Effects of parameters of sol–gel process on the phase evolution of sol–gel-derived hydroxyapatite

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Materials Chemistry and Physics 106 (2007) 310–316

Effects of parameters of sol–gel process on the phase evolution of sol–gel-derived hydroxyapatite Hossein Eshtiagh-Hosseini ∗ , Mohammad Reza Housaindokht, Mohammad Chahkandi Department of Chemistry, Ferdowsi University, P.O. Box 91775-1436 Mashad, Iran Received 15 November 2006; received in revised form 20 May 2007; accepted 3 June 2007

Abstract It has been established that hydroxyapatite powders can be produced using an alkoxide-based sol–gel technique. Nanocrystalline powders of hydroxyapatite (HA) were prepared from Ca(NO3 )2 ·4H2 O and PO(OC2 H5 )3 as calcium and phosphorus precursors, respectively, using a sol–gel route. For a number of samples, sol of phosphorus was first hydrolyzed for 24 h with distilled water. The sol temperature, aging time and heat treatment temperature on apatite formation were systematically studied. Increasing the aging time affected the reducing of CaO. Also, increasing the mixed sol solution temperature up to 80 ◦ C had a positive effect on the disappearance of impurity phases. With the increase of the calcination temperature >600 ◦ C, calcium phosphate impurity phases disappeared. Structural evolution during the synthesis of hydroxyapatite is investigated by using infrared (IR) analysis, X-ray diffraction (XRD), thermal behavior (DTA), and elemental analysis of electron microscopy examination (SEM). X-ray diffraction with the aid of Scherrer and Williamson–Hall equations has been used to characterize the distributions of crystallite size and micro-strain of HA powders .The results indicated that mean crystallite size increased and micro-strain decreased significantly with the rise in firing temperature. © 2007 Elsevier B.V. All rights reserved. Keywords: Hydroxyapatite; Sol–gel; Aging; Structural evolution; Sol temperature

1. Introduction Tissue diseases and defects, particularly bone disease are serious health condition that directly affects the quality of the life of the suffers. Within the last four decades a revolution has occurred in the use of ceramics to improve the quality of life. This revolution is the innovative use of specially designed ceramics for the repair and reconstruction of diseased or damaged parts of the body [1]. Ceramics used for this purpose are termed bioceramics. Replacement of tissues has two alternatives: (1) transplantation and (2) implantation. The significant advantages of bioceramics as implants over transplants are availability, reproducibility, reliability; moreover, they do not pose any viral or bacterial risk to patients [2,3]. Bioceramics can be classified into two large groups: bioinert and bioactive. Bioinert ceramics have almost no influence on the surrounding living tissue, and their finest examples would be alumina and zirconia. Bioactive ceramics, by contrast, are capable of bonding with living osseous tissues;

∗

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several calcium phosphates, e.g., hydroxyapatite (HA) and certain compositions of glasses and ceramic glasses exhibit such a feature [4]. Synthetic hydroxyapatite has long been recognized as one of the most important bone substitute materials in orthopedics and dentistry over the past few decades because of its chemical and biological similarity to the mineral phase of human bones [5–7]. As a result, this inorganic phosphate has been studied extensively for medical applications in the form of powders, composites or even coatings [8–10]. Powders of HA have been synthesized in two main ways: dense and porous [2]. There are several different synthetic methods used to generate HA as reported in the literature including aqueous colloidal precipitation [11], sol–gel route [12–14], solid-state reaction [15], and the hydrothermal method [16]. Sol–gel approaches have attracted much attention recently [17–19] because of the well-known inherent advantages of the sol–gel technique that can generate glass, glass-ceramic and ceramic powders. These advantages include homogeneous molecular mixing, comparatively low synthesis temperature, high purity of product and the ability to generate nanosized particles and thin films. However, sol–gel-derived hydroxyapatite, due to poorly crystalline

