Formation Mechanisms in Î²-Ca3(PO4)2âZnO ... - ACS Publications

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Formation Mechanisms in β‑Ca3(PO4)2−ZnO Composites: Structural Repercussions of Composition and Heat Treatments Ponnusamy Nandha Kumar,† José Maria da Fonte Ferreira,‡ and Sanjeevi Kannan*,† †

Centre for Nanoscience and Technology, Pondicherry University, Puducherry 605 014, India Department of Materials and Ceramics Engineering, University of Aveiro, CICECO, Aveiro 3810 193, Portugal

‡

S Supporting Information *

ABSTRACT: Composites with varied proportions of βCa3(PO4)2 and ZnO were obtained through an in situ aqueous precipitation method under slightly basic (pH ≈ 8) conditions. The formation of β-Ca3(PO4)2 phase starts at an early heattreatment stage (∼800 °C) and incorporates Zn2+ ions at both Ca2+(4) and Ca2+(5) sites of the lattice up to its occupancy saturation limit. The incorporation of Zn2+ in the β-Ca3(PO4)2 lattice enhances its thermal stability delaying the allotropic βCa3(PO4)2→α-Ca3(PO4)2 phase transformation. The excess zinc beyond the occupancy saturation limit precipitates as Zn(OH)2 and undergoes dehydroxylation to form ZnO at elevated temperatures. The presence of ZnO in the β-Ca3(PO4)2 matrix yields denser microstructures and thus improves the mechanical features of sintered composites up to an optimal ZnO concentration beyond which it tends to exert an opposite effect.

1. INTRODUCTION Composites based on the combination of bioactive and bioinert ceramics have been the topic of impending interest among the researchers for the past three decades in the field of biomedicine. The bioactive components are generally based on hydroxyapatite [HAP, Ca10(PO4)6(OH)2] and β-tricalcium phosphate [β-TCP, β-Ca3(PO4)2], whereas alumina (Al2O3), zirconia (ZrO2), and titania (TiO2) fall into the category of bioinert components.1−5 Generally the motivation behind using bioinert reinforcement is to enhance the mechanical properties while keeping the bioactive features, thus taking advantages from the composite concept. In this context, several works on the synthesis and fabrication of composites based on HAP− Al2O3, HAP−ZrO2, and HAP−TiO2 were undertaken aiming at coping with the limited mechanical stability of the HAP component. However, the aimed targets were not successfully accomplished due to the thermal degradation of the bioactive component upon sintering.6−9 The dehydroxylation of HAP component during heat treatment yields free CaO, which then reacts with the bioinert component (Al2O3, ZrO2, or TiO2) to form the corresponding secondary phases such as CaAl2O4, CaZrO3, and CaTiO3. Zinc oxide (ZnO) has also been recently proposed as an alternative bioinert component to reinforce with HAP10,11 due to the attractive features of ZnO in the area of bone remodelling applications. From the medical science viewpoint, zinc is considered as one of the essential trace elements for skeletal development and plays a vital role in the stimulation of osteoblasts differentiation in bone remodelling.12,13 Moreover, ZnO is also proven to possess superior antibacterial features, © XXXX American Chemical Society

having the potential to prevent any kind of microbial attack at the implant site.14,15 In terms of materials perspective, ZnO exists in three kinds of allotropes, namely, hexagonal wurtzite, cubic zinc blende, and cubic rocksalt (B1).16 Among these allotropes, hexagonal wurtzite is considered a thermodynamically stable system with higher mechanical properties in comparison to the other ZnO polymorphs. Some literature reports focused on the development of HAP−ZnO composites reported on their promising features in terms of osteoblast adhesion, antibacterial features, cellular biocompatibility, and mechanical behavior.13,14,17 However, to the best of our knowledge, no research work has already reported on the synthesis and fabrication of β-Ca3(PO4)2−ZnO composites. Investigations have shown that a critical amount of Zn2+ could be substituted at the crystal lattice of β-Ca3(PO4)2 without imposing significant structural distortions to yield a solid solution.18−20 Here in the current investigation, we intend to form a structural and mechanically stable β-Ca3(PO4)2−ZnO composite possessing varied proportions of individual βCa3(PO4)2 and ZnO components. A simple in situ precipitation route was used to synthesize the various compositions, and the effects of increasing ZnO content on the structure and mechanical stability of β-Ca3(PO4)2−ZnO composite were evaluated and thoroughly discussed. Received: October 7, 2016

A

DOI: 10.1021/acs.inorgchem.6b02445 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

one in site 6a). The CIF files from Wilson24 and Wyckoff25 were taken as standards for refining the ZnO and HAP. The structural parameters of ZnO taken for refinement were given as space group P63mc, a = 3.24950 Å, and c = 5.2069 Å. The structural parameters of HAP taken for refinement were given as space group P63/m, a = 9.4172 Å, and c = 6.8799 Å.

