Structural and Magnetic Phase Transformations of Hydroxyapatite

Article pubs.acs.org/JPCC

Structural and Magnetic Phase Transformations of HydroxyapatiteMagnetite Composites under Inert and Ambient Sintering Atmospheres Sunil Kumar Boda,‡ Anupama A. V.,‡ Bikramjit Basu,* and Balaram Sahoo* Materials Research Centre, Indian Institute of Science, Bangalore 560012, India S Supporting Information *

ABSTRACT: The present work reports the impact of sintering conditions on the phase stability in hydroxyapatite (HA)−magnetite (Fe3O4) bulk composites, which were densified using either pressureless sintering in air or by rapid densification via hot pressing in inert atmosphere. In particular, the phase abundances, structural and magnetic properties of the (1−x)HA-xFe3O4 (x = 5, 10, 20, and 40 wt %) composites were quantified by corroborating results obtained from Rietveld refinement of the X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Mössbauer spectroscopy. Post heat treatment phase analysis revealed a major retention of Fe3O4 in argon atmosphere, while it was partially/completely oxidized to hematite (α-Fe2O3) in air. Mössbauer results suggest the hightemperature diffusion of Fe3+ into hydroxyapatite lattice, leading to the formation of Fe-doped HA. A preferential occupancy of Fe3+ at the Ca(1) and Ca(2) sites under hot-pressing and conventional sintering conditions, respectively, was observed. The lattice expansion in HA from Rietveld analysis correlated well with the amounts of Fe-doped HA determined from the Mössbauer spectra. Furthermore, hydroxyapatite in the monoliths and composites was delineated to exist in the monoclinic (P21/b) structure as against the widely reported hexagonal (P63/m) crystal lattice. The compositional similarity of iron doping in hydroxyapatite to that of tooth enamel and bone presents HA-Fe3O4 composites as potential orthopedic and dental implant materials. nonequivalent lattice sites, four Ca2+ along the 3-fold screw axis referred to as Ca2+(1) and another six situated symmetrically about the 6-fold screw axes referred to as Ca2+(2).4 There are also two types of oxygens in the hydroxyapatite structure, namely, those comprising the phosphate tetrahedra and those bonded to the hydrogen atom (represented as OH). A single unit cell of HA consists of 44 atoms. Considering the different sites occupied by Ca and O in hydroxyapatite (as described above), the chemical formula can be rewritten as Ca(1)4Ca(2)6[PO(1)O(2)O(3)2]6(OHH)2.5 Two stable phases are reported in literature for the crystal structure of hydroxyapatite. One phase corresponds to a P63/m hexagonal symmetry, which is generally associated with nonstoichiometric HA containing F− or Cl− impurities. These impurities give rise to disorder in the arrangement of OH− ions. In the hexagonal structure of HA, the reflection of the OH− ions in mirror planes located at z = 1/4 and 3/4 leads to a superposition of oxygen atoms separated by 0.7 Å. This anomaly is corrected by the disordered distribution of hydroxyls about the plane of calcium triangle, which sets the occupancy for OH− ions equal to 0.5 for structure refinement. However, this local disorder leads to electrostatic repulsion

1. INTRODUCTION Among the different calcium apatites Ca5(PO4)3X (X = Cl, F, or OH), hydroxyapatite Ca10(PO4)6(OH)2 is notable for biological applications due to its structural and chemical resemblance to the inorganic constituent of bone and teeth. Precisely, hydroxyapatite in the tooth enamel does not correspond to the above formula due to the presence of other ions such as Mg2+, Na+, Cl−, and CO32−. Among the anionic impurities, carbonate comprises ∼3 wt % of the enamel and, hence, the tooth mineral is better-described as carbonateapatite.1 Furthermore, some literature reports also speculate that the presence of iron in traces ranging from 0 to 157 ppm prevents desorption of calcium from bone and enamel from teeth.2 Up to 1200 ppm of Fe3+ was incorporated into the structure of carbonate-substituted apatite by coprecipitation and the Mössbauer spectroscopic data (isomer shift, quadrupole splitting, and hyperfine magnetic fields) were observed to match with FeOOH, the form in which iron is present in bones and teeth in vivo.2 In the above context, the development of iron-doped hydroxyapatite presents interesting prospects as bone/dental replacement material. The crystal structure of hydroxyapatite (HA) is described as a pseudohexagonal network of phosphate (PO4) tetrahedra with Ca2+ at the interstices and the anions oriented along the columns of the c-axis.3 The bonding characteristics in hydroxyapatite indicate that calcium ions occur in two © XXXX American Chemical Society

Received: November 14, 2014 Revised: February 23, 2015

A

DOI: 10.1021/jp5114027 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 1. Heat treatment cycles used for sintering of HA-Fe3O4 ball-milled powders under (A) conventional sintering in air and (B) hot pressing in argon atmosphere at 70 MPa.

prolonged sintering.8 However, the presence of a second phase such as hydroxyapatite is believed to stabilize the γ-Fe2O3 crystals in HA composites even after calcination at 600 °C, without the formation of hematite.9 Nevertheless, the high temperature phase stability of HA and Fe3O4 in such composites remains an issue. In the present study, magnetic composites of hydroxyapatite and magnetite, (1−x)HA-xFe3O4 (x = 5, 10, 20, and 40 wt % designated as HA5Fe, HA10Fe, HA20Fe, and HA40Fe, respectively) were prepared under two different sintering conditions, namely, (i) conventional sintering at 950 °C, in air for 1 h and (ii) hot-pressing in an inert argon atmosphere with a holding time of 6 min at 950 °C, 70 MPa. The phase assemblage of the ball-milled and sintered HA-Fe 3 O 4 composites were characterized by Rietveld refinement of the X-ray diffraction data and the various magnetic phases by Mössbauer spectroscopy.

between the hydroxyl groups. The other phase corresponds to P21/b monoclinic structure, which is the low-temperature stable phase, usually adopted by synthetic or stoichiometric HA.1,3 Apatites, such as hydroxyapatite, present interesting derived structures due to their structural stability and potential for cationic and anionic isomorphous substitution. A theoretical and experimental approach was applied to study Fe2+/Fe3+ substitution into the calcium sites in hydroxyapatite by electronic calculations, electron paramagnetic resonance (EPR), and Mössbauer spectroscopy. Fe3+ substitution led to a nonstoichiometric defected HA with Fe3+ replacing Ca2+ in the 4-fold Ca(1) site, while Fe2+ substitution occurred in a stoichiometric manner due to charge balance. The Ca(2) site substitution of Fe2+ was more energetically favorable compared to Ca(1) due to the greater stability of 6-fold coordination.5 For the synthesis of iron-doped HA, the coprecipitation of iron precursors such as FeF2, FeI2, or FeCl2/FeCl3 along with Ca(NO3)2,4 sol−gel, and solid state reaction of HA with iron oxides are the possible methods. Between the two, the latter is less explored. In the former case, the sintering reaction products of the coprecipitated Fe-HA vary drastically depending on the atmospheric conditions used. Sintering in air was observed to predominantly form hematite (α-Fe2O3), while sintering in N2 atmosphere led to iron-doped hydroxyapatite confirmed by an increase in lattice parameters of HA by Rietveld analysis.6 In another instance, a mixture of HA and 5 wt % iron oxide calcined at 900 °C for 5 h resulted in the formation of iron-doped hydroxyapatite, Fe2O3, and Ca2Fe2O5 (an oxygen-deficient perovskite) as revealed by Mössbauer spectroscopy.7 The sintering temperature, atmosphere, and time significantly influence the phase transformation of Fe3O4 during the heat-treatment processes. The phase transformations between maghemite, magnetite, and hematite under oxidizing and reducing atmospheric conditions is known to occur in the reaction conditions as shown in the scheme below, O2 ,302 ° C

≫ 350 ° C

H 2 − Ar,294 ° C

Δ

2. EXPERIMENTAL SECTION 2.1. Materials and Methods. 2.1.1. Powder Processing. Hydroxyapatite (HA) was synthesized from calcium oxide and phosphoric acid via the suspension precipitation route as described elsewhere.10 In brief, 0.17 M phosphoric acid (H3PO4) was added dropwise to CaO dispersed in water at a concentration of 18.6 g/L and stirred on a magnetic hot plate at 80 °C for 3−4 h. Subsequently, concentrated ammonia (NH4OH) was added dropwise until the pH of the solution reached 10. The product was allowed to precipitate from the reaction mixture at room temperature for 24 h followed by filtration. The synthesized powders were calcined at 800 °C and ball-milled to obtain fine powder. Magnetite (Fe3O4) nanopowder with particle size 98% purity, catalog no. 637106). The above two powders were mixed to obtain various powder compositions as (1−x)HA-xFe3O4, where x = 5, 10, 20, and 40 wt % of Fe3O4 and the powder mixtures were ball-milled for 16 h in a planetary ball mill (Fritsch Pulveristee, Germany) with a ball (agate) to powder ratio of 4:1. After ball-milling in ethanol, the resultant powder slurry was dried in a hot air oven at 100 °C. The dry powders were mixed well in an agate mortar and pestle before sintering. The ball-milled samples have been designated as HA5Fe_B, HA10Fe_B, HA20Fe_B, and HA40Fe_B, and referred to similarly throughout the paper.