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and presence of carbonate ions in the crystal lattice, has showed very high bioactivity in comparison to others [20]. However, reports on the sol–gel-derived HA have indicated that synthesis of HA is always accompanied by a secondary phase of calcium oxide (CaO) [21–23]. Since CaO is harmful to the biocompatibility of HA, attempts to overcome this problem are thus the current research interest. In the present investigation, we focus on mixed sol solution temperature, aging time and heat treatment temperature of the sol–gel system and their affect on phase purity of the synthetic hydroxyapatite. Triethyl phosphate and calcium nitrate were selected as P and Ca precursors, respectively. Insufficient aging caused the appearance of impurity phases, such as CaO or CaCO3 . The distribution of crystallite size and microstrain of HA powders were calculated by using Scherrer and Williamson–Hall equations.

Table 1 Compositions and processing variables of precursors and comparison of XRD peak height ratios, HA (2 1 1) vs. CaO (2 0 0) for appropriate calcined samples

2. Materials and methods

All peak heights are measured directly from XRD pattern using a metric ruler, considering the background subtraction.

Samples

Temperature (◦ C)

Aging

Heat treatment temperature (◦ C)

Peak height ratio HA (2 1 1)/CaO (2 0 0)

S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13

80 80 80 80 80 45–50 45–50 45–50 20–25 20–25 20–25 20–25 20–25

2h 4h 16 h 24 h 5 days 2h 4h 16 h 8h 24 h 1h 4.5 h 24 h

700 700 700 700 700 700 700 700 600 600 500 500 500

0.444 0.388 0.328 0.282 0.200 0.485 0.455 0.359 0.606 0.588 0.366 0.365 0.333

2.1. Sol–gel synthesis Four molars triethyl phosphate ((C2 H5 O)3 PO, TEP, Fluka) diluted in anhydrous ethanol was added to 3 M Ca(NO3 )2 ·4H2 O (Merck) dissolved in distilled water (Ca/P = 1.67) slowly at a rate of 6 ml min−1 . The flow chart of the synthesis is depicted in Fig. 1. Mixed sol solution continuously was agitated for 60 min at different temperatures (temp. (◦ C), Table 1). Detailed information for each sample is given in Table 1. The phosphorus precursor for samples S11–S13 was first hydrolyzed for 24 h at ambient temperature with a fixed amount of distilled water (the molar ratio of water to phosphate is fixed at 8) in a parrafilm-sealed glass container under vigorous stirring. The pH values of solutions of samples S11–S13 before and during aging were recorded. These aged sols were then

subjected to thermal treatment at 150 ◦ C until a white dried gel was obtained. The dried gels were further calcined at 500 ◦ C for 10 min or at 600 ◦ C for 30 min or 700 ◦ C for 2 h at a constant heat rate of 2 ◦ C min−1 (Table 1). In order to study the phase evolution and formation of crystalline HA, the as-dried powders were analyzed by using differential thermal analysis (DTA, Netzsch, Germany) and IR spectroscopy (Buck 500, KBr) in the range of 500–4000 cm−1 . Phase identification of the calcined gels was performed using the X-ray diffractometer (XRD, Philips, X’pert Pro, Cu K␣) at a scanning speed of 1◦ 2θ min−1 from 20◦ to 50◦ . Scanning electron microscopy (SEM, S 360, Oxford-England) was used for elemental analysis of Ca and P.

2.2. Crystallite size determination Particle size and micro-strain of the synthesized HA powders with the aid of Scherrer and Williamson–Hall equations [24] were determined. To assess the instrumental line broadening, the XRD pattern of a standard microcrystalline quartz powder was recorded.

3. Results and discussion 3.1. Characterization of the mixed sol Phosphorus alkoxides have frequently been used as the phosphorus precursors for sol–gel HA synthesis in recent years. Triethyl phosphate and triethyl phosphite are major precursors among them [25–27]. The hydrolysis activity of the triethyl phosphate is relatively poor and a higher solution temperature together with a prolonged time period (of several days) is needed to form the HA phase [21]. Hydrolysis reaction of triethyl phosphate (TEP) proceeds as follows: OP(OC2 H5 )3 + H2 O → OP(OC2 H5 )3−x (OH) x + xC2 H5 OH (1)

Fig. 1. Flow chart of hydroxyapatite preparation by the sol–gel route.