2. EXPERIMENTAL METHODS 2.1. Synthesis of Powders. An aqueous precipitation technique was adopted to synthesize a wide range of β-Ca3(PO4)2−ZnO composite powders. Analytical grade Ca(NO3)2·4H2O, Zn(NO3)2· 6H2O, and NH4H2PO4 procured from Sigma-Aldrich, India, were used as precursors. Increasing amounts of Zn(NO3)2 were added to constant concentrations of Ca(NO3)2 and NH4H2PO4 to obtain five different compositions. The detailed planned compositions and the respective sample codes are presented in Table 1. In brief, for each

3. RESULTS The present study was aimed at synthesizing β-Ca3(PO4)2− ZnO powders from precursor solutions and the in situ formation of composites upon heat-treating the starting powders. The X-ray diffraction (XRD) patterns of powders heat-treated at 800 °C reveal the formation of both βCa3(PO4)2 and ZnO phases (Figure 1). A steady upsurge of the

Table 1. Molar Concentrations of the Precursors Used in the Synthesis of β-TCP−ZnO Composites molar concentration of the precursors sample code TCP 1CPZN 2CPZN 3CPZN 4CPZN 5CPZN

Ca(NO3)2· 4H2O 0.5 0.5 0.5 0.5 0.5 0.5

M M M M M M

NH4H2PO4 0.3333 0.3333 0.3333 0.3333 0.3333 0.3333

M M M M M M

Zn(NO3)2· 6H2O 0.1 0.2 0.3 0.4 0.5

M M M M M

Ca/P ratio

(Ca + Zn)/P ratio

1.5 1.5 1.5 1.5 1.5 1.5

1.500 1.800 2.100 2.400 2.700 3.000

composition a discretely prepared NH4H2PO4 stock solution was added in a dropwise manner to the stock solution mixture comprising Ca(NO3)2 and Zn(NO3)2 under constant stirring (250 rpm) at the operating temperature of 100 °C. After the completion of addition, the pH was enhanced to ∼8.0 by adding the required amounts of concentrated NH4OH solution, and continuous stirring was kept for further 2 h. The as-obtained precipitate was separated by vacuum filtration and then dried at 120 °C overnight. The dried precipitates were crushed to fine powders and hereafter designated as prepared powders. 2.2. Characterization. The as-prepared powders were subjected to a heat treatment in air atmosphere using a heating rate of 5 °C min−1 to achieve predetermined temperatures in the range of 800−1300 °C, followed by 4 h of dwelling time, and then they were cooled again to room temperature at 5 °C min−1. The as-heat-treated powders were subjected to characterization studies. The phase analysis was determined using a high-resolution X-ray diffractometer (Rigaku, Ultima IV, Japan) with Cu Kα radiation (λ = 1.5406 Å) produced at 40 kV and 30 mA to scan the diffraction angles (2θ) between 5 and 90° with a step size of 0.02° 2θ per second. Phase determinations were made using Standard International Centre for Diffraction Data (ICDD) Card Nos. 00−009−0169 for β-Ca3(PO4)2, 00−036−1451 for ZnO, 00−009−0348 for α-Ca3(PO4)2, and 00−009−0432 for HAP. Fourier transform infrared spectroscopy (FT-IR) in the transmission mode using an FT-IR spectro-photometer (PerkinElmer, USA) in the IR region (4000−400 cm−1) was performed to determine the functional groups in the powders. The vibrational modes of the composite powders were determined using back scattering geometry of confocal Raman microscope (Renishaw, U.K.). All the powder samples were excited at a wavelength of 785 nm by semiconductor diode laser (0.5% of power) with the data acquiring time of 30 s. The microstructural features of β-Ca3(PO4)2−ZnO composites were determined through high-resolution scanning electron microscopy (HRSEM; FEI-Quanta, FEG-200, Netherlands). The mechanical study was performed in accordance with the procedure described in our previous reports.21,22 Quantitative phase analyses through Rietveld refinement for selected compositions were performed using GSAS-EXPGUI software package. The initial refinement of pure β-Ca3(PO4)2 was done with the standard crystallography information file (CIF) from Yashima et al.23 The structural parameters of β-Ca3(PO4)2 taken for refinement were given as space group R3c, Z = 21, a = 10.4352 Å, and c = 37.4029 Å, 18 independent atomic positions: five Ca positions (three in site 18b and two in site 6a at one-half occupancy), three P positions (two in site18b and one in site 6a), and 10 O positions (nine in site 18b and

