-assembly process were completed. Then the pH value was adjusted to 4.5 by adding pure acetic acid. After that, 0.05 mole of Ca(NO₃)₂.4H₂O was dissolved in 175 ml of deionized water and proper amount of PEG400 was simultaneously added to the solution under constant stirring for 30 min. Then the mixed solution of Ca(NO₃)₂.4H₂O and PEG400 was added to the latter dropwise under continuous magnetic stirring in air. The final milky suspension was transferred to an autoclave, sealed tightly and hydrothermally treated in an oven at different temperatures of 90,120 and 150 °C for 22 h. The resultant precipitates were separated from the suspension by centrifuging, washed three times with deionized water to remove the residual CTAB and PEG and then oven-dried at 90 °C for 22 h to yield white powders.

Figure 1. XRD diffraction patterns of the HAp powders obtained at 90 ˚C (A; down), 120 ˚C (B; middel) and 150 ˚C (C; top).

2.3. Characterization

      Phase composition of powders was identified by a 3003 PTS Seifert X-ray diffractometer, using Cu-Kα with wavelength 1.5406 A. Fourier transform infrared spectroscopy (FT-IR) studies were carried out by using a Bruker IFS 48. The morphology and particle size of HA powders were investigated by a XL30 Philips SEM.

3. Results and discussion

       The XRD patterns of HA samples obtained at 90, 120 and 150 °C are shown in Figure 1. In all samples, all the diffraction peaks could be perfectly indexed to the hexagonal HA with lattice constants of a=9.418 A and c=6.884 A (JCPDS card file no. 09-432). No impurity other than HA is detected by XRD which indicates that the products prepared at different temperatures are monophase of HA. There is also a sign of directional growth in the XRD patterns. In HA standard pattern, intensity of diffracted X-ray relating to (211) plane is assumed to be 100 units and intensity of diffracted X-ray relating to (002) plane will be calculated 40 units. So that the ratio of I₍₂₁₁₎ to I₍₀₀₂₎ is equal to 2.5. But when the crystal has preferential orientation in one direction, the intensity of scattered X-ray in plane of that preferred direction will be increased in comparison with intensity of other planes. In HA powders synthesized at 90 °C I211/I002 is equal to 1.92, where for HA powders obtained at 120 and 150 °C is 2.12 and 2.16, respectively, which indicates that directional growth along (002) occurred in all samples and it is more striking in HAppowders synthesized at 90 °C as distin-guishable in SEM photographs.

       Figure 2 shows the infrared (IR) absorption spectrum of the HA samples obtained at 90, 120 and 150 °C. The FT-IR spectra of all HA samples display the same profile. In all as-prepared powders absorption peaks at 882, 1421 and 1460 cm^-1 for carbonate ions [11-13] have not been seen. The two peaks at 2852 and 2921 cm^-1 are attributed to residual CTAB [7], which shows organic surfactant has not been washed away completely and remains in obtained samples. By a simple heat treatment (1 h in 450°C) the residual organic materials will be omitted. The peak at 471.2 cm^{- 1} resulted from υ₂ phosphate mode [7, 14, 15] while broad bands at 1633 and 3204 cm^-1 correspond to adsorbed water [7] and lattice H₂O [7], respectively. The characteristic bands for PO₄^3- appeared at 565 (PO₄^3- bend υ4 [16]), 602.7 (P-O modes [7]), 961 (υ₁ symmetric P-O stretching vibration of the PO₄^3- [5, 15]), 1031 (PO₄^3-stretching mode [17]) and 1096 cm^-1(PO₄^3-bend υ₃ [16]) . The band at 1382 cm ^-1 is assigned to the N-O stretch of NO₃^-[17]. The broad bands at 3197 and 1655 cm^-1 correspond to adsorbed water [5, 16]. The two medium sharp peaks at 633 and 3565 cm^-1 attributed to the O-H bending deformation mode [7] and structural OH [16], respectively, are more distinguishable in HA sample obtained at 150 °C. For powders obtained at 90 °C, they are relatively indistinct which indicates lower crystallinity. So we can conclude that as the hydrothermal temperature increases, crystallinity of obtained powders enhances. Figure 3 illustrates the SEM photographs of HA powders obtained at 90, 120 and 150 °C. As can be seen, all samples have uniform rod-like particles with different aspect ratios and sizes. SEM observations of HA powders synthesized at 90 °C (Figurer 3a) show that the HA crystals are rod-like with straight and flat faces. Also HA particles have a typical diameter of about 40-80 nm and an average aspect ratio of about 16-18. For HA samples obtained at 120 °C (Figure 3b), the long rod-like crystals with mean particle size of about 60-90  nm  in  diameter  and  aspect  ratio  of about 12-15 are observed clearly, but they are aggregated. From the SEM micrographs of HA powders synthesized at 150 °C (Figure 3b), aspect ratio of HA particles is equal to about 4 with the crystal diameter and length of  about  80-100  nm  and  320-400  nm, respectively. It is also observed that particles have curved and round edges. These results proved that there is not too much difference in the morphology of HA samples obtained at different hydrothermal temperatures, but sizes are heterogeneous. At 90 °C HA particles are much thinner and longer and nano-rods are more distinguishable compared to HA sample obtained  at  150  °C. Also  they  are  more uniform size-distributed and separated from each other very well. In all these samples, the mixture of CTAB and PEG has acted as a growth-controlling agent in crystallization of HA.