Increasing the mixed sol solution temperature and aging time will accelerate reaction (1). A continuous pH measurement of the sol solution of samples S11–S13 during aging showed a decrease of pH value, normally from pH ∼ 6 at the beginning to pH < 4.5 after 48 h aging. It is indicative of liberation of H+ upon

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H. Eshtiagh-Hosseini et al. / Materials Chemistry and Physics 106 (2007) 310–316 Table 2 Ca/P molar ratio of samples S1–S13

Fig. 2. IR spectrum collected from as-dried gel of sample S5 wavenumber range of 500–4000 cm−1 .

polymerization reaction between Ca and hydrolyzed phosphate precursors based on the partial charge model, this positively charged group, i.e. H+ , is essentially a good leaving group. After protonation of the alkoxide ligands (–OR) and removal of the charged ligand (–OR)+ , P–(OR) hydrolyzed to form P–(OH), following interaction with Ca precursor to develop the apatite structure. Since calcium nitrate is dissolved in water to form ionic derivative; the reaction may proceed as follows: OP(OEt)3−x (OH)x + Ca2+ + NO3 − → (NO3 )− (OH)–Ca– O– PO(OEt)3−x + H+ + C2 H5 OH + H2 O

(2)

According to the partial charge model, the polymerization of the phosphate will be limited to a certain degree of polymerization due to progressively weakly charged hydroxyl ligands. The polymerization seizes when the partial charge of the hydroxyl groups approaches zero or positive charge. Further heating causes the removal of the solvents, accompanied by accelerated thermal dehydration [28] or polymerization/condensation [14] between these derivatives units, resulting in the formation of more (–Ca–O–P–)-containing bonds in the dry gels. IR spectra from as-dried gel of samples (for example S5, Fig. 2) ascertain that the hydrolyzed phosphorus precursor is incomplete. The bands at 812, 1041 and 2995, and 1232 cm−1 are attributed to PO4 −3 and OEt groups, respectively [14,29]. The bands at 1370 cm−1 correspond to the CH3 (bending) in ethyl alcohol and NO3 − is related to the bands at 910, 746 and 1445 cm−1 [30]. Water in the precursor causes the absorption at 3485 and 1641 cm−1 [29]. Although the absorption band is not very significant, the hydroxyl bending band is shown at 3570 cm−1 , supporting the presence of HA in the as-dried gel precursor [31]. Therefore, the IR spectra are indicative of the presence of amorphous HA as well as calcium nitrate in the as-dried precursors. 3.2. Effect of parameters of sol–gel process on the phase purity of calcined powder The hydrolysis products of TEP form a complex with calcium ions dissolved in the solution. In terms of optical appearance of mixed sol solution, the as-prepared mixed sol solutions without aging are transparent colorless solutions, but the aged mixed