Figure 1. XRD patterns recorded for all the powders heat-treated at 800 °C. The diffraction lines of standard β-TCP, HAP, and ZnO, corresponding, respectively, to the ICDD Card Nos. 00−009−0169, 00−009−0432, and 00−036−1451, are also plotted.

X-ray reflections pertaining to ZnO with increasing added amounts of zinc precursors is clearly evident. The most intense XRD peaks belong to β-Ca3(PO4)2, which was formed from the calcium-deficient apatite with Ca/P molar ratio of 1.5 as endorsed by the previous studies.26,27 A gradual intensity reduction in the corresponding X-ray reflections of βCa3(PO4)2 and a concomitant strengthening of the ZnO reflections is also noted from Figure 1. The presence of any apatite at trace level could not be confirmed from the XRD results, as both HAP and ZnO phases share common reflections in the 2θ region, especially the maximum intensity peak of HAP overlap with ZnO peak ∼31.8° 2θ. The Raman spectra recorded for powder compositions heattreated at 800 °C (Figure 2) display the characteristic peaks of β-Ca3(PO4)2 and ZnO phases, in good correlation with the XRD results. The following vibrations characteristic of PO4 tetrahedra from β-Ca3(PO4)2 in TCP composition were determined in the Raman spectral regions at 947 and 969 cm−1 (υ1-symmetric P−O stretching), 406, 441, and 481 cm−1 (υ2-doubly degenerate O−P−O bending), 1046 and 1089 cm−1 (υ3-triply degenerate asymmetric P−O stretching), and 548, 612, and 627 cm−1 (υ4-triply degenerate O−P−O bending).28,29 In the presence of ZnO, the characteristic vibrations of PO4 tetrahedra from β-Ca3(PO4)2 appear slightly shifted toward higher wavenumber region especially for the 947 cm−1 υ1 band that showed a uniform shift to 952 cm−1 for all the ZnO-containing compositions. Moreover, it is also noted from B


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cm−1.34,35 The FT-IR spectra also ascertain the presence of apatite phase by the virtue of its characteristic sharp −OH band observed at 3572 cm−1.36,37 This means an incomplete transformation of calcium deficient apatite to pure βCa3(PO4)2 phase at 800 °C. The observed steady gain in intensity of this −OH band with increasing zinc content suggests a concomitant increase in the amount of apatite phase formed. The FT-IR patterns of all powders heat-treated at 800 °C also illustrate the invariable presence of −OH moieties at 3488 cm−1 attributed to its chemical linkage with ZnO.38,39 Further increasing the heat-treatment temperature to 1100 °C enhanced the crystallization of both β-Ca3(PO4)2 and ZnO phases as deduced from the gains in intensity of their X-ray reflections (Figure 4) in comparison to their respective patterns

Figure 2. (a) Raman spectra recorded for all the powders heat-treated at 800 °C; (b) enlarged region (900−1000 cm−1) to highlight the typical β-Ca3(PO4)2 Raman doublet.

the Raman spectra that the doublet peaks at 947 and 969 cm−1 determined for TCP tend to gradually merge with incremental amounts of ZnO in the composition. The Raman spectral bands typical of ZnO are also noticed from the peaks at 438 and 588 cm−1.30,31 The intensity of the 438 cm−1 band seems to steadily increase with increasing ZnO content. However, for all ZnO-containing compositions, the presence of apatite phase in the powders heat-treated at 800 °C could not be confirmed from either Raman or XRD. The FT-IR spectra recorded for the powders heat-treated at 800 °C (Figure 3) confirmed the presence of both β-Ca3(PO4)2

Figure 4. XRD patterns recorded for all the powders heat-treated at 1100 °C. The diffraction lines of standard β-TCP, HAP, and ZnO, corresponding, respectively, to the ICDD Card Nos. 00−009−0169, 00−009−0432, and 00−036−1451, are also plotted.