Fe3O4 XooooooooooooooY γ − Fe2O3 ⎯⎯⎯⎯⎯⎯⎯⎯→ α − Fe2O3

(The symbol Δ represents continuous heating of the sample above 350 °C). It is also known that magnetite (Fe3O4) undergoes oxidation to maghemite (γ-Fe2O3) upon heat treatment in air or oxygen, and further conversion to hematite (α-Fe2O3) is possible upon B


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magnetite (Fd3̅m), maghemite (P4332), and the rhombohedral crystal structures of hematite (R3̅c) were used to generate the calculated XRD patterns for the composites. Apart from the symmetry and structural considerations, a scale factor for adjusting the relative intensities of reflections for the constituent phases, parameters describing the background and peak profile and parameters simulating instrumental aberrations arising from particle size, preferred orientation of the crystallites, and particle strain-related effects were optimized for the simulated XRD pattern. The fit between the calculated and observed XRD patterns was evaluated by the Rietveld agreement factors such as R-pattern factor (Rp), R-weighted pattern factor (Rwp), χ2, and goodness of fit (GOF).12 The phase abundances of the constituent phases in the composites were calculated from the refined scale factors of the experimental data. 2.2.2. Microstructure. Transmission electron microscopy was used to determine any preferred arrangement/orientation of crystallites or grains in the hot-pressed samples. Electron transparent samples were prepared either in the form of a lamella with dimensions of 10 × 4 μm2 and 100 nm thickness using UHR dual beam Focused ion beam system (FIB, Helios NanoLab 600i, FEI)) or by Precision ion polishing (PIPS, Gatan 600A). Tecnai F30 transmission electron microscope, operated at an accelerating voltage of 300 kV was used to examine the microstructure of select samples (HA5Fe_HP and HA40Fe_HP). The preferred arrangement and orientation of the crystallites in the ultrathin specimens were inspected by high-resolution transmission electron microscopy (HRTEM). Gatan Digital micrograph software was used for the analysis of the HRTEM data. 2.2.3. Mössbauer Spectroscopy. Mössbauer spectroscopy was employed to distinguish and quantify different iron oxide phases present in the HA-Fe3O4 composites. The interconversion between the iron oxide phases (Fe3O4, magnetite; γ-Fe2O3, maghemite; and α-Fe2O3, hematite) and the diffusion of iron into the hydroxyapatite lattice were investigated using Mössbauer spectroscopy. The Mössbauer spectroscopy measurements, in the velocity range from ±11.1 mm/s, were recorded at room temperature in transmission geometry with a conventional constant-acceleration Doppler drive using a 57Co/ Rh source with a specific activity of ∼15 mCi. The detection of the 14.4 keV γ-rays was carried out with a Kr-proportional counter. The spectra were collected at room temperature in zero magnetic field, and the spectra were fit by NORMOS written by R.A. Brand.13,14 From the fit data, the abundances of different iron oxide phases were quantified based on the characteristic Mössbauer spectral parameters−isomer shift (IS), quadrupole splitting (QS), hyperfine field (Bhf), and spectral area. The data were also compared with the room temperature, zero-field spectra of pure iron oxide phases as standard reference Mössbauer spectra, including the spectra of the Fe3O4 precursor sample. All isomer shift values are given w.r.t. the 57Co/Rh matrix source. For comparison of the isomer shifts relative to α-Fe (BCC) at RT, its isomer shift of 0.106 mm/s from the source must be added. 2.2.4. Surface Characterization. In order to further confirm the phase composition, hot-pressed HA-Fe3O4 composites were investigated using X-ray photoelectron spectroscopy (XPS). The sample surfaces were etched by an argon ion beam to remove surface contamination. XPS (AXIS ULTRA) equipped with an Al Kα source (1486.6 eV) was used for acquiring the wide survey spectrum scan, over 1 × 1 mm2 surface area of each

2.1.2. Sintering/Heat Treatment Conditions. The sintering of the HA-Fe3O4 composites was performed via two heat treatment methods. (i) By conventional sintering wherein the dry ball-milled powders were compacted into green bodies in 10 mm steel die by applying a load of 25 kN. The powder compacts were kept in quartz boats and sintered in a muffle furnace (PSM Scientific Pvt Ltd., India). The heating rate was 10 °C/min, and sintering was carried out at 950 °C with a holding time of 1 h, following the heat treatment cycle shown in Figure 1A. After the heat treatment cycle, the samples were cooled down to RT inside the furnace. (ii) Hot pressing of the powders was carried out in 10 mm graphite die using an induction furnace (in house assembled) in Ar atmosphere. The sintering was carried out at T = 950 °C and P = 70 MPa with a holding time of 6 min. An infrared (IR)-based optical pyrometer was used to monitor the temperature during hot pressing. The heat treatment cycle is shown in Figure 1B. The following sample nomenclature was adopted throughout the paper for (1) conventionally sintered samples: HA5Fe_S, HA10Fe_S, HA20Fe_S, and HA40Fe_S and (2) hot pressed samples: HA_HP, HA5Fe_HP, HA10Fe_HP, HA20Fe_HP, and HA40Fe_HP. 2.2. Material Characterization. For determining the phase abundances of the iron oxides, both Rietveld refinement of XRD data and Mössbauer spectroscopy were employed. The Rietveld refinement of the powder XRD patterns was used for determining the composition of the crystalline phases and information on the lattice parameters of the constituent phases. Complementary information on iron oxide phase composition, including amorphous phases (not detected in XRD), were studied by Mössbauer spectroscopy and X-ray photoelectron spectroscopy. Fourier transform infrared (FTIR) spectroscopy was used to trace carbonate impurities and adsorbed CO2 and H2O in the samples. The presence of any order or preferred arrangement of the crystallites/grains in the hot-pressed samples was investigated by transmission electron microscopy (TEM). The magnetic behavior of the composites was also deciphered using Mössbauer spectroscopy. The carbon and hydrogen content of the samples were determined by CHNS analysis by heating the samples to 950 °C in a ThermoFinnigan FLASH EA 1112 CHNS analyzer. 2.2.1. X-ray Diffraction and FT-IR. The characterization of the phase assemblage in the sintered pellets was carried out using XRD. The data was recorded on a Bruker D8 powder diffractometer with a θ − 2θ angular geometry of the goniometer. The scans were performed using a Cu Kα (1.5418 Å) source, in the Rietveld mode at a typical scan rate of 0.15°/min with an angular step size of 0.0263° and 2θ ranging between 20−80° or 5−120°. The phases present were identified with the help of the Inorganic Crystal Structure Database (ICSD). The FT-IR spectra of the ball-milled and heat-treated samples were recorded by a Bruker Alpha FT-IR spectrophotometer, and the diffuse reflectance spectra were scanned (n = 256 scans) over a range of 400−4000 cm−1. Rietveld Analysis. The least-squares fitting of the experimentally observed XRD data with the calculated patterns of the assumed phases was performed with the help of the computer program “FullProf”.11 The least-squares refinement procedure was carried out for all the reflections in the range 2θ = 20−80° or 5−120° for each of the constituent phases. For the structure refinement, the room temperature stable structure of hydroxyapatite (HA) was considered to exist in a monoclinic (P21/b) structure. The face-centered cubic structures of C