Samples

Determined Ca/P molar ratio

S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13

2.46 2.39 2.26 2.23 1.95 2.52 2.44 2.32 2.45 2.39 2.12 2.09 2.04

sol solutions are transparent pale-yellow solutions, indicating a homogeneous solution containing a dissolved complex. In the present research, the presence of calcium oxide has been indicated from XRD patterns of HA calcined powders (Fig. 3a, Table 1). The last column of Table 1 compares the effect of parameters of sol–gel process including mixed sol solution temperature, aging time and heat treatment, on the CaO amount in the samples S1–S13, semi-quantitatively estimated from the XRD peak height ratio HA (2 1 1)/CaO (2 0 0) among calcined powders. Ca/P molar ratio for samples S1–S13 obtained from SEM elemental analysis is given in Table 2. TGA analysis demonstrated by Chai and Bin-Nissan [32] indeed reveals that phosphoruscontaining precursors have high potential for volatilization [33]. Direct evidence of the phosphorus volatilization during preparation of calcium phosphate includes the presence of CaO peaks in XRD pattern and the positive derivation of the Ca/P molar ratio. From the phase diagram of the CaO–P2 O5 binary system, pure HA exists only in a limited range around a Ca/P ratio of 1.67. 3.2.1. Mixed sol solution Similar to that previously mentioned, increasing the mixed sol solution temperature of calcium and phosphate under vigorous stirring causes the acceleration of the hydrolysis reaction of the phosphorous precursor and progress of Eq. (1) towards the right side leads to the formation of amorphous Ca–P intermediate. Therefore, further completion of these reactions was found to reduce CaO content and Ca/P molar ratio of calcined samples approaches stoichiometric value 1.67 since the Ca(NO3 )2 molecule has almost been incorporated into the complex. According to Table 1, the comparison of samples S1 with S6, S2 with S7 and S3 with S8, provides supporting evidence for this. Samples S9–S13 have the same sol temperature, whilst the phosphorous precursor for samples S11–S13 was first hydrolyzed for 24 h, so their CaO content in comparison to samples S9 and S10 has reduced greatly. Also according to Table 2, by increasing the mixed sol solution temperature, Ca/P molar ratio approached stoichiometric HA of 1.67. Compare sample S1 with S6, S2 with S7 and S3 with S8.

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Fig. 3. XRD patterns of samples (a) S1–S5; (b) S9 shown: () HA; () CaO; () ␤-TCP; (♦) Ca2 P2 O7 .

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Fig. 4. DTA curve of the dried gel from sample S5.

3.2.2. Aging time The reaction of TEP with Ca(NO3 )2 ·4H2 O results in the formation of a gel, and with the progress of the reaction a viscous liquid was obtained. It can be speculated that Ca(NO3 )2 ·4H2 O probably results in the generation of alkoxy-nitrate salts that participate in a polymerization reaction with the partially hydrolyzed phosphate precursors (reaction (2)). The polymerization reaction, thereby, results in the gel. Since all the calcium nitrate is not transformed into alkoxy-nitrates, for those gels subject to short-term aging, the presence of CaO phase results primarily from incomplete reaction (2), leaving a residual nitrate. Since the hydrolysis of triethyl phosphate is much slower than alkyl phosphate [34], short-time aging, for example 2–16 h, may not be helpful in hydrolysis and the P–O–Ca linkage formation. Moreover, according to Tables 1 and 2, this time, with the decreasing of sol temperature to room temperature, increased to 24 h. However, for samples S11–S13, the decrease of CaO/HA content even from short-time aging 1 h was observed. With the increase of aging time for samples S1–S5, the impurity of CaO amount reduced (Fig. 3a). Also, according to Tables 1 and 2 the comparison of samples S6–S13 proves these ideas. 3.2.3. Heat treatment Fig. 4 shows the DTA curve of the dried gel from sample S5. A weak exothermic peak was observed at ∼400 ◦ C, which according to XRD patterns of dried gel from sample S5 was calcined in 300 and 400 ◦ C (Fig. 5a and b, respectively) is attributed to the formation of crystalline apatite. XRD patterns of the gels (Fig. 5a and b) show amorphous characteristics with peaks of Ca(NO3 )2 anhydride, which is an undesirable precipitate representing incomplete incorporation of calcium ions into the

Fig. 5. XRD patterns of sample S5 calcined in (a) 300 ◦ C and (b) 400 ◦ C shown: (䊉) Ca(NO3 )2 (H2 O)2 ; () Ca(NO3 )2 ; () HA; () CaO; (♦) ␤-TCP.