at 800 °C. The same intensification effects are also evident in the Raman spectra upon increasing the heat-treatment temperature from 800 to 1100 °C (Figure 5). The merging trend of 947 and 969 cm−1 Raman doublet at 800 °C is witnessed only for the 5CPZN that possesses the highest ZnO content, whereas all the other compositions indicated sharp and well-resolved doublet at 1100 °C. The FT-IR spectra recorded at 1100 °C (Figure 6) enable inferring about the absence of apatite phase for all the compositions except the 5CPZN that still indicates a low-intensity band corresponding to −OH group of apatite at 3572 cm−1. The FT-IR band of ZnO at 482 cm−1 also indicates a steady rise in intensity with increasing ZnO content in the compositions. The XRD patterns of the powders heat-treated at 1300 °C are displayed in Figure 7. It can be seen that the TCP sample underwent the β-Ca3(PO4)2→α-Ca3(PO4)2 allotropic transformation. This transformation corroborates well with similar observations in previous literature reports.40,41 Interestingly, the thermal stability of all the ZnO-containing β-Ca3(PO4)2 powders was significantly enhanced in comparison to that of TCP sample as deduced from the absence of α-Ca3(PO4)2 after heat-treating at 1300 °C. This absence means that the allotropic β-Ca3(PO4)2→α-Ca3(PO4)2 phase transformation was hindered upon heating, or if occurred in some extent, it was easily reversed upon cooling, similarly to what happens in Mgdoped tricalcium phosphate reported elsewhere.42 The Raman

Figure 3. FT-IR spectra recorded for all the powders heat-treated at 800 °C.

and ZnO in all the compositions. The specific vibrations of PO4 tetrahedra of TCP are determined at 943 and 973 cm−1 (υ1symmetric P−O stretching), 1122 and 1192 cm−1 (υ3-triply degenerate asymmetric P−O stretching), and ∼555 and ∼610 cm−1 (υ4-triply degenerate O−P−O bending).32,33 The typical vibrations of ZnO appear in the IR region at 461 and 554 C


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Figure 7. XRD spectra recorded for all the powders heat-treated at 1300 °C. The diffraction lines of standard β-TCP, α-TCP, and ZnO, corresponding, respectively, to the ICDD Card Nos. 00−009−0169, 00−009−0348, and 00−036−1451 are also plotted.

Figure 5. (a) Raman spectra recorded for all the powders heat-treated at 1100 °C. (b) Enlarged region (900−1000 cm−1) to highlight the typical β-Ca3(PO4)2 Raman doublet.

Figure 8. Raman spectra recorded for all the powders heat-treated at 1300 °C.


vibrations unambiguous of both β-Ca3(PO4)2 and ZnO were determined at 1300 °C (Figure 8). The merging behavior of 947 and 969 cm−1 Raman doublet witnessed for 5CPZN at 1100 °C resulted in the formation of a well-defined doublet at 1300 °C. FT-IR spectra recorded at 1300 °C (Figure 9) display good coincidence with their corresponding XRD and Raman results, as the distinct vibrations characteristic of both βCa3(PO4)2 and ZnO appear well-noticed. The Rietveld refinement was performed for the XRD patterns of powders heat-treated at both 1100 and 1300 °C aiming at providing quantitative information about their phase compositions. The refinement at 1100 °C was performed with the inclusion of all the CIF files of Ca10(PO4)6(OH)2, βCa3(PO4)2, and ZnO based on the indication of apatite presence detected from FT-IR results. The refinement results at


D


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Table 2. Refined Structural Parameters and Phase Fractions Determined from Refinement of Various β-TCP−ZnO Composites after Heat Treatment at 1100 °C β-Ca3(PO4)2

phase fractions (wt %) sample code

β-Ca3(PO4)2

TCP 1CPZN 2CPZN 3CPZN 4CPZN 5CPZN

100 93.91 84.70 76.90 67.25 54.60

ZnO 06.90 15.30 19.90 26.05 31.45

HAP

03.20 06.70 13.95

a = b-axis (Å) 10.4384 10.3514 10.3492 10.3499 10.3503 10.3508

(3) (3) (4) (5) (3) (5)

ZnO

c-axis (Å) 37.4010 37.1735 37.1778 37.1789 37.1723 37.1744

(1) (2) (2) (1) (3) (2)

a = b-axis (Å) 3.2455 3.2464 3.2480 3.2485 3.2473

(2) (1) (4) (9) (5)

HAP c-axis (Å) 5.1983 5.2002 5.2009 5.2041 5.2013

(5) (3) (2) (8) (2)

a = b-axis (Å)

c-axis (Å)

9.4651 (9) 9.4673 (3) 9.4623 (4)

6.1395 (1) 6.1345 (4) 6.1375 (3)

Figure 10. Refined diffraction patterns of the five different β-Ca3(PO4)2−ZnO composites heat-treated at 1300 °C: (a) 1CPZN; (b) 2CPZN; (c) 3CPZN; (d) 4CPZN; (e) 5CPZN. Calculated (red lines); background (yellow lines); difference (pink lines); Bragg of β-Ca3(PO4)2 (green ticks); Bragg of ZnO (blue ticks).