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3.1.2. Ball-Milled Powders. According to the Rietveld refinement of the XRD data, all the ball-milled samples predominantly consist of hydroxyapatite and magnetite (Fe3O4) phases. The quantitative phase abundance for each of the constituent phases determined from the refined scale factors indicate the phase compositions of the composites to be within the experimental error associated with sample preparation. The slight alteration of the phase compositions compared to the starting powder compositions can be rationalized by the formation of amorphous phases during ball-milling. Also, mechanical milling/grinding of Fe3O4 in ambient atmosphere was reported to trigger the oxidation of Fe3O4 to γ-Fe2O3 and further to α-Fe2O3 upon prolonged milling for 35 h.16 In our study, the comparatively shorter milling time of 16 h in ambient atmosphere could explain the minor transformations between Fe3O4 and γ-Fe2O3, as confirmed from the Mössbauer data (Section 3.3.1). Figure S2A of the Supporting Information shows the Rietveld refined XRD patterns of the ball-milled HA-Fe3O4 composites. The phase abundance and lattice parameters of the phases present in the ball-milled composites are summarized in Table 2. The lattice parameters and unit cell volume of HA and Fe3O4 phases were nearly constant for x ≥ 10 wt % among the HA-xFe3O4 ball-milled composites (Figure 3A). The unit cell volume of HA5Fe_B is similar to that of hot-pressed HA. Figure S4A of the Supporting Information depicts the IR absorption bands of the calcined and ball-milled HA-Fe3O4 powders. Strong IR bands for carbonate (1400−1500 cm−1) and faint adsorbed CO2 (2347 cm−1) peaks are to be noted from Figure S4A of the Supporting Information. Further, it can be observed from Table S2 of the Supporting Information, which shows the results of CHNS analysis that the carbonate content in the ball-milled powders was greater than that of the heat-treated samples. These impurities can have a bearing on the observed lattice parameters obtained from the Rietveld data fits. 3.1.3. Air Sintered Composites. Rietveld refinement of X-ray diffraction patterns measured in the 5−120° (2θ) range for all the air-sintered composites confirmed the retention of monoclinic HA without transformation to α-/β-tricalcium phosphates (TCP). Heating monolithic HA to temperatures between 1000 and 1300 °C in air is known to induce phase transformation to oxyhydroxyapatite (OHA) and further heating above 1400 °C leads to tricalcium phosphate (αTCP) and tetracalcium phosphate (TTCP).17 Hence, the sintering was performed at 950 °C to avoid any such phase transitions in HA. Further, the presence of the secondary iron oxide phase did not trigger the dehydration of HA to TCP. However, for all the air-sintered composites, phase analysis revealed partial oxidation of magnetite (Fe3O4) to maghemite (γ-Fe2O3) and/or further transformation to hematite (αFe2O3) upon heat treatment. For HA5Fe_S composition, incomplete oxidation of Fe3O4 led to the formation of γ-Fe2O3 as the predominant iron oxide phase with minor amounts of αFe2O3. For HA10Fe_S, HA20Fe_S, and HA40Fe_S, sintering in air caused complete oxidation of magnetite to hematite with trace amounts of γ-Fe2O3. This can be confirmed from the decreasing sample density with increasing Fe3O4 content (Table S1 of the Supporting Information). The greater porosity in the HA10Fe_S, HA20Fe_S, and HA40Fe_S samples replenished oxygen for the near complete oxidation of Fe3O4 to α-Fe2O3. Figure S2B of the Supporting Information shows the Rietveld refined XRD patterns of the conventionally sintered composites. Table 3 summarizes the phase abundance

sample, with binding energy range from 0 to 1200 eV. High resolution scans of the constituent elements identified from the wide spectrum scans were carried out with pass energy of 20 eV, dwell time of 200 ms, and a step size of 0.1 eV. All the binding energies were referenced to the C 1s line of 285.0 eV. The peaks obtained in the high-resolution scans were deconvoluted and peak fitted using Fityk software. Quantitative analysis was performed by comparing the areas under the fit peaks. The relative areas under the deconvoluted peaks were used to ascertain the electronic environment and oxidation states of the constituent elements.

3. RESULTS 3.1. Phase Abundance by Rietveld Refinement. 3.1.1. Phase Pure HA. Rietveld refinement of the X-ray diffraction data in the 5−120° (2θ) range was carried out by using FullProf. The experimental data gave a better fit for the monoclinic (P21/b) phase as against the standard hexagonal (P63/m) lattice, in the calculated XRD pattern. Table 1 is a Table 1. Rietveld Refinement of Monoclinic [P21/b] Hydroxyapatite (HA)a sample

phase

assynthesized

HA

calcined

HA

hot-pressed

HA

a

lattice parameters (Å) a = 9.4259(2) b = 18.8143(7) c = 6.8858(1) γ = 120.028(5)° V = 1057.24(2) a = 9.4097(4) b = 18.8371(4) c = 6.8808(1) γ = 120.053(4)° V = 1055.06(6) a = 9.4140(4) b = 18.8302(8) c = 6.8782(1) γ = 119.976(3)° V = 1056.18(7)

Rp; Rwp; Rexp

χ2

goodness of fit (GOF)

15.2; 12.8; 8.66

2.1

1.2

15.2; 12.8; 8.24

2.4

1.3

15.5; 13.2; 8.79

2.2

1.2

The numbers in brackets indicate errors of the last digit.

summary of lattice constants for the as-synthesized, calcined, and hot-pressed hydroxyapatite, all of which comply with the monoclinic structure. The coprecipitation of impurities such as nitrate and carbonate incorporated into the structure of HA during synthesis lead to a larger lattice parameter. The removal of these impurities as gaseous products by calcination leads to the formation of stochiometric HA.15 The calcined and hotpressed HA exhibited similar unit cell volumes confirming the purity of the calcined hydroxyapatite powders. The experimental as well as calculated diffraction data along with the difference between them and the Bragg reflection positions for the synthesized, calcined and hot-pressed hydroxyapatite (HA_HP) are depicted in Figure 2A. Furthermore, the refined XRD data of the three hydroxyapatite samples have been presented as an overlay in the (2θ) range of 5−55° along with the simulated diffraction patterns for the monoclinic and hexagonal structures of hydroxyapatite (Figure S1 of the Supporting Information). The characteristic planes of HA in the experimental data along with the simulated monoclinic and hexagonal structures have been indexed in Figure S1 of the Supporting Information for comparison. D


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Figure 2. (A) Rietveld refinement of monoclinic hydroxyapatite, as synthesized, calcined, and hot-pressed (HP). The observed, calculated, and difference profiles for the XRD data of HA_HP along with Bragg reflection positions are marked for illustration. (B) Rietveld refinement of hotpressed (HP) HA-Fe3O4 composites with the insets showing the indices for the reflections of the constituent phases.

attributed to the diffusion of Fe2+/Fe3+ ions into the hydroxyapatite lattice. As reported by Morrissey et al., the unit cell volumes of iron incorporated HA, sintered in air were comparable to those sintered in inert atmosphere.6 Further FTIR spectra (Figure S4B of the Supporting Information) of the air-sintered composites exhibited intense peaks for carbonate (1400−1500 cm−1), adsorbed CO2 (2347 cm−1), and adsorbed H2O (3400−3200 cm−1) apart from the expected symmetric and antisymmetric P−O stretch in PO43− (900−1200 cm−1), P−O bending modes (500−700 cm−1), and lattice hydroxyl (3572 cm−1). However, a significant reduction in the carbon content of the air sintered composites, in comparison to the ball-milled samples was recorded using Thermo Finnigan FLASH EA 1112 CHNS analyzer (Table S2 of the Supporting

and lattice parameters of the phases obtained from Rietveld refinement of the XRD data. The lattice volume of HA in the air sintered composites (1057.35−1059.55 Å3), unaided by external pressure, was larger than that of the ball-milled compositions (1056.32−1058.83 Å3) and as synthesized HA (1057.24 Å3). The unit cell volume change was observed to increase from HA5Fe_S to HA20Fe_S, indicating a systematic increase in Fe incorporation into HA lattice. Although the lattice volume of HA in the HA40Fe_S composite was greater than as-synthesized HA, all the iron oxide was oxidized to hematite and no Fe incorporation into the Ca lattice sites of HA was observed from Mössbauer spectra (Section 3.3.2). This can be explained by the least density/maximum porosity of this composite. The unit cell expansion observed in HA can be E


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Table 2. Phase Abundance of the Constituents of HA-Fe3O4 Ball-Milled Powders Calculated from Rietveld Refinement of XRD Dataa sample HA5Fe_B

HA10Fe_B

HA20Fe_B

HA40Fe_B

a

Rp; Rwp; Rexp

χ2


Fe3O4 (4.25%) a = b = c =8.3541(5) V = 583.04(2)

16.5; 13.0; 8.8

2.1

1.5

Fe3O4 (8.80%) a = b = c = 8.3681(3) V = 585.97(1)

16.2; 12.0; 8.51

1.99

1.4

Fe3O4 (20.38%) a = b = c = 8.3685(1) V = 586.06(4)

18.6; 12.6; 10.12

1.56

1.2

Fe3O4(38.76%) a = b = c = 8.3675(1) V = 585.85(4)

28.6; 17.2; 15.71

1.2

1.1

phase abundance and lattice parameters (Å) of HA

phase abundance and lattice parameters of iron oxide

HA (95.75%) a = 9.4027(2) b = 18.8420(8) c = 6.8828(1) γ = 119.972(6)° V = 1056.33(1) HA (91.20%) a = 9.4122(2) b = 18.8619(4) c = 6.8874(1) γ = 120.014(4)° V = 1058.76(2) HA (79.62%) a = 9.4116(3) b = 18.8598(6) c = 6.8881(1) γ = 120.001(5)° V = 1058.83(2) HA (61.24%) a = 9.4117(6) b = 18.8645(9) c = 6.8867(2) γ = 120.025(9)° V = 1058.64(4)


Figure 3. (A) Changes in the unit cell volume (±0.05 Å3) and (B) % of Fe-doped HA in HA−Fe3O4 composites with variation in heat treatment conditions. The solid lines are guides to the eyes.