complex. Fig. 5b indicates the formation of crystalline apatite. In addition, ␤-tricalcium phosphate (␤-TCP) phase observes which is identical to that observed in some reports [35]. The phase transformation from amorphous to crystalline apatite took place at 400 ◦ C, or rather somewhere in the range of 300–400 ◦ C consistent with some communications [35]. These samples were pale-yellow chunks, and were very hygroscopic when exposed to air. Therefore, this sign ascertains existence of calcium nitrate in gels. It should be noted, however, that the reflection intensity of the major peaks in gels was improved by increasing the temperature, indicating an increase in HA content that is attributed to both the removal of the organic residuals and the improved crystallinity of the apatite. Apatite can evolve from either the transformation of the amorphous Ca–P intermediate or the reactions among Ca2 P2 O7 , Ca3 (PO4 )2 (␤-TCP), CaCO3 , and Ca(NO3 )2 , or a combination of both when the calcinations temperature reach about 600 ◦ C

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Table 3 Mean crystallite size and micro-strain from Scherrer and Williamson–Hall equations

[36]. This may be grossly represented as follows: 1 2 Ca2 P2 O7

+ Ca3 (PO4 )2 + Ca(NO3 )2 + CaCO3 + 21 H2 O

→ Ca5 (PO4 )3 (OH) + CaO+2NO2 +CO2 + 21 O2

(3)

A strong endotermic peak appearing at ∼600 ◦ C in Fig. 4 suggests reaction (3). So calcination temperature can affect the reduction of calcium phosphate impurity phases. The impurity phases such as Ca2 P2 O7 , Ca3 (PO4 )2 , originally present in lower temperature calcined gels, disappeared at 700 ◦ C, suggesting a result caused by thermally activated reactions (Fig. 3a). One possible reason for TCP formation is the acidity of the sol solution that favors more acidic calcium phosphate formation, having a lower Ca/P ratio than stoichiometry. The presence of Ca2 P2 O7 in XRD patterns (Fig. 3b, other patterns omitted) is believed to originate from the condensation between the particles of phosphorus sol, which may lead to formation of pyrophosphate: 2PO(OC2 H5 )2 OH → (C2 H5 O)2 OP–O–PO(OC2 H5 )2 + H2 O (4) The calcite within the gels may result from the reaction between air carbon dioxide and calcium ions during sol–gel synthesis. In fact, both Ca2 P2 O7 and TCP phases are not necessarily detrimental components of bioceramic, since they dissolve faster than apatite in physiological environments. According to a recent interpretation by Daculsi [37], this would be an advantage from the viewpoint of bioresorption and so bioactivity.

The breadth of the four strongest isolated peaks, which are those corresponding to the HA crystal planes (0 0 2), (1 0 2), (2 2 2) and (0 0 4), were used as an indicator of crystal dimension in the direction of the scattering vector, i.e. planes parallel to the film surface. The reflection broadening in the XRD pattern is attributed to the contributions of the crystallite size, micro-strain, and the instrument itself [38]. The crystal size D is inversely proportional to the peak breadth, according to the Scherrer formula [39]: 0.9λ D cos(θ)

Calcination temperature (◦ C)

D (nm)

Micro-strain (◦ )

500 600 700

40.9 ± 2.86 48.6 ± 3.16 98.3 ± 3.43

0.00303 0.00294 0.00241

Fig. 6. Crystal size of the gels calcined at different temperatures, calculated based on the Scherrer equation.

Stokes and Wilson [40]: Wε = 4εS tan θ

(6)

Combining Eqs. (5) and (6), a separation of the two broadening effects is possible by the so-called Williamson–Hall plot of W cos θ versus sin θ [24]: λ + 4εs sin θ D In this representation, W is assumed to be the addition of the line broadening due to micro-strains and due to particle size. Therefore, if we plot W cos θ versus sin θ, micro-strain is calculated from the slope and the particle size is obtained from the intercept. Table 3 lists the results for HA powders calcinations temperatures (500–600–700 ◦ C). With a rise in temperature, crystallite size D increased but micro-strain decreased. Fig. 6 shows the calculated D of gels as a function of calcinations temperature. Almost a two-fold increase of the crystallite size D at 700 ◦ C suggests this temperature as suitable sintering temperature. W cos θ =