1100 °C (Supporting Information) deliberated the presence of both β-Ca3(PO4)2 and ZnO phases in all the compositions. However, 3CPZN, 4CPZN, and 5CPZN compositions perceived the presence of Ca10(PO4)6(OH)2 along with the β-Ca 3 (PO 4 ) 2 and ZnO. The refined lattice data for Ca10(PO4)6(OH)2, β-Ca3(PO4)2, and ZnO crystallized in the hexagonal unit cell [P63/m (No. 176)-space setting], the rhombohedral unit cell [R3c̅ (No. 167)-space setting], and the

hexagonal unit cell [P63/mc (No. 186)-space setting], respectively, are reported in Table 2. The refinement results also contemplated a steady increment in the ZnO content with respect to its enhanced additions during the synthesis. In other words, β-Ca3(PO4)2 content envisaged a steady decline with incremental amounts of ZnO. Moreover, Ca10(PO4)6(OH)2 indicated a steady increment in its content in the order 3CPZN < 4CPZN < 5CPZN. The refinement results at 1300 °C E


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Table 3. Refined Structural Parameters and Phase Fractions Determined from Refinement of Various β-TCP−ZnO Composites after Heat Treatment at 1300 °C β-Ca3(PO4)2

phase fractions (wt %) sample code a

TCP 1CPZN 2CPZN 3CPZN 4CPZN 5CPZN a

β-Ca3(PO4)2 100 94.50 85.20 77.30 72.00 67.70

ZnO

a = b-axis (Å) 10.4384 10.3531 10.3472 10.3547 10.3552 10.3543

05.50 14.80 22.70 28.00 32.30

(3) (3) (3) (4) (4) (4)

ZnO

c-axis (Å) 37.4010 37.1703 37.1719 37.1729 37.1712 37.1730

a = b-axis (Å)

(1) (2) (4) (1) (3) (2)

3.2433 3.2466 3.2469 3.2468 3.2472

(2) (1) (4) (3) (2)

refinement parameters c-axis (Å)

cRwp

cRp

χ2

RBragg

5.1964 5.1992 5.1994 5.1999 5.2000

07.15 06.94 05.89 05.76 05.08 05.20

08.44 05.24 04.49 04.35 03.77 03.81

1.114 1.341 1.269 1.446 1.246 1.428

4.29 05.43 05.03 06.41 05.41 05.09

(3) (3) (2) (1) (2)

The refined data of TCP are accounted at 1100 °C.

(Figure 10 and Table 3) enunciated the distinct presence of βCa3(PO4)2 and ZnO in all the compositions. Attempts were also made to refine all the compositions by the inclusion of Ca10(PO4)6(OH)2, and the corresponding results indicated a significant misfit, and this implies its absence in all the compositions heat-treated at 1300 °C. In accordance with the phase content determined at 1100 °C, a steady increment in the phase content of ZnO with simultaneous decline in the βCa3(PO4)2 content is noticed at 1300 °C. The preferential occupancy of Zn2+ at the lattice sites of β-Ca3(PO4)2 was also determined through refinement. The refined occupancy values are attributed to the preferential accommodation of Zn2+ at both the Ca2+(4) and Ca2+(5) lattice sites of β-Ca3(PO4)2 (Table 4).