under inert atmosphere and short duration of the heat treatment cycle considerably reduced the transformation of Fe3O4 to γ-Fe2O3 and/or α-Fe2O3. However, heating of the samples in graphite dies establish reducing sintering atmospheres for the powder compacts, leading to the formation of the wüstite (Fe1−xO) phase in all the hot-pressed composites. The proportion of wüstite varied between 3.5−5 wt % approximately for all the samples, except for HA5Fe_HP. Figure 2B is the XRD pattern of the hot-pressed composites after Rietveld refinement of the data. The (111) reflection of wüstite and (222) reflection of Fe3O4 can be seen from the insets in Figure 2B, while HA40Fe_HP shows additional (110)

Information). The iron oxide vibrational bands are concealed within the P−O bending modes of HA. Considering that the ionic radius of Ca2+ is 114 pm, the low spin state of Fe3+ 69 pm, and the high spin state is 78.5 pm, while the low spin state of Fe2+ is 75 pm and the high spin state 92 pm, the substitution of Ca2+ by Fe2+/Fe3+ at the lattice sites of HA is surprisingly accompanied by significant changes in the unit cell volume of hydroxyapatite (Figure 3A). The rationale for this volume expansion has been presented in the discussion. 3.1.4. Hot-Pressed Composites. In case of the hot-pressed composites, a much greater retention of the magnetite along with the HA phase was observed. The sintering of the powders F


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The Journal of Physical Chemistry C Table 3. Phase Abundance of Air Sintered HA-xFe3O4 Composites Determined by Rietveld Analysisa sample HA5Fe_S

HA10Fe_S

HA20Fe_S

phase abundance and lattice parameters of HA (Å)

phase abundance and lattice parameters (Å) of iron oxide

HA (95.38%) a = 9.4175 (7) b = 18.8356(11) c = 6.8827(1) γ = 120.013(5)° V = 1057.90(5)

γ-Fe2O3 (2.79%) a = b = c = 8.3971(4) V = 593.64(25)

HA (93.3%) a = 9.4173(5) b = 18.8339(9) c = 6.8843(1) γ = 120.007(5)° V = 1057.35(8)

HA (82.76%) a = 9.4203(5) b = 18.8493(8) c = 6.8874(1) γ = 120.058(2) V = 1058.50(9)

HA40Fe_S

HA (51.0%) a = 9.4081(8) b = 18.8620(29) c = 6.8910(2) γ = 119.951(17)° V = 1059.55(9)

a

α-Fe2O3(1.83%) a = b = 5.0081(37) c = 13.9807(10) V = 303.67(3) γ-Fe2O3 (1.1%) a = b = c = 8.3806(6) V = 589.35(10) α-Fe2O3(5.4%) a = b = 5.0352(9) c = 13.7404(20) V = 301.74(9) γ-Fe2O3 (1.57%)

Rp; Rwp; Rexp

χ2


15.7; 12.4; 9.65

1.65

1.1

27.0; 17.4; 14.2

1.5

1.1

23.9; 14.5; 12.36

1.37

1.3

30.5; 25.0; 13.63

3.0

1.1

a = b = c = 8.3922(7) V = 591.06(2) α-Fe2O3(15.67%) a = b = 5.0365(1) c = 13.7460(3) V = 301.86(28) γ-Fe2O3 (0.24%) a = b = c = 8.3900(20) V = 592.70(16) α-Fe2O3(48.75%) a = b = 5.0361(1) c = 13.7465(2) V = 301.93(26)


reflection of α-Fe2O3. The FT-IR spectra of the hot-pressed composites exhibited very weak or no absorptions for carbonate/adsorbed CO2 (Figure S4C of the Supporting Information). This is probably due to the thermal decomposition of Fe3O4 to Fe1−xO (wüstite), triggered by carbonate in the apatite as depicted in the below scheme:

and volume of hydroxyapatite with variation in Fe3O4 content can be observed among the composites (Figure 3A). 3.2. TEM Analysis of Microstructure−Morphology and Crystallite Orientation. The morphological features of the hot-pressed specimens were observed under TEM in the bright field mode. Figure 4 (panels A and B) are representative bright field images of HA5Fe_HP and HA40Fe_HP, respectively. Both of them show lighter HA grains and dark iron oxide grains, with grain sizes in the range of 150−200 nm. Selected area diffraction patterns (SADPs) acquired from the darker contrast grains in the HA40Fe_HP sample were indexed using Gatan digital micrograph software. The indexed SADPs match well with Fe3O4 and γ-Fe2O3 phases in the sample (data not shown). The ordered arrangement of the crystallites and their preferred orientation was revealed by HRTEM in the HA40Fe_HP sample. Figure 4C is a nearly perfect translational Moiré fringe pattern formed by the overlap of planes from HA and Fe3O4 crystals. Figure 4D is the intensity profile of the Moiré fringe pattern. The overlapping planes giving rise to the fringe pattern were determined using the following equation for the interference of crystal planes:

Fe3O4 + CO32 − → 3FeO + 2CO2 + O2 −

Also, the data in Table S2 of the Supporting Information reveals a significant decrease in the carbon content of the hotpressed composites as compared to the ball-milled powders. However, the carbon content in the hot-pressed composites was greater than the conventionally sintered samples. This anomalous increase can be attributed to the diffusion of minor amounts of carbon from the graphite sheet and die, within which the green compacts were hot-pressed. In the HA40Fe_HP sample, a mixture of iron oxide phases resulted in the retention of magnetite (Fe3O4) as the major phase, while maghemite (γ-Fe2O3), hematite (α-Fe2O3), and wüstite (Fe1−xO) were the minor phases formed by the decomposition of magnetite. Table 4 provides a summary of the phase abundance and lattice parameters of the phases obtained from Rietveld refinement of the XRD data for all the hot-pressed composites. A systematic increase in the unit cell parameters

D = d 2*d1/(d 2 − d1) G


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The Journal of Physical Chemistry C Table 4. Phase Abundance of Hot-Pressed HA-Fe3O4 Composites Determined by Rietveld Analysisa sample HA5Fe_HP

HA10Fe_HP

HA20Fe_HP

a

χ2


FeO (0.72%) (Fm3̅m) a = b = c = 4.2964(2) V = 79.30(2)

7.36; 6.64; 2.80

5.0

2.4

FeO (3.34%) a = b = c = 4.3029(3) V = 79.67(2)

13.1; 10.5; 4.52

5.4

2.3

16.5; 13.8; 23.09

0.36

0.6

18.9; 13.2; 6.48

4.1

2.0

phase abundance and lattice parameters (Å) of iron oxide phases

HA (99.28%) a = 9.4187(2) b = 18.8370(4) c = 6.8841(1) γ = 119.998(2) V = 1057.76(4) HA (95.45%) a = 9.4180(3) b = 18.8374(6) c = 6.8811(1) γ = 119.993(3) V = 1057.30(5) HA (82.38%) a = 9.4319(2) b = 18.8457(9) c = 6.8844(1) γ = 120.027(5) V = 1059.47(4)

HA40Fe_HP

Rp; Rwp; Rexp

phase abundance and lattice parameters (Å) of HA

HA (55.02%) a = 9.4274(6) b = 18.8437(9) c = 6.8894(1) γ = 119.993(2) V = 1059.91(2)

Fe3O4 (1.21%) a = b = c = 8.4026(4) V = 593.25(2) FeO (3.48%) a = b = c = 4.2999(6) V = 79.50(4) Fe3O4 (14.14%) a = b = c = 8.4098(1) V = 594.77(5) FeO (5.26%) a = b = c = 4.2957(1) V = 79.27(5) γ-Fe2O3 (7.27%) a = b = c = 8.4525(5) V = 603.89(2) Fe3O4 (27.45%) a = b = c = 8.4117(1) V = 595.19(2) α-Fe2O3 (4.7%) a = b = 5.0348(1) c = 13.7401(5) V = 301.64(7)


magnetite (Fe3O4).18 The higher hyperfine field (Bhf) for the sextet S1, 50 T can be attributed to the γ-Fe2O3 phase while Fe3O4 exhibited sextets, S2 and S3, with hyperfine fields of 44 T and 48 T for the octahedral and tetrahedral iron species, respectively. Table 5 presents a summary of the Mössbauer subspectral parameters for the ball-milled powders. It seems that the precursor Fe3O4 was partially oxidized to γ-Fe2O3. This can also be inferred from the lattice parameters obtained from Rietveld refinement (Table 2). The hyperfine fields of the tetrahedral and octahedral iron species at room temperature is also reported to vary in a narrow range with the particle size. The spin relaxation of the smaller particles (d < 50 nm) occurs at a much faster rate compared to the larger ones (d > 100 nm).19 Also, the area ratios for the tetrahedral: octahedral site occupancy of iron in Fe3O4 was in the range from 1:2 to 1:3, indicating greater iron vacancies at the tetrahedral sites. The small quadrupole splitting values for the 57Fe nuclei in the composites indicate the retention of cubic symmetry of Fe3O4 with mild distortion, without any traces of α-Fe2O3. It was possible to fit even the Mössbauer spectrum of the precursor Fe3O4 NPs with d ≤ 50 nm (Sigma), which was used for the preparation of the composite in an identical manner to that of the ball-milled powders. Hence, the initial precursor iron oxide