3.3. Determination of crystallite size and micro-strain by XRD

(2θ) =

315

(5)

where (2θ) represents the peak width at half-maximum intensity (in radian), λ the wavelength for Cu K␣ (λ = 0.15418 nm), and D is the crystal size in nanometers. The contribution to the peak breadth from instrumental broadening was determined to be ∼0.12◦ (0.002 rad), independent of 2θ. Therefore, this amount was subtracted from the total peak width prior to applying the Scherrer formula. Instrumental broadening arising from slit width, penetration in sample, imperfect monochromaticity, and imperfect focusing is generally observed. The additional line broadening arising from micro-strain εS that causes the effects of distortion of the lattice plane and the imperfection of grain boundaries between crystallites can be expressed according to

4. Conclusions We synthesized HA through the new synthetic conditions water-based sol–gel process, such as mixed solvents ratio, rate addition and different parameters of sol–gel process. Structural evolution from sol-to-gel and from gel-to-ceramic during the synthesis of hydroxyapatite was investigated using infrared, XRD, and thermal analyses. Effects of parameters of sol–gel process including mixed sol solution temperature, aging time, and heat treatment on phase evolution to synthesize calcium phosphates was elaborately and with details studied. Impurity phases such as CaO, Ca2 P2 O7 , Ca3 (PO4 )2 , and CaCO3 evolve if the aging and heat treatment parameters are not properly controlled. The secondary phase of CaO was originated

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from the decomposition of undesirable Ca(NO3 )2 precipitate during calcining. In the aging step, calcium nitrate reacts with triethyl phosphate to form a complex which is stabilized during the subsequent drying step. From X-ray and DTA analyses of powders calcined at different temperatures, the formation temperature of HA was confirmed to be 400 ◦ C. Firing temperature did show a significant effect on crystallite size and micro-strain of powders calcined. With arise in temperature, crystallite size D increased but micro-strain decreased. The HA powders obtained after heat treatment consist of nanocrystalline HA particles in the size range of 40–100 nm. The calculation method used in this paper is an inexpensive and reliable method for crystallite size and micro-strain measurement. The apatitic structure developed was identified to be nano-structured, low-crystalline, and carbonated apatite, which closely resembles that of human bone apatite. References [1] L.L. Hench, J. Am. Ceram. Soc. 74 (7) (1991) 1487–1510. [2] L.L. Hench, J. Wilson (Eds.), Introduction to Bioceramics, World Scientific, Singapore, 1993. [3] B.D. Ranter, A.S. Hoffman, F.J. Schoen, J.E. Lemons (Eds.), Biomaterials science. An Introduction to Materials in Medicine, 1996, p. 484. [4] M. Vallet-Regi, J.M. Gonzalez-Calbet, Progress in Solid State Chemistry 32 (2004) 1–31. [5] A.S. Posner, Physiol. Rev. 49 (4) (1969) 760–792. [6] E.E. Berry, J. Inorg. Nucl. Chem. 29 (1967) 317–327. [7] J.E. Easton, The chemical composition of bone, in: C. Long (Ed.), Biochemists Handbook, Van Nostrand, Princeton, NJ, 1961, pp. 715–720. [8] W. Paul, C.P. Sharma, J. Mater. Sci. Mater. Med. 10 (1999) 383–388. [9] C.S. Chai, B. Ben-Nissan, S. Pyke, L. Evans, Mater. Manuf. Process. 10 (1995) 205–216. [10] E. Milella, F. Cosentino, A. Licciulli, C. Massaro, Biomaterials 22 (2001) 1425–1431. [11] A. Slosarczyk, E. Stobierska, Z. Paszkiewicz, M. Gawlick, J. Am. Ceram. Soc. 79 (1996) 2539–2544. [12] P. Layrolle, A. Ito, T. Tateishi, J. Am. Ceram. Soc. 81 (1998) 1421–1428. [13] G. Kordas, C.C. Trapalis, J. Sol-Gel Sci. Technol. 9 (1997) 17–24. [14] K. Hwang, Y. Lim, Surf. Coat. Technol. 115 (1999) 172–175.

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