4. DISCUSSION The results from this investigation confirmed the discrete formation of β-Ca3(PO4)2−ZnO composites at 1300 °C from the aqueous precipitated powders. However, the characterization results ascertained that composite formation is not a single-step process but rather depends on the concentrations of the precursors, heat-treatment temperatures, and the thermal stabilizing role of zinc in the β-Ca3(PO4)2 structure. Note that all the reactions were performed in slightly basic conditions (pH ≈ 8), and hence the precipitated powders are expected to possess excess adsorbed water and lattice-bound water in the as-dried conditions. The phase analysis of the powders confirmed the formation of calcium-deficient apatite phase and zinc hydroxide in the as-dried conditions. The adsorbed water on the surface of the precipitated powders is easily eliminated at low temperatures, whereas high thermal energy is required to eliminate the lattice-bound water. Crystalline βCa3(PO4)2 and ZnO phases have been already formed in the powders heat-treated at 800 °C, but some lattice-bound water still remained under these conditions. ZnO ensures its chemical linkage with OH moiety to form zinc hydroxide at 800 °C as confirmed by the FT-IR spectral band determined at 3488 cm−1. The presence of calcium-deficient apatite was confirmed from its typical −OH vibration at 3572 cm−1 IR region in all the compositions at 800 °C. Generally, the Raman spectral band typical of apatite phase is determined at ∼960 cm−1.43,44 The presence of apatite phase in the powders heat-treated at 800 °C was perceived by the merging behavior of typical βCa3(PO4)2 947 and 969 cm−1 Raman doublet. A gradual merging tendency of this particular doublet with enhanced ZnO content ensures the increased apatite presence. However, TCP composition that is devoid of any added ZnO confirms the absence of −OH group. This signals the fact that the transformation of calcium-deficient apatite into β-Ca3(PO4)2 gets accomplished at 800 °C in good agreement with the previously reported results.26,27 Thus, the detection of apatite phase at 800 °C in all the ZnO-containing compositions enunciates the delaying role of ZnO in the formation of βCa3(PO4)2. The characterization results of powders heat-treated at 1100 °C still affirm the presence of apatite in the compositions 3CPZN, 4CPZN, and 5CPZN. The apatite signs become more evident as ZnO content increases, and the FT-IR −OH band at 3572 cm−1 appears well-defined for 5CPZN. Moreover, the merging trend of 947 and 969 cm−1 Raman doublet distinctive of β-Ca3(PO4)2 is witnessed only for 5CPZN. The characterization results at 800 and 1100 °C converge toward a decreasing tendency of apatite formation, as the heat-treatment temperature increases. This tendency was kept for the

Table 4. Zn2+ Occupancy at the Crystal Lattice of β-TCP Component in β-TCP−ZnO Composites Sintered at 1300 °C and the Refined Zn-Doped β-TCP Compositions Zn2+ occupancy at β-TCP sample code TCP 1CPZN 2CPZN 3CPZN 4CPZN 5CPZN

Ca2+(4) site 0.13 0.14 0.20 0.19 0.21

(2) (1) (2) (3) (2)

Ca2+(5) site 0.46 0.54 0.46 0.45 0.47

(2) (2) (3) (4) (3)

refined compositions Ca2.82Zn0.18(PO4)2 Ca2.81Zn0.19(PO4)2 Ca2.81Zn0.19(PO4)2 Ca2.81Zn0.19(PO4)2 Ca2.80Zn0.20(PO4)2

The morphological features observed by SEM analysis (Figure 11) reveal the formation of dense microstructures and the absence of pores in all the composites. The SEM microstructures consist of small ZnO grains formed preferentially at the grain boundaries of the relatively large β-Ca3(PO4)2 grains, the size of which tend to decrease with increasing ZnO content in the composites. The elemental mapping analysis also confirmed the uniform distribution of Ca2+, Zn2+, P5+, and O2− throughout the microstructure (Supporting Information). The selective mechanical properties of hardness and Young’s modulus determined from nanoindentation are presented in Table 5. The mechanical properties of the investigated βCa3(PO4)2−ZnO composites were noticeably enhanced with the first ZnO addition (1CPZN), followed by a roughly a steady increasing trend to maximum values for 3CPZN, and then by a gradual regression for higher ZnO contents. From the TCP to the 3CPZN samples, Young’s modulus and hardness increased for ∼3 and 1.5 times, respectively. The load versus displacement curves of all the β-Ca3(PO4)2−ZnO composites deliberated from indentation tests display smooth loading and unloading curve for all the specimens (Figure 12). F


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Figure 11. SEM images of selected β-Ca3(PO4)2−ZnO composite specimens sintered at 1300 °C: (a, b) 1CPZN; (c, d) 3CPZN; (e, f) 5CPZN.