where D is the thickness of the fringes formed by the overlap of crystal planes with d-spacing (Å) d1 and d2. In consideration of the lattice parameters of HA (P21/b) to be a = 9.4214 Å, b = 18.8428 Å, and c = 6.8814 Å and Fe3O4 (Fd3̅m) as a = b = c = 8.393 Å, the interfering planes of HA and Fe3O4 in the Moiré pattern were determined to be (221) and (311), respectively. Incidentally, the (221) and (311) planes of HA and Fe3O4, respectively, have the highest structure factor values and peak intensities in XRD. Such Moiré fringe patterns were abundant throughout the regions of the specimen examined. Thus, a preferred arrangement and orientation of crystallites/grains was observed in the hot-pressed samples by transmission electron microscopy. 3.3. Magnetic Phase Analysis by Mössbauer Spectroscopy. 3.3.1. Ball-Milled Powders. While assessing the magnetic structure, Mössbauer spectroscopy was used to identify the different chemical environments of iron in the HA-Fe3O4 ball-milled powders (see Figure S3A of the Supporting Information). The Mössbauer spectra were fit by three sextets with hyperfine fields (Bhf in Tesla), ranging from 43.0 to 50.5 T. The isomer shifts of 0.3 mm/s due to Fe3+ in the tetrahedral sites and 0.4−0.6 mm/s due to Fe2+/Fe3+ at the octahedral sites are characteristic of the iron occupancies in H


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Figure 4. TEM microstructure of hot-pressed (A) HA5Fe_HP and (B) HA40Fe_HP samples. The bright field images show HA (light) and iron oxide (dark) grains. (C) HRTEM of HA40Fe_HP showing Moiré fringe pattern formed by overlapping HA and Fe3O4 crystal planes. (D) Intensity profile of the marked region in (C) and deconvolution of the translational fringes into the overlapping crystal planes.

was a mixture of defect-filled Fe3O4 and γ-Fe2O3 phases. Figure S3 (panels A and B) of the Supporting Information show the Mössbauer spectra fit with subspectra for the different iron species in the ball-milled composites and pristine iron oxides, respectively. 3.3.2. Air-Sintered Composites. For the HA-Fe 3 O 4 composites sintered in air at 950 °C, the Mössbauer spectra are shown in Figure 5A. In the case of HA5Fe_S, the spectra was fit with two sextets, S1 and S2, with hyperfine fields (Bhf) of 48.5 T for hematite (α-Fe2O3) and 44.1 T for defect-filled maghemite (γ-Fe2O3) with a smaller particle size range of 5−10 nm. This is commensurate with the IS, QS of 0.25 mm/s, 0.26 mm/s for S1, and 0.20, 0.19 mm/s for S2 in HA5Fe_S. For HA10Fe_S and HA20Fe_S, identical subspectra were fit as S1 (IS = 0.26−0.37 mm/s, QS = 0.22 mm/s, and Bhf = 51.7 T) for hematite (α-Fe2O3), S2 (IS = 0.18−0.31 mm/s, QS = 0.02 mm/s, Bhf = 47 T) for iron oxide NPs with d = 10−50 nm, and S3 (IS = 0.17−0.32 mm/s, QS = 0.09 mm/s, Bhf = 40 T) for iron oxide NPs with d = 5−10 nm. The decrease in Bhf from S2 to S3 is due to the faster spin relaxation of the smaller particles saturating at lower fields. Also, the large quadrupole splittings of 0.22−0.26 mm/s for rhombohedral hematite (R3̅c) enable its distinction from cubic maghemite (P4332) in all the samples.

Table 5. Mössbauer Isomer Shift (IS), Quadrupole Splitting (QS), and Hyperfine Magnetic Field (Bhf) Parameters of the Fit Curves of Ball-Milled HA-Fe3O4 Powders sample HA5Fe_B

HA10Fe_B

HA20Fe_B

HA40Fe_B

Fe3O4 NPs

subspectrum designation

IS (mm/s) (±0.01)

QS (mm/s) (±0.01)

Bhf (T) (±0.5)

area (%) (±0.5)

S1 S2 S3 S1 S2 S3 S1 S2 S3 S1 S2 S3 S1 S2 S3

0.32 0.33 0.57 0.33 0.32 0.45 0.32 0.32 0.42 0.32 0.33 0.42 0.33 0.32 0.44

0.01 −0.03 −0.34 0.01 −0.02 0.01 0.01 −0.03 −0.09 −0.01 −0.03 −0.00 0.01 −0.03 −0.13

50.1 48.4 44.4 50.4 48.7 44.7 50.3 48.6 43.5 50.4 48.8 44.9 50.2 48.5 43.1

48.7 33.9 17.2 29.6 53.7 16.7 38.9 41.1 19.9 34.2 45.4 20.4 33.2 52.6 14.1

I


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Figure 5. Room temperature, zero field recorded Mössbauer spectra of HA-Fe3O4 composites: (A) conventionally sintered in air and (B) hotpressed in Ar atmosphere. The black dots (●) represent the measured data and the red line is the least-squares fitting to the measured data according to the model assumed. The difference between the measured data and the curve fit is shown above each spectrum.

the 5- and 6-fold coordinated Fe3+ were in the range of 0.8− 1.08 mm/s for all the samples (Table 6). This indicates that the occupancy of Fe3+ in different coordination symmetries has a greater bearing on the isomer shift than quadrupole splitting in Fe-doped HA. The high-temperature diffusion of 57Fe nuclei into HA lattice resulted in 0.3 to 0.6 wt % of the Fe-doped HA phase (Figure 3B), which was formed during the solid-state reaction between HA and Fe3O4 (5−20 wt %). In the case of HA40Fe_S, Fe3O4 was completely oxidized to α-Fe2O3 due to greater replenishment of oxygen resulting from higher porosity of the sample. Hence, it was not possible to fit doublets for Fedoped HA in the Mössbauer spectra for this sample. This explains the missing data point in Figure 3B. Further, the distorted/defect-filled iron oxide particles detected in all compositions except HA40Fe_S were designated as SPIONS. They displayed small isomer shifts of 0.1 mm/s and large quadrupole split values of 1.26 mm/s for the doublet D2 and singlet (Si in Table 6) which indicate faster magnetic spin relaxation in comparison to the nuclear transitions in the 57Fe

However, there are reports of tetragonal maghemite (P41212) arising from a mild distortion of cubic symmetry that can lead to a slightly different Mössbauer spectral parameter.20 The spectra were also fit with doublets arising from Fe3+substitution at the Ca2+ lattice sites of hydroxyapatite (HA) and doublet/ singlet for superparamagnetic iron oxide nanoparticles (SPIONS), respectively. For the HA40Fe_S sample, a sextet S1 with a high hyperfine field of 51.6 T gave a perfect fit, suggesting the near complete oxidation of Fe3O4 to α-Fe2O3. The doublet arising from the substitution of Ca2+ with Fe3+ at the Ca(2) site in different coordination/geometries were matched with the theoretical calculations made for the doping of natural hydroxyapatite (HA) with Fe.5 The isomer shift for the doublet D1 assigned for Ca2+ substitution by Fe3+, in the HA5Fe_S and HA10Fe_S samples was 0.55 mm/s, arising from a symmetric 6-fold coordination by oxygen. In the case of the HA20Fe_S composite, Fe3+ occupancy in lower 5-fold coordination symmetry increased the isomer shift to 0.8 mm/s. However, the quadrupole splitting for the doublets arising from J


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able to detect minor amounts of amorphous phases such as the Fe-doped HA and SPIONS. 3.3.3. Hot-Pressed Composites. The room temperature, zero-field Mössbauer spectra for various compositions of the hot-pressed HA-Fe3O4 composites are portrayed in Figure 5B. The Mössbauer spectral parameters and the phase abundances of the magnetic components in the hot-pressed composites are listed in Tables 8 and 9, respectively. A first glance at the

Table 6. Mössbauer Spectral Parameters for the Conventional Air Sintered HA-Fe3O4 Composites sample HA5Fe_S

HA10Fe_S

HA20Fe_S

HA40Fe_S


IS (mm/s) (±0.01)

QS (mm/s) (±0.01)

S1 S2 D1 D2 S1 S2 S3 D1 D2 S1 S2 S3 D1 Si S1

0.25 0.20 0.53 −0.10 0.26 0.18 0.17 0.56 −0.13 0.37 0.31 0.32 0.80 0.40 0.37

−0.26 0.19 0.79 1.29 −0.22 −0.01 −0.10 1.08 1.26 −0.22 −0.04 −0.09 0.80 −0.22

Bhf (T) (±0.5)

area (%) (±0.5)

48.5 44.1

27.6 55.3 10.6 6.6 32.0 17.6 27.0 8.8 14.6 64.9 11.7 10.9 5.2 7.4 100.00

51.6 47.2 41.6

51.7 46.3 40.2

51.6

Table 8. Mössbauer Spectral Parameters for the Hot-Pressed HA-Fe3O4 Composites sample HA5Fe_HP HA10Fe_HP

HA20Fe_HP

nuclei. The Mössbauer spectral parameters and the phase abundances of the magnetic components in the air sintered composites are listed in Tables 6 and 7, respectively. It may be observed that XRD analysis slightly overestimates the phase abundance of the nanocrystalline iron oxide phases, while Mössbauer spectroscopy with a superior detection limit was

HA40Fe_HP

Table 7. Phase Abundance of Iron Oxides in Air Sintered HA-Fe3O4Composites Determined from XRD and Mössbauer Spectroscopya sample HA5Fe_S

S1

Fe3+

S2

[Fe3+]5−10

D1 D2

Fe3+ Ca(2), CN:6 SPIONS

S1

Fe3+

S2

[Fe3+]10−50

S3 D1 D2

[Fe3+]5−10 Fe3+ Ca(2), CN:6 SPIONS

S1

Fe3+

S2

[Fe3+]10−50

S3 D1 Si

[Fe3+]5−10 Fe2+ Ca(2), CN:5 SPIONS

S1

Fe3+

HA10Fe_S

HA20Fe_S

HA40Fe_S

a

site occupancy of Fe


Mössbauer analysis γ-Fe2O3 (2.21%) α-Fe2O3 (1.1%) Fe doped HA (0.3%) SPIONS (0.26%) α-Fe2O3 (2.56%) γ-Fe2O3 (3.56%) Fe-doped HA (0.5%) SPIONS (2.3%) α-Fe2O3 (10.36%) γ-Fe2O3 (3.61%) Fe-doped HA (0.6%) SPIONS (1.2%) α-Fe2O3 (31.94%)


IS (mm/s) (±0.01)

QS (mm/s) (±0.01)

D1 D2 S1 S1 D1 D2 S1 S2 D1 D2 S1 S2 S3 D1 D2

1.01 0.31 0.20 0.58 0.86 0.31 0.29 0.64 0.95 0.31 0.20 0.54 0.25 1.14 0.31

1.10 0.37 −0.01 0.05 0.90 0.37 −0.02 0.01 0.84 0.37 −0.02 −0.01 −0.19 0.80 0.37

Bhf (T) (±0.5)

48.4 45.6

48.9 45.6

49.0 45.7 51.7

area (%) (±0.5) 91.1 8.9 9.2 9.4 72.7 8.6 29.0 43.5 22.6 5.0 31.4 44.4 12.3 9.0 3.0

Mössbauer spectra reveals the gradual passage from ferromagnetic to paramagnetic behavior with decreasing content of Fe3O4 in the composite. For the HA40Fe_HP, HA20Fe_HP, and HA10Fe_HP compositions, the spectra were fit with two sextets S1 and S2 with isomer shifts of ∼0.28 and ∼0.6 mm/s along with hyperfine fields of ∼48.5 and ∼45.6 T, respectively, assigned to the tetrahedral and octahedral sites of Fe3O4. The above compositions were also fit with doublets D1 (IS ∼ 1.01 mm/s, QS ∼ 0.8 mm/s) for wüstite (Fe1−xO) and D2 (IS = 0.31 mm/s, QS = 0.37 mm/s) for Ca2+ substitution by Fe3+ in a 6-fold symmetry at the Ca (1) site. The nonstoichiometric wüstite was identified from the large isomer shift values of 0.9− 1.0 mm/s and quadrupole splitting of 0.8−1.1 mm/s.21 The presence of Fe3+ in the nonstoichiometric Fe1−xO causes a distortion of the cubic FeO, leading to structural asymmetry which gives rise to a quadrupole split doublet. The isomer shift of 0.31 mm/s for Fe3+ at the Ca(2) site in hydroxyapatite matches the generally low isomer shift associated with Fe3+ as compared to the Fe2+ state. Also, a quadrupole splitting of 0.37 mm/s may be comparable to a distorted tetrahedral symmetry as a result of a four-coordinated arrangement about Fe3+. For the HA10Fe_HP composition, weak sextets appear with prominent doublets, D1 and D2. In the case of HA5Fe_HP, it was possible to fit only a pair of doublets D1 and D2, indicating the predominant paramagnetic nature of the sample. This is in contrast to the ferro/antiferromagnetic sextets fit for identical compositions of ball-milled and air-sintered composites. An additional sextet S3 was fit at a higher hyperfine field of 51.7 T for HA40Fe_HP, indicating the formation of small amount of α-Fe2O3. With respect to the stability of the magnetite, major proportions of the Fe3O4 phase (11.2 and 24.0 wt %) were retained in the HA20Fe_HP and HA40Fe_HP composites, respectively. Similar to the conven-

XRD analysis γ-Fe2O3 (2.79%) α-Fe2O3 (1.83%)

α-Fe2O3 (5.4%) γ-Fe2O3 (1.1%)

α-Fe2O3 (15.67%) γ-Fe2O3 (1.57%)

α-Fe2O3 (48.75%) γ-Fe2O3 (0.24%)

The subscript [Fe3+]x−y indicates the particle size range of iron oxides. K


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respectively, and the appearance of these peaks indicate the presence of α-Fe2O3 and γ-Fe2O3. The retention of Fe3O4 is indicated by the characteristic Fe 2p3/2 and Fe 2p1/2 peaks fit around 710.6 and 724.0 eV, respectively. In general, all the hotpressed composites were fit with Fe2+ (2p sat) and Fe3+ (2p sat) peaks. The Fe3+:Fe2+ ratios for the peak-fitted Fe 2p signals were calculated from the areas under the curves. The XPS spectra recorded from HA5Fe_HP and HA10Fe_HP composites were fit to a much higher Fe2+ content (90 at % and 70 at %, respectively), while Fe3+ (53 at % and 68 at %) surpassed Fe2+ in the HA20Fe_HP and HA40Fe_HP composites, respectively. The binding energy values for the elements in their different oxidation states have been listed in Tables 10 and 11. The calculated ratios of Fe3+:Fe2+ are in good agreement with Mössbauer spectroscopy results. Due to the incorporation of the ferric ion into the hydroxyapatite lattice and the formation of mixed valence iron oxides, the binding energy values for the fit Fe2+ and Fe3+ states are an average of the contribution from the resultant oxides. In the case of the retention of Fe3O4 in the composite, the fitting becomes simple, as reported earlier in the Fe3O4−Al2O3 granular system.22 The characteristic phase transformations of magnetite in the hydroxyapatite matrix during hot-pressing are reflected by the Fe3+:Fe2+ ratio.

Table 9. Phase Abundance of Iron Oxides in Hot-Pressed HA-Fe3O4Composites Determined from XRD and Mössbauer Spectroscopya sample


HA5Fe_HP

D1

FeO

D2

Fe3+Ca(1), CN = 6

S1 S1

[Fe3+]Th [Fe2+,Fe3+]Oh

D1

FeO

D2

Fe3+Ca(1), CN = 6

S1 S2

[Fe3+]Th [Fe2+,Fe3+]Oh

D1

FeO

D2

Fe3+Ca(1), CN = 6

S1

[Fe3+]Th

S2

[Fe2+,Fe3+]Oh

S3

Fe3+

D1

FeO

D2

Fe3+Ca(1), CN = 6

HA10Fe_HP

HA20Fe_HP

HA40Fe_HP

a

site occupancy of Fe

Mössbauer analysis

XRD analysis

FeO (3.28%) Fe doped HA (0.25%)

FeO (0.72%)

Fe3O4 (1.44%) FeO (5.2%)

Fe3O4 (1.21%) FeO (3.34%)

Fe-doped HA (0.48%) Fe3O4 (11.21%) FeO (3.25%) Fe-doped HA (0.56%) Fe3O4 (23.5%)

α-Fe2O3 (3.7%) FeO (2.60%) Fe-doped HA (0.67%)

Fe3O4 (14.14%) FeO (3.48%)

Fe3O4 (27.45%) γ-Fe2O3 (7.3%) α-Fe2O3 (4.7%) FeO (5.26%)

4. DISCUSSION In the present work, a number of material characterization tools were used to understand the phase stability/dissociation of HA and Fe3O4 phases in the sintered composites. In particular, the complementary results from the Rietveld analysis of XRD data, Mössbauer spectroscopy and X-ray photoelectron spectroscopy were used to determine the phase abundances and magnetic structure of the samples. While XRD provides information pertaining only to the crystalline phases present in the sample, it is being increasingly realized that the detection of amorphous iron oxides is equally important. This is especially in the field of catalysis, wherein amorphous Fe2O3 performs as an excellent catalyst for sonochemical synthesis, oxidation of cyclohexane derivatives, and photocatalytic splitting of H2O.23 In the present study, we have demonstrated the application of Mössbauer spectroscopy to characterize the extent of iron doping in HA and to estimate the phase abundance of crystalline and amorphous iron oxide phases with greater accuracy as compared to XRD. The major phase component of the ball-milled and sintered composites in the present study, hydroxyapatite (HA) has a chemical resemblance with the inorganic component of natural bone and has intrigued considerable interest in the study of the crystal structure of monolithic and Fe-doped HA. The thermodynamically stable phase of HA (whether hexagonal or monoclinic) at room temperature is still a matter of debate. Density functional theory (DFT) calculations showed the monoclinic phase of HA has a slightly lower energy, which is about 22 meV/cell lower than that of the hexagonal phase.24 The phase transition between the monoclinic and the hexagonal phases of HA was shown to occur at 473 K by high-temperature XRD and DSC.25 Although, in the present study, we have determined the structure of HA to be monoclinic, a closer look at the lattice parameters will reveal a mild distortion of HA from hexagonal to monoclinic crystal structure. Among the phase pure HA samples, the larger unit cell volume of synthesized HA can be rationalized by the trapping of carbonate and nitrate impurities during synthesis.

The subscript “Th” = Tetrahedral and “Oh” = Octahedral.

tionally sintered samples, 0.25 to 0.67 wt % of Fe-doped HA phase was formed, as a result of high-temperature Fe diffusion into the HA lattice. In summary, there was a systematic increase in the proportion of Fe-doped HA for the increasing amount of Fe3O4 in the composite (Figure 3B). Minor amounts of oxidized products, α-Fe2O3 and γ-Fe2O3, were detected in the HA40Fe_HP composite. The inert sintering atmosphere explains the greater retention of the magnetite phase and the formation of wüstite respectively, for all the composites. 3.4. Surface Chemical Analysis by XPS. The XPS scan of the hot-pressed composites revealed characteristic Ca 2p, P 2p, Fe 2p, O 1s, and C 1s (reference) signals at binding energy values around ∼348 and 350 eV, 133.6 eV, ∼710 and 725 eV, ∼531 and 285 eV, respectively (data not shown). The XPS peak fits corresponding to Ca 2p, O 1s, and Fe 2p are represented in Figure 6 (panels A, B, and C, respectively). Due to the formation of mixed valence iron oxide phases, it was not possible to discriminate between the Fe3+ and Fe2+ contribution from each of the phases. In general, the Ca 2p and Fe 2p signals were split into spin doublets of 2p3/2 and 2p1/2 with intensity ratios of 2:1, due to spin−orbit coupling of the core level 2p electrons. In the cases of HA5Fe_HP and HA10Fe_HP composites, large proportions of Fe2+ compared to the Fe3+ signal gave good fits for the acquired data. The Fe 2p3/2, Fe 2p1/2, and satellite peaks at 709.4, 723.0, and 715.5 eV, respectively, are characteristic of the nonstoichiometric wüstite (Fe1−xO) phase. In addition, peaks at 711.0, 724.5, and 718.7 eV correspond to Fe 2p3/2, Fe 2p1/2, and satellite peaks, L


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Figure 6. Deconvoluted and peak-fitted XPS spectra of (A) Ca 2p, (B) O 1s, and (C) Fe 2p from polished and Ar plasma etched surfaces of HAFe3O4 hot-pressed composites.

cm−1 in calcined and ball-milled powders, which is also reflected as higher carbon content as compared to the heattreated samples, in CHNS analysis. These absorptions are relatively weaker among the conventionally sintered and hotpressed composites (Figure S4C of the Supporting Information). In the case of the air-sintered samples, the carbonate was partially oxidized to CO2, while carbonate reduced magnetite to

As against an earlier study that reported the formation of stoichiometric HA after heating to 1100 °C,15 the synthesized hydroxyapatite powders in the present study were calcined at 800 °C, which resulted in partial/incomplete removal of anionic radicals as gaseous products. Figure S4 (panels A and B) of the Supporting Information shows strong infrared (IR) absorption peaks for carbonate/adsorbed CO2 around 1500 M


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Table 10. Binding Energy (eV) Values Obtained from Deconvoluted Peak-Fitted XPS Spectra of Hot-Pressed HA-Fe3O4 Composites Using C 1s (285.0 eV) as the Reference binding energy (eV) Ca 2p

O 1s

P 2p

sample

Ca 2p3/2

Ca 2p1/2

PO43−

Ads. H2O

Fe−O

O−H

HA5Fe_HP HA10Fe_HP HA20Fe_HP HA40Fe_HP

348.0 347.7 347.2 347.7

351.4 351.2 350.8 351.3

531.4 531.5 531.9 532.1

533.1 533.1 533.2 533.2

530.0 530.5 530.1 530.2

532.3 532.2 532.6 531.7

133.6 133.6 133.6 133.6

Table 11. Binding Energy (eV) Values of Deconvoluted Peak-Fitted Fe 2p XPS Peak Showing the Peak Positions of Fe3+ and Fe2+ Along with Fe3+:Fe2+ Ratio in Hot-Pressed HA-Fe3O4 Composites Binding Energy (eV) of Fe 2p Fe

3+

Fe2+

Fe3+:Fe2+

sample

2p3/2

2p1/2

2p sat

2p3/2

2p1/2

2p sat

HA5Fe_HP HA10Fe_HP HA20Fe_HP HA40Fe_HP

711.0 711.0 710.7 710.5

724.5 724.8 723.7 723.7

717.0 718.6 719.5 718.7

710.0 709.5 709.4 709.4

723.2 723.0 722.8 722.9

715.5 715.5 715.6 715.7

1:8.6 1:2.4 1.2:1 2.1:1

panels A and B) formed, as calculated from the corresponding Mössbauer doublets. The charge difference between Fe3+ and Ca2+ leads to the formation of nonstoichiometric Fe-doped HA. However, the ionic radii difference (0.64 Å for Fe3+ and 0.99 Å for Ca2+) presents ambiguity over the lattice expansion of HA due to Ca2+substitution by Fe3+. Additional Fe3+ incorporation into the lattice interstices of HA, introducing nonstoichiometry, might explain the lattice parameter changes of HA lattice. In a previous work, Jiang et al.5 experimentally determined and calculated the mean interatomic distances of Ca−O using GSA (generalized simulated annealing) simulations. The simulations were also performed for Fe doping at different Ca sites and in various coordination geometries. The mean Fe−O interatomic distances were calculated. In the case of relaxed structures for Fe3+ substitution at the Ca(1) and Ca(2) sites in HA, the mean interatomic distances for Fe−O were less than that of Ca−O. For the unrelaxed structures with Fe-occupying Ca sites of higher coordination (CN: 5, 6) geometry in HA, the interatomic distances for Fe−O > Ca−O. These unrelaxed or metastable structures could arise from the initial diffusion of Fe2+ into the Ca sites of HA followed by oxidation to Fe3+. In Table 12, we present a few selected data from the paper by Jiang et al.5 Corroborating the Fe occupancies at Ca(1) and Ca(2) sites for Fe-doped HA with the data given in Table 12, the observed increase in lattice parameters can be justified. The ion-exchange properties of hydroxyapatite have attracted considerable attention in the field of orthopedic biomaterials.

wüstite in an inert atmosphere. Further, an increase in the carbon content from calcined HA to hot-pressed HA indicates that around 0.1 atom % of carbon from the graphite sheet and die diffused into the powders during hot-pressing of the composites. Thus, the higher carbon content in the hot-pressed composites in comparison to air-sintered samples can be rationalized. Also, the air-sintered composites show IR absorptions for adsorbed H2O between 3400 and 3200 cm−1, which is commensurate with their higher hydrogen content in comparison to the hot-pressed samples, as determined from CHNS analysis. The variations in the adsorbed/trapped chemical species can explain the differences in the observed lattice parameters of phase pure HA. Rietveld refinement of the XRD data for the sintered and hot-pressed composites also indicates an increase in unit cell constants/lattice volume of HA due to iron diffusion into the lattice. The paramagnetic doublets in Mössbauer spectroscopy provide complementary information to the observations in XRD along with quantifying the percentage of Fe-doped HA. The ferric ion incorporated into the apatite lattice has been presented as a percentage of Fedoped HA in Figure 3B. The diffusion of iron into the apatite crystal during sintering would most certainly have temperaturedependence. However, a much greater control over iron incorporation into mineral apatites can be achieved by wet chemical or sol−gel methods wherein a stoichiometric amount of iron salt can be added before precipitation of the product. In this case too, there is added risk of the formation of FeOOH and the like as side-reaction products. In line with the above hypothesis, an earlier study on the synthesis of ferric-ion-doped HA by wet chemical precipitation reported that Fe3+ is attached to apatite in the form of FeOOH.2 However, in the present study, no traces of FeOOH were detected either by XRD or Mössbauer spectroscopy. Another possibility of the antiferromagnetic Ca2Fe2O5 phase formation as a result of sintering was ruled out due to the mismatch of the Mössbauer spectral parameters of the composites to those of Ca2Fe2O5 reported earlier.26 An increase in the unit cell volume/lattice parameters observed from the Rietveld analysis presented a good correlation with the percentage of Fe-doped HA (Figure 3,

Table 12. Comparison of Ca−O and Fe−O Mean Interatomic Distances for Fe Occupancy at the Ca(1) and Ca(2) Sites in HA

N

Ca−O bonds in HA

bond length (Å)

Ca(1)−O(1) Ca(1)−O(2)

2.405 2.438

Ca(2)−O(2) Ca(2)−O(3) Ca(2)-OH

2.357 2.378 2.387

Fe3+ at Ca sites in HA Fe3+ Ca(1) CN = 6, unrelaxed Fe3+ Ca(2) CN = 6 CN = 5, unrelaxed

mean interatomic distance (Å) 2.43 (±0.022)

2.42 (±0.078)


Article


Table 13. Comparison of the Results Obtained in the Present Study with Earlier Reports on Iron-Doped Hydroxyapatite sample composition

synthesis/heat treatment

iron uptake into carbonate apatite HA doped with 5 wt % iron

aqueous precipitation in presence of Fe salt precipitation of HA with Fe precursor

stoichiometric and calcium deficient iron apatites HA-5 wt % Fe2O3

sintering of coprecipitated iron apatites at 1150 °C in air/ nitrogen calcination of ball-milled powders at 900 °C for 5 h

(1 − x) HA-xFe3O4; x = 0, 5, 10, 20, and 40 wt %

conventional sintering/hotpressing in argon at 950 °C.

characterization methods XRD and Mössbauer spectroscopy electron paramagnetic resonance (EPR), Mössbauer spectroscopy and theoretical calculations Rietveld refinement of XRD data

Rietveld analysis, Mössbauer spectroscopy and Vibrating sample magnetometry (VSM) Rietveld analysis, Mössbauer spectroscopy, and X-ray photoelectron spectroscopy (XPS)

comments

reference

Fe incorporated into apatite as FeOOH, up to 1200 ppm theoretical and experimental determination of Fe2+/ Fe3+ substitution at Ca sites in HA

Mayer et al.2 Jiang et al.5

hematite formed in air sintered samples; increase in the c axis lattice parameter

Morrissey et al.6

Formation of Ca2Fe2O5 (oxygen deficient perovskite) along with HA and Fe2O3

Silva et al.7

increase in unit cell volume of HA commensurate with % of Fe-doped HA; preferential site occupancy of Fe in HA

present study

the application of formidable pressure of 70 MPa, helped to retain a considerable amount of the parent Fe3O4, with minor transformation to Fe1−xO (wüstite) and Fe-doped HA. The Mössbauer spectra for HA40Fe_HP and HA20Fe_HP are fit with two distinct sextets for the octahedral and tetrahedral Fe sites of Fe3O4. Summarizing, the present experimental results show that the lattice expansion of hydroxyapatite determined from the Rietveld analysis correlate well with the percentage of Fedoped HA formed, as analyzed using Mössbauer spectroscopy. Also, the present study illustrates the effective use of complementary characterization tools to understand the finer details of phase stability and magnetic structure of the HAFe3O4 composites. A comparison of the results of the present study with notable findings on iron-doped hydroxyapatite reported earlier is summarized in Table 13.

The detection of ppm levels of Fe in the enamel of teeth and bone has led to the development of Fe-doped HA as a potential bone or dental replacement material. Apart from bioceramic applications, hydroxyapatite is also known to be an efficient scavenger of heavy metals due to its excellent cation-exchange property.27 This property of HA has been exploited to study the electronic environment and local coordination of the Ca in the crystal structure of HA. Atomistic calculations were performed in order to determine the Mössbauer spectral parameters for Fe2+/Fe3+ substitution at the Ca(1) and Ca(2) sites in hydroxyapatite.5 In our experimental study, a predominant ferric ion substitution of Ca led to the formation of nonstoichiometric defect-filled Fe-doped HA. In the case of the air-sintered composites, one of the room temperature doublets with IS ∼ 0.55 mm/s and QS ∼ 0.9 mm/s was assigned to Fe3+ at the Ca(2) site in an octahedral symmetry while IS = 0.8 mm/s and QS = 0.8 mm/s was assigned similarly except for a 5-fold symmetry. In the hot-pressed samples, the paramagnetic doublets arising at IS = 0.31 mm/s and QS = 0.37 mm/s were designated as an Fe3+ substitution at Ca(1) in a 6fold symmetry. The site occupancy of Fe was assigned based on the theoretical calculations of the Mö ssbauer spectral parameters for Fe occupancy at the two Ca sites, as reported earlier.5 The level of iron doping into the apatite lattice has not been extensively investigated. Khudolozhkin et al. have reported the formation of stoichiometric Fe1.5Ca8.5(PO4)6F2 at 1200 °C by solid state synthesis from Ca2P2O7, CaO, FeO, and CaF2 precursors.28 However, it has not been possible to accurately determine the level of Fe doping in most studies due to the formation of nonstoichiometric apatite. In the present study, the phase transformations of magnetite during the two types of heat treatment are controlled by the sintering atmospheres. Ball-milling in ambient atmosphere has little effect on the phase stability of Fe3O4. The three sextets fit for the ball-milled samples correspond to γ-Fe2O3, and the tetrahedral and octahedral sites of Fe3O4. The faster spin relaxations with decreasing particle size led to the smaller hyperfine fields. A size-dependent ferromagnetic and superparamagnetic behavior of Fe3O4 NPs was reported earlier, wherein the Mössbauer spectra were fit with two sextets for 150 nm, one sextet for 50−10 nm, and paramagnetic doublet for 5 nm.19 Among the air-sintered composites, the driving force for the partial or complete transformation of Fe3O4 to α-Fe2O3 was the lower density of the sintered sample, especially in case of HA10Fe_S, HA20Fe_S, and HA40Fe_S (Table S1 of the Supporting Information). The greater porosity in these samples enabled the facile oxidation of magnetite to hematite. In the hot-pressed samples, heat treatment in inert atmosphere and

5. CONCLUSIONS The critical assessment of X-ray diffraction results using Rietveld refinement indicates the room temperature stability of hydroxyapatite in the monoclinic (P21/b) structure than the widely reported hexagonal (P63/m) symmetry in all the investigated monoliths and HA-Fe3O4 composites. An increase in the unit cell volume and lattice parameters of HA deduced from Rietveld analysis indicate the incorporation of Fe into the apatite lattice, while FT-IR and CHNS analysis reveal greater adsorption of CO2 and H2O in calcined HA and the ball-milled powders. Mössbauer spectral parameters suggest the nonstoichiometric substitution of Ca2+ by Fe3+ in hydroxyapatite. The iron occupancy in Fe-doped HA seemed to prefer the Ca(2) site during conventional sintering and Ca(1) site for hotpressing. Under ambient atmosphere sintering, oxidation of Fe3O4 resulted in hematite (α-Fe2O3) and maghemite (γFe2O3) as the major iron oxide phases. Hot-pressing in Ar atmosphere led to a major retention of the Fe3O4 phase with minor conversion to wüstite (Fe1−xO). The implications of the current study are that the similarity of iron doping in hydroxyapatite to that of tooth enamel and bone present the sintered HA-Fe3O4 composites as potential orthopedic biomaterials. The iron oxide phases can be incorporated into bone implant scaffolds for bone tissue engineering29,30 and hyperthermia treatment of cancerous tissues.31

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ASSOCIATED CONTENT

* Supporting Information S

Densification of the composites prepared by conventional sintering and hot-pressing routes (Table S1). Carbon and O


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hydrogen content of the ball-milled, air sintered, and hotpressed samples determined by CHNS analysis (Table S2). Figure S1 are the Rietveld refinements of the phase pure HA, while Figure S2 corresponds to Rietveld fits for the ball-milled and conventionally sintered composites. Figure S3 shows the fit Mössbauer spectra for the ball-milled powders and pristine iron oxides, while Figure S4 exhibits the FT-IR spectra for all the samples. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +91-80-2293 3256. Fax: +91-80-2360 7316. *E-mail: [email protected]. Tel: +91-80-2293 2943. Fax:+91-80-2360 7316. Author Contributions ‡

S.K.B. and A.A.V. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the support from Department of Science and Technology (DST), Department of Biotechnology (DBT), Govt. of India. The authors are grateful to Dr. Girish Kunte, Suma (MNCF, CeNSE, IISc), Prof. N. Ravishankar (MRC, IISc), and the AFMM facility for helping with TEM sample preparation and microscopy. The authors also thank the Dept. of Organic Chemistry, IISc for CHNS analysis. One of the authors, Sunil Kumar B [09/079 (2501)/ 2011-EMR-I dt. 16-11-2011], acknowledges the Council for Scientific and Industrial Research (CSIR) for providing scholarship during the period of study.

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Structural and Magnetic Phase Transformations of Hydroxyapatite

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