Table 5. Mechanical Data of β-TCP−ZnO Composites Determined from Nanoindentation sample code TCP 1CPZN 2CPZN 3CPZN 4CPZN 5CPZN

Young’s modulus (GPa) 37.456 99.379 96.343 115.054 107.368 91.485

± ± ± ± ± ±

5.839 6.012 5.313 7.260 6.353 5.487

Ca3(PO4)2 component in the composites indicate strong contractions along all axes in comparison to the lattice data determined for TCP sample. The observed contractions are ascribed to the accommodation of lower-sized Zn2+ (0.745 Å for sixfold coordination with O) at Ca2+ (1.00 Å for sixfold coordination with O) sites in the β-Ca3(PO4)2 structure. Moreover, the contractions of lattice parameters were uniform and independent of the ZnO content. This suggests that Zn2+ saturation occupancy limit has been already achieved for 1CPZN composite. The refined lattice data for ZnO in all the compositions displayed good uniformity and coherence with the lattice data of standard CIF used for refinement. Attempt to refine the possibility of Ca2+ occupancy at the ZnO lattice resulted in negative results, and from this we infer that ZnO component of the composite is distinct and that its structure is not subjected to any alterations. The refined occupancy factors approve the Zn2+ occupancy at the Ca2+(4) and Ca2+(5) sites of β-Ca3(PO4)2 lattice. Similarly to the uniform contraction undergone by the lattice

hardness (GPa) 2.665 3.666 3.510 4.105 4.032 3.006

± ± ± ± ± ±

0.410 0.463 0.219 0.164 0.141 0.322

composite powder calcined at 1300 °C, where only the discrete presence β-Ca3(PO4)2 and ZnO components were identified. While pure TCP signaled its allotropic transformation from β-Ca3(PO4)2 to α-Ca3(PO4)2 at 1300 °C, this transformation was prevented in all the composite systems. The gradual increment in the ZnO phase content and a concomitant decline in the β-Ca3(PO4)2 component coincides well with our experimental plan. The refined lattice parameters of the βG


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Figure 12. Nanoindentation plots of β-Ca3(PO4)2−ZnO composites sintered at 1300 °C: (a) 1CPZN; (b) 2CPZN; (c) 3CPZN; (d) 4CPZN; (e) 5CPZN.

parameters of β-Ca3(PO4)2 due to Zn2+ incorporation, the refined occupancy values also displayed good uniformity for all the β-Ca3(PO4)2 component of the five different composites. Further, the refined compositions (Table 4) also reveal an almost uniform saturation level of Zn2+ in the β-Ca3(PO4)2 lattice. The Zn2+ occupancy at the β-Ca3(PO4)2 lattice is also justified by the uniform shift in 947 cm−1 Raman band of TCP toward higher wavenumber of 952 cm−1 in all the composites. This Raman shift toward higher wavenumber due to the substitution of lower-sized ion for Ca2+ in β-Ca3(PO4)2 is in accordance with the previous studies.45,46 It has been stated that β-Ca3(PO4)2 forms solid solution by the accommodation of ∼10 mol % Zn2+ at its lattice.47 However, the saturation limit of Zn2+ occupancy at β-Ca3(PO4)2 was not explored in that particular study. The synthesis in the present study was performed with Ca/P molar ratio of 1.5 and hence results in the formation of β-Ca3(PO4)2. But its defective nature with five

equivalent Ca2+ sites23,48 enables a selective uptake of Zn2+ in the structure. The results from the refinement support the absence of free CaO in any of the investigated compositions. The absence of CaO after the replacement of Ca2+ by Zn2+ could be explained on the basis of our previous study using excess Mg2+ beyond its saturation occupancy limit, which did not reveal any sign of free CaO but the preferential formation of MgO.46 The preferential accommodation of Zn2+ at the βCa3(PO4)2 lattice is well-known.45,49,50 Oppositely, the partial replacement of Ca2+ by other ions such as Sr2+ released from the HAP lattice, beyond stoichiometric ratio of 1.67, results in the formation of CaO.36 Thus, the calcium phosphate phase formed is a trade-off with the Ca/P ratio, the added amount of doping element, and particularly its ionic size that determines the fitting easiness in the crystalline lattice sites. HAP and βCa3(PO4)2 that possess different Ca/P ratios display contrasting behaviors on doping with Zn2+ ions. Zn2+ occupies the H


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Inorganic Chemistry

achieved at 1300 °C; (vi) zinc enhanced densification, enabled achieving composites with uniform sintered microstructures devoid of pores and cracks, and improved mechanical properties (hardness and Young’s) up to 3CPZN; (vii) the salient features of β-Ca3(PO4)2−ZnO composites make them very promising for further investigations aiming at exploring their potential for biomedical applications.

interstitial sites of HAP, while its preferential accommodation at the β-Ca3(PO4)2 lattice is reported.45,49,50 Beyond its saturation limit, the freely available Zn2+ crystallizes to form a stable hexagonal wurtzite ZnO component. Since majority of PO43− ions have been consumed by Ca2+ to form β-Ca3(PO4)2, the free Zn2+ is left with negligible PO43− to yield any form of zinc phosphates. The results also revealed that Zn2+ accommodation preserved the structural stability of β-Ca3(PO4)2 lattice until 1300 °C without its allotropic transformation to form α-Ca3(PO4)2. This inference corroborates the enhanced thermal stability of βCa3(PO4)2 derived from the incorporation of impurities in its lattice.51−53 The morphological analysis revealed the formation of dense microstructures that are devoid of pores in all the βCa3(PO4)2−ZnO composites. The mapping results also perceived a uniform distribution of Ca2+, Zn2+, P5+, and O2− elements throughout the microstructures. The squeezing of the excess ZnO from the initial homogeneous compositions at the atomic level led to the preferential crystallization of ZnO at the grain boundaries of the first crystallized β-Ca 3 (PO 4 ) 2 component. The as-formed ZnO inclusions are likely to exert an arresting effect of the grain boundaries and a consequent delay in grain growth of β-Ca3(PO4)2 as observed in Figure 11. The Young’s modulus and hardness data attained for pure TCP are in good accordance with similar indentation data reported for stoichiometric β-Ca3(PO4)2.54 The strong improvements in mechanical properties of β-Ca3(PO4)2−ZnO composites make them promising for load-bearing applications. All of them exhibit superior Young’s modulus and hardness in comparison to those reported for human bones.55,56 The improvement in mechanical properties can be attributed to the synergetic combination of different factors including: (i) the enhanced thermal stability of Zn-doped β-Ca3(PO4)2 that prevents the formation of α-Ca3(PO4)2 phase in the samples sintered at 1300 °C; (ii) the enhanced densification of composites promoted by the presence of ZnO, enabling porosity to be more effectively eliminated;57 (iii) the reinforcing role of ZnO preferentially segregated as secondary phase at the grain boundaries58 of the main β-Ca3(PO4)2 phase.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02445. Refined diffraction patterns of the five different βCa3(PO4)2−ZnO composites heat-treated at 1100 °C. Calculated, background, difference, Bragg of βCa3(PO4)2, Bragg of ZnO. Scanning electron micrographs and the corresponding elemental mappings for the composites 1CPZN and 5CPZN (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 0091-413-2654973. ORCID

José Maria da Fonte Ferreira: 0000-0002-7520-2809 Sanjeevi Kannan: 0000-0003-2285-4907 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful for the financial support received from the Council of Scientific and Industrial Research, India (CSIR Scheme No. 22(612)/12/EMRII). The Instrumentation facility availed from the Central Instrumentation Facility of Pondicherry Univ. is greatly acknowledged. The support of CICECO-Aveiro Institute of Materials (Ref. No. UID/CTM/ 50011/2013), funded by FEDER funds through the Operational Programme Competitiveness Factors (COMPETE 2020) and the Portuguese Foundation for Science and Technology is acknowledged.

5. CONCLUSIONS We have shown that a simple aqueous precipitation method is suitable for the synthesis of β-Ca3(PO4)2−ZnO composites with varied proportions of their individual components. On the basis of a broad array of characterization techniques (XRD, FTIR and Raman spectroscopies, Rietveld refinement, SEM analysis, and indentation tests) the following conclusions can be drawn: (i) The β-Ca3(PO4)2 is early formed from the calcium-deficient apatite at temperatures as low as 800 °C; (ii) zinc ions readily occupy the Ca2+(4) and Ca2+(5) sites of βCa3(PO4)2 lattice up to its saturation limit and exert a thermal stabilizing effect, delaying the allotropic β-Ca3(PO4)2→αCa3(PO4)2 phase transformation; (iii) the excess of zinc in the composition is squeezed out of the structure and concentrates preferentially at the grain boundaries, initially combined with OH moieties, and then transforms into ZnO as heat-treatment temperature increases; (iv) discrete βCa3(PO4)2−ZnO composites start appearing at 1100 °C for the lower zinc content, while higher temperatures are required to accomplish the composite phase formation for the higher added amounts of zinc; (v) highly crystalline β-Ca3(PO4)2− ZnO composites devoid of additional by products have